CHAPTER II
ABSORPTION, ASSIMILATION, AND THE PROCESSES OF METABOLISM
Topics: Physiological peculiarities in absorption. Chemical changes in epithelial walls of intestine. Two pathways for absorbed material. Function of the liver as a regulator of carbohydrate. Absorption of proteid products. Assimilation of food products. Anabolism. Katabolism. Metabolism. Processes of metabolism. Older views regarding oxidation. Discoveries of Lavoisier. The views of Liebig. Theory of luxus consumption. Oxidation in the body not simple combustion. Oxygen not the cause of the decompositions. Oxidation not confined to any one place. Intracellular enzymes. Living cells the guiding power in katabolism. Some intermediary products of tissue metabolism. Chemical structure of different proteids. Decomposition products of nucleoproteids. Relation to uric acid. Action of specific intracellular enzymes. Creatin and creatinin. Relation to urea. Proteid katabolism a series of progressive chemical decompositions. Intracellular enzymes as the active agents.
Digestion being completed, and the available portion of the foodstuffs thereby converted into forms suitable for absorption, the question naturally arises, In what manner are these products transported from the alimentary tract to the tissues and organs of the body? In attempting to answer this question, we shall find many illustrations of the precise and undeviating methods which prevail in the processes of nutrition. For example, it would seem plausible to assume that the different forms of sugar entering into man’s ordinary diet, all of them being soluble, would be directly absorbed and at once utilized, but such is far from being the case. Milk-sugar and cane-sugar, both appearing in greater or less degree in our daily dietaries, if introduced directly into the blood, are at once excreted through the kidneys unchanged. The body cannot use them, and they are gotten rid of as speedily as possible, much as if they were poisons. When taken by way of the mouth, however, they are utilized, simply because in the intestine two enzymes are present there, known as lactase and invertase, which break each of the sugars apart into two smaller molecules. In other words, milk-sugar and cane-sugar are disaccharides, and if they are to be absorbed in forms capable of being made use of by the body they must be split apart into simpler sugars, viz., monosaccharides, such as dextrose, levulose, etc. The great bulk of the carbohydrate food consumed by man is in the form of starch, and this, as we have seen, is converted into maltose by the action of saliva and pancreatic juice. Maltose, however, like cane-sugar, is a disaccharide, and the body has no power to burn it or utilize it directly; but in the intestine and elsewhere is an enzyme termed maltase, which breaks up maltose into two molecules of the monosaccharide dextrose, and this the body can use. Man frequently consumes starch to the extent of a pound a day, and if utilized it must all undergo transformation into maltose, and then into dextrose. There is no apparent reason why maltose should not be absorbed and assimilated as readily as dextrose, but so urgent is the necessity for this conversion into dextrose that in the blood itself there is present maltase, to effect the transformation of any maltose that may gain entrance there. We are here face to face with a simple fact in nutrition. The body cannot utilize disaccharides directly. Why it is so we cannot say, but the fact is a good illustration of the principle that nothing can be taken for granted in our study of nutrition.
For years, physiologists assumed that the ordinary physical laws of osmosis, imbibition, and diffusion were quite adequate to explain the passage of digested food materials into the blood and lymph. If a substance was soluble and diffusible, that was sufficient; it would quite naturally be absorbed in harmony with its diffusion velocity. This, however, is not wholly true, since experiment shows that the rapidity of absorption of diffusible substances through the wall of the intestine is by no means always proportional to the diffusion velocity of the substance. The lining membrane of the small intestine, where absorption mainly takes place, is not to be compared to a dead parchment membrane. On the contrary, it is made up of living protoplasmic cells; absorption is not a physical, but a physiological, process, in which the living epithelium cells stand as guardians of the portals, ready to challenge and, if need be, modify the rate of passage. Osmosis and diffusion undoubtedly play some part in absorption, but they alone are not sufficient to account for what actually takes place in the absorption of digestion products, and other substances from the living intestine.
The primary products formed in the digestion of proteid foods—the proteoses and peptones—afford another illustration of physiological peculiarity in absorption. These bodies are readily soluble and quite diffusible, yet they are never found to any extent in the circulating blood and lymph during health. It is of course possible, as has been previously suggested, that as soon as formed they undergo transformation into simpler decomposition products in the small intestine; but this is by no means certain. If proteoses and peptones are injected directly into the blood, they cause a marked disturbance, influencing at once blood-pressure, affecting the coagulability of the blood, and in many other ways exhibiting a pronounced deleterious action which at once indicates they are out of their normal environment. They are not at home in the circulating blood, and the latter medium gets rid of them as speedily as possible; they behave like veritable poisons, and yet they are the primary products formed in the digestion of all proteid foodstuffs. On the basis of all physical laws governing diffusion they should be absorbed, and help to renew the proteids of the blood and later the proteids of the tissues. Yet, as we have said, they are not normally present in the blood or lymph. Apparently, in the very act of absorption, as they pass through the epithelial cells of the intestinal wall, before they gain entrance to the blood stream, they undergo transformation into serum-albumin and globulin, the characteristic blood proteids. The other alternative is that, as previously mentioned, they are completely broken down in the intestine into amino-acids, etc., and these simpler products synthesized, as they pass through the intestinal wall toward the blood, into serum-albumin and globulin. Certainly as yet, there is no evidence that the amino-acids, as such, go through the epithelial cells of the intestine; they are not found in the blood or lymph to any appreciable extent, yet the proteids of the blood are reinforced in some manner by the products of proteid digestion. Whichever view is correct, one thing is perfectly obvious, viz., that in the act of absorption the products resulting from the gastric and pancreatic digestion of proteid foods are exposed to some influence, presumably in the epithelial cells of the intestinal wall, by which there is a reconstruction of proteid. Further, the proteid substances so formed are of the type peculiar to the blood of that particular species of animal. The proteids of beef, mutton, chicken, oatmeal, or bread go to make the proteids of human blood.
From these statements, it is obvious that what we term absorption is something more than a simple diffusion of soluble substances from the alimentary tract into the blood current. The process is much more complex than appears on the surface, and our lack of definite knowledge, in spite of numerous efforts to unravel the mystery, merely strengthens the view that we are dealing here with an obscure physiological problem, and not a simple physical one. Digestion induces a splitting up of the food proteid into fragments, large or small, while incidental to absorption there is apparently a reconstruction, or synthesis, of proteid from the fragments so formed. The process seems somewhat costly, physiologically speaking, yet when one considers the variety of proteids consumed as food, it is easy to comprehend how essential it is that in some manner, as in absorption, there be opportunity for construction of the specific proteids of the blood and lymph.
We find an analogous process in the absorption of fats. As we have seen, the fats of the food are broken apart in the small intestine into glycerin and free fatty acid, a portion of the latter, and perhaps all, combining with the alkali of the intestinal juices to form soluble soaps, or sodium salts of the respective fatty acids. The neutral fats present in animal and vegetable foods are all alike in containing the glyceryl radicle, but they differ in the character of the fatty acids present. Further, one form of animal fat, like that from beef, may contain quite a different proportion of stearin, palmitin, and olein than is present in the fat of another animal, like mutton. By digestion, however, they are all broken apart into fatty acid and glycerin. These acids and their salts can be readily detected in the intestine, but they are not found in the blood or lymph, yet shortly after fatty food is taken the lymph is seen to be milky from fat. Obviously, the fatty acids liberated in the intestine are absorbed, either as soluble soaps or as free fatty acids dissolved in bile, but as they pass through the epithelial cells of the intestine into the lacteal radicles, there is a synthesis or reconstruction of fat; and as a result, neutral fats and not soaps are found in the lymph. Here, then, we have a process quite analogous to what apparently occurs in the absorption of proteid, though less complex; and it is possible that this is one of the factors which aids in the formation of a specific fat mixture corresponding, in a measure, to the type of fat present in the particular species. It is well understood that the fat of an animal’s tissues may be modified somewhat by the character of the fat fed, yet in spite of this there is a certain degree of constancy in composition which calls for explanation. Sheep and oxen feeding in the same pasture have fat widely different in the proportion of stearin, palmitin, etc. The fat of man’s tissues is fairly definite in composition, yet he eats a great variety of fatty foods. One man may consume large amounts of hard mutton fat with its relatively large content of stearin, while another individual may take his fat mainly in the form of the soft butter fats, with their relatively large content of olein and palmitin. In both cases, the fat of the man’s tissues will be essentially the same. To be sure, the changes that take place in the tissue cells, reinforced by the construction of fat from other sources, may be partly responsible for this constancy of composition, but the transformations incidental to absorption are quite possibly, in some measure, helpful thereto.
The great bulk of the digested food material is absorbed from the small intestine, and there are two pathways open through which the absorbed material can gain access to the blood. The one path leads directly to the liver, and substances taking this course are exposed to the action of this organ, before they enter into the general circulation. The other path is through the lacteal or lymphatic system, and constitutes a roundabout way for substances to enter the blood stream, since they must first pass through the thoracic duct before entering the main circulation. As a general truth, it may be stated that fats are absorbed through the latter channel, while carbohydrates and proteids follow the first path. The innumerable blood capillaries in the villi of the intestine take up the products resulting from the digestion of proteids and carbohydrates, through which they are passed into the portal vein, and thereby distributed throughout the liver. This means that both carbohydrates and proteids—or their decomposition products—are exposed to a variety of possible changes in this large glandular organ, before they can enter into the tissues of the body. As we have seen, practically all carbohydrate food is converted into a monosaccharide, principally dextrose, in the alimentary tract; and it is in this form of a simple sugar that the carbohydrate passes into the blood. This might easily mean a pound of sugar absorbed during the twenty-four hours, and would obviously give to the blood a high degree of concentration, unless the excess was quickly disposed of. Sugar is very diffusible, and if it accumulates to any extent in the blood it is quickly gotten rid of by excretion through the kidneys. This, however, is wasteful, physiologically and otherwise, and does not ordinarily occur except in diseased conditions. Further, physiologists have learned that a certain small, but definite, amount of sugar in the blood is a necessary requirement in nutrition, and it is the function of the liver to maintain the proper carbohydrate level.
We must again emphasize the great importance of carbohydrate food; there is a far larger amount of starchy food consumed than of any other foodstuff, and it is more readily available as a source of energy. Its presence in the blood, in the form of sugar, is constantly demanded, but it must be kept within the proper limits for the uses of the different tissues and organs of the body. The liver serves as an effective regulator, maintaining, in spite of all fluctuations in the supply and demand, a definite percentage of sugar such as is best adapted to keep the tissues of the body in a normal and healthy condition. This regulation by the liver is rendered possible through the ability of the hepatic cells to transform the sugar brought to the gland into glycogen, so-called animal starch, which is stored up in the liver until such time as it is needed by the body. The process is one of dehydration, the reverse of what takes place in the intestine when ordinary starch is converted into maltose and dextrose. The efficiency of this regulating mechanism depends also upon the ability of the liver to transform glycogen into sugar, presumably through the agency of an enzyme in the hepatic cells. Hence, glycogen may be looked upon as a temporary reserve supply of carbohydrate, manufactured and stored in the liver during digestion, when naturally large amounts of sugar are passing into the portal blood, and to be drawn upon whenever from any cause the content of sugar in the blood threatens to fall below normal. Obviously, there must be some delicate machinery for the adjustment of these opposite changes in the liver, and we may well believe that it is associated with the composition of the blood itself, which in some fashion stimulates and inhibits, as may be required, the functional activity of the liver, or its component cells. In any event, we have in this so-called glycogenic function of the liver a most effective means for accomplishing the complete and judicious utilization of all the sugar formed from the carbohydrates of the food, after it has once passed beyond the confines of the alimentary tract into the blood; preventing all loss, and at the same time guarding against all danger, from undue accumulation of sugar in the circulation. We see, too, how wise the provision that all sugar should pass from the alimentary canal into the portal circulation and not by way of the lymphatics, since by the latter channel the regulating action of the liver would be mainly lost. Further, recalling how soluble and diffusible sugar is, we may well marvel that it practically all passes from the intestine by way of the blood, and escapes entry into the lymphatics. Surely, this marked shunning of the other equally accessible pathway affords a striking illustration of selective action such as might be expected in a physiological process, but not in harmony with the ordinary physical laws of osmosis or diffusion. In conformity with this statement, it may be mentioned that appropriate experiments have clearly demonstrated that the different sugars available as food are not absorbed from the intestine in harmony with their diffusion velocity, but show deviations therefrom which can be explained only on the ground that the intestinal wall exercises some selective action, due to the living cells composing it. Likewise interesting in their bearing on nutrition are the observations of Hofmeister,[13] who finds by experiments on dogs that the assimilation limit of the different sugars shows marked variation. Thus, dextrose, levulose, and cane-sugar have the highest assimilation, while milk-sugar is far less easily and completely assimilated. If this is equally true of man, it indicates that starchy foods, with their ultimate conversion into dextrose, are to be ranked as having a high assimilation limit, thus affording additional evidence of their high nutritive value.
In the absorption of proteid products, their passage from the intestine by way of the portal circulation insures exposure to the action of the hepatic cells, before they are distributed by the general circulation throughout the body. It is only under conditions of an excessive intake of proteid foods that their products are absorbed by way of the lymphatics. These points are clearly established, and there is every ground for believing that substantial reasons exist to account for this single line of departure. Just what the liver does, however, is uncertain. In fact, as already indicated, there is lack of definite knowledge as to how far the proteid foods are broken down in digestion, prior to absorption. The combined action of pepsin, trypsin, and erepsin, if sufficiently long continued, can accomplish a complete disruption of the proteid molecule. We are inclined to assume in a general way that the “proteids taken as food cannot find a place in the economy of the animal body till they have been, as it were, melted down and recast.”[14] How far this melting down or disruption extends in normal digestion, we do not at present know. As already stated, neither proteoses and peptones, nor the amino-acids, are found in the blood stream in sufficient amounts, or with that frequency, to suggest absorption in these forms. Possibly, as some physiologists have suggested, the amount of any of these products to be found at any one time in a given quantity of blood is too small for certain recognition, yet in the twenty-four hours the amount passing from intestine to liver might be sufficiently large to equal the total proteid absorbed. We can, however, at present only conjecture, and must rest content with the simple statement that in the digestion of the proteid foodstuffs, proteoses, peptones, and amino-acids are formed, and that by transformation or total reconstruction of these products, special types of proteid are manufactured either in the epithelial cells of the intestinal walls during absorption, or elsewhere in the body after absorption. If this latter is the case, the liver might readily be regarded as a likely spot for the synthesis to occur.
Bearing in mind what has been said regarding the production of specific types of proteid by every species of animal, we can the more readily conceive of a synthesis “out of fragments of the original molecules rearranged and put together in new combinations, by processes in which the intestine can hardly be supposed to play a part.” This, the liver might well be assumed as capable of accomplishing, and if we were disposed to accept this view we might use as an argument the fact that the products of proteid digestion are taken directly to this organ, before being cast loose in the tissues and organs of the body. There is perhaps as good ground for assuming that a synthesis or reconstruction of proteid takes place all over the body; that, as suggested by Leathes, “the synthesis of proteids is a function of every cell in the body, each one for itself, and that the material out of which all proteids in the body are made is not proteid in any form, but the fragments derived from proteids by hydrolysis, probably the amido-acids, which in different combinations and different proportions are found in all proteids, and into which they are all resolved by the processes, autolytic or digestive, which can be carried out in every cell in the body.” It is certainly a reasonable hypothesis, and since we lack positive knowledge it cannot at present be disproved. All that we can affirm in the light of established fact is that the products of proteid digestion are absorbed from the intestine by way of the portal circulation, and that either in their passage through the intestinal wall, or later on in the liver or elsewhere, there is a construction of new proteid to meet the wants of the body. The liver, indeed, may be effective in both construction and destruction of proteid, but there is no way of telling at present just how far it acts in either direction.
Regarding the absorption of fats, a single statement will suffice, in addition to what has already been said. Fats gain access to the general circulation by passing from the intestine into the lacteal radicles, thence into the lymphatics, whence they move onward into the thoracic duct, and from there are emptied into the great veins at the neck. A small amount is apparently absorbed in the form of soap by the portal circulation, but by far the larger amount of fat gains access to the blood stream without going through the liver.
In these ways, the blood and lymph are continually supplied with proteid, fat, and carbohydrate from the ingested food, and as these fluids surround and permeate the organized elements of the tissues, the latter are enabled to gain what they need to maintain their nutritive balance. Living matter is essentially unstable; it is the seat of chemical changes of various kinds, anabolic or constructive, and katabolic or destructive. The more comprehensive term “metabolic” is applied to all of these changes that take place in living matter. In anabolism, the dead, inert proteids, fats, and carbohydrates are more or less assimilated and made a part of the living matter of the tissue cells, while at the same time a certain amount of the food material, probably the larger amount, is simply stored as such, or left to circulate in the blood and lymph, without being raised to the higher level of living protoplasm. In katabolism, this accumulated material, and in some degree the living substance itself, is broken down or disintegrated with liberation of the stored-up energy, which manifests itself in the form of heat and mechanical work. At times, the anabolic processes predominate and there is a relatively large accumulation of stored-up materials; while at other times, katabolism, with its attendant chemical decompositions, predominates, and the body loses correspondingly. The point to be emphasized here is that the living body, with its multitude of living cells, is the seat of incessant change. Construction and destruction are continually going forward side by side; sometimes the one and sometimes the other predominating, according to existing conditions. The living protoplasm with its attendant storage material is, under ordinary conditions, constantly being made good from the assimilated food, a part of which is raised to the dignity of living matter and becomes an integral part of the living cells, while the larger portion is simply stored for future uses, or circulates in the blood and lymph which bathe them. Doubtless, this storage or circulating material is the main source of the energy which constantly flows from the cells in the form of heat and of work, as a result of the disruptive changes that constitute katabolism.
Worthy of special notice is the fact that cell protoplasm is essentially proteid in nature; water and proteid make up the larger part of its substance, to which are added small proportions of carbohydrate, fat, and mineral matter. Proteid is the basis of cell protoplasm; it is the chemical nucleus of living matter, and owing to the large size of its molecule, with its large number of contained atoms, is capable of many combinations and many alterations. Most of the reactions characteristic of katabolism centre around this proteid, but the disruptive changes that occur undoubtedly involve more largely the circulating materials present in the blood and lymph, and which bathe the cells, rather than the so-called fixed, or organ proteid, of the cell substance itself. Still, while the circulating blood and lymph furnish largely the substances which are made to undergo disintegration in katabolism, the living protoplasmic cell is the controlling power which regulates the extent and character of the decompositions, and proteid matter is the chemical basis of protoplasm. From these statements, we again have suggested the significant importance of the proteid foods in nutrition, since they alone can furnish the material which constitutes the chemical basis of living cells. The human body, which represents the highest form of animal life, is merely, as stated by another, “literally a nation of cells derived from a single cell called the ovum, living together, but dividing the work, transformed variously into tissues and organs, and variously surrounded by protoplasm products” (Waller).
The processes involved in metabolism are not easily unravelled. The word itself is simple, but it is employed to designate that complex of “chemical changes in living organisms which constitute their life, the changes by which their food is assimilated and becomes part of them, the changes which it undergoes while it shares their life, and finally those by which it is returned to the condition of inanimate matter. Gathered together under this one phrase are some of the most intricate and inaccessible of natural phenomena. It implies also, and gently insists on the idea, that all the phenomena of life are at bottom chemical reactions” (Leathes). Regarding the processes of anabolism, as in the construction of living protoplasm out of inert food materials, we can say nothing. This is altogether beyond our ken at present, and doubtless will remain so, since it involves a chemical alteration, or change, akin to that of bringing the dead to life. With the processes of katabolism, however, we may hope for more satisfactory results; and, indeed, to-day we have considerable information of value as to some of the methods, at least, which are the cause of this phase of nutrition. This knowledge, however, has been slow of attainment.
In the earlier years of the sixteenth century, when anatomy and physiology were beginning to make progress, the savants of that day, hampered as they were by grave misconceptions and by the lack of any understanding of chemical phenomena, could not take advantage, naturally, of the suggestion that as wood burns or oxidizes in the air with liberation of heat, so might the food substances, absorbed by the body, undergo oxidation in the tissues and thus give rise to animal heat. Such suggestions were at that time as a closed book, and so we find Vesalius, in 1543, teaching the Galenic doctrines in physiology then prevalent. The conception of heat production, as it existed at that time, may be inferred from the following quotation:[15] “The parts of the food absorbed from the alimentary canal are carried by the portal blood to the liver, and by the influence of that great organ are converted into blood. The blood thus enriched by the food is by the same great organ endued with the nutritive properties summed up in the phrase ‘natural spirits.’ But blood thus endowed with natural spirits is still crude blood, unfitted for the higher purposes of the blood in the body. Carried from the liver by the vena cava to the right side of the heart, some of it passes from the right ventricle through innumerable invisible pores in the septum to the left ventricle. As the heart expands it draws from the lungs through the vein-like artery air into the left ventricle. And in that left cavity, the blood which has come through the septum is mixed with the air thus drawn in, and by the help of that heat, which is innate in the heart, which was placed there as the source of the heat of the body by God in the beginning of life, and which remains there until death, is imbued with further qualities, is laden with ‘vital spirits,’ and so fitted for its higher duties. The air thus drawn into the left heart by the pulmonary vein, at the same time tempers the innate heat of the heart and prevents it from becoming excessive.” In other words, heat was considered as a divine gift, and as can readily be seen, there was an utter lack of appreciation of the use of air in breathing. Even van Helmont, who lived in 1577–1644, and was in a sense an alchemist, still gave credence to the spirits, viz., that the food absorbed from the stomach and intestine is in the liver endued with natural spirits, while in the heart the natural spirits are converted into vital spirits, and in the brain the vital spirits are transformed into animal spirits.[16] Later, Malpighi discovered the true structure of the lungs, and Borelli, in 1680, exposed the erroneous views then prevalent regarding the purpose of breathing. It is not true, says Borelli, that the use of breathing is to cool the excessive heat of the heart or to ventilate the vital flame, but we must believe that this great machinery of the lungs, with their accompanying blood vessels, is for some grand purpose. In a long and vigorous argument, he contends that the “air taken in by breathing is the chief cause of the life of animals, far more essential than the working of the heart and the circulation of the blood.” He quotes the experiments of Boyle, who showed in 1660 “that even in a partial vacuum brought about by his air pump, flame was extinguished and life soon came to an end; the candle went out and the mouse or the sparrow died.”
At this time, and for long afterwards, the belief was prevalent that the air taken up by the blood in the lungs was the air of the atmosphere in its entirety. No one appears to have thought of the possibility of only a part of the air being used, for at that time there was no suspicion that air was a mixture of substances. Mayow, however, in 1668, showed that it was not the whole air which was employed for respiration, but a particular part only. At this time, great attention was being given to a study of nitre or saltpetre; its wonderful properties in combustion were being recognized, and Mayow, who was a chemist of repute, claimed that it had its origin partly in the air and partly in the earth. The air “which surrounds us, and which, since by its tenuity escapes the sharpness of our eyes, seems to those who think about it to be an empty space, is impregnated with a certain universal salt, of a nitro-saline nature, that is to say, with a vital, fiery, and in the highest degree fermentative spirit,” to which the name of “igneo-aereus” was applied. Nitre was shown to be composed of a sal fixum or sal alkali,—potash as it is now called,—and was obviously derived from the earth, while the other part of nitre was made up of the spiritus acidus, or nitric acid. For a time it was supposed that the whole of this spiritus acidus was contained in the atmosphere, but it was soon recognized that this could not be the case, since nitric acid was found to be a corrosive liquid, destructive to life and quite incapable of supporting combustion. Hence, Mayow concluded that only a part of the acid exists in the atmosphere, viz., that part which he termed spiritus nitro-aereus. In combustion, there is something in the air which is necessary for the burning of every flame, unless perchance igneo-aereal particles should pre-exist in the thing to be burnt. These igneo-aereal particles form “the more active and subtle part of air which is thus necessary for combustion, exist in nitre and indeed constitute its ‘more active and fiery part.’” Mayow fully recognized that burning and breathing involved in a measure the same process; both consisted in the consumption of the igneo-aereal particles present in the air. “If a small animal and a lighted candle be shut up in the same vessel, the entrance into which of air from without be prevented, you will see in a short time the candle go out, nor will the animal long survive its funeral torch. Indeed, [says Mayow] I have found by observation that an animal shut up in a flask together with a candle will continue to breathe for not much more than half the time than it otherwise would, that is, without the candle.” Something contained in the air, necessary alike for supporting combustion and for sustaining life, passes from the air into the blood. Mayow expressed his thoughts in these words: “And indeed it is very probable that certain particles of a nitro-saline nature, and those very subtle, nimble, and of very great fermentative power, are separated from the air by the aid of the lungs and introduced into the mass of the blood. And so necessary for life of every kind is that aereal salt (constituent) that not even plants can grow in earth the access of air to which is shut off. But if that same earth be exposed to air and so forthwith impregnated with that fecundating salt, it at once becomes fit again for growing.”[17] Mayow fully appreciated the importance of his nitro-aereal particles in the processes of life; he had indeed a fairly accurate conception of a sound theory of animal heat; he saw that they were equally necessary for burning, or combustion, and for respiration, and so was enabled to draw a parallelism between the two processes; he pointed out that they were essential for the ordinary activity of the muscles of the body, that as muscle work was increased more particles from the air were required; indeed, he clearly foresaw the need which the body had for these igneo-aereal particles in all the chemical processes of life. And thus was foreshadowed a conception of oxidation, a hundred years before Priestley evolved his phlogiston theories and Lavoisier discovered oxygen.
The discoveries of Lavoisier, published in 1789, led to a clear understanding of combustion as a process of oxidation, and paved the way for a fuller knowledge of the part played by the oxygen of the air in the chemical reactions going on in the animal body. Lavoisier showed that the oxygen drawn into the lungs with the air breathed was used in the body for the oxidation of certain substances, carbon being transformed thereby into carbon dioxide, and hydrogen into water. Further, he noted that these oxidations were carried forward on a large scale, and he emphasized the importance of oxygen as being the true cause of the varied decompositions taking place in the living body. The larger the amount of oxygen inspired, the more extensive the oxidation, and consequently the rate of respiration as modifying the intake of oxygen served in his opinion as a regulator to control the extent of the oxidative processes. He pointed out that a definite relationship existed between the amount of work done by the body and the oxygen consumed; greater muscular activity, lower temperature of the surrounding air, the activities attending the digestive functions, all seemed to be associated with a greater utilization of oxygen. Oxidation was the pivot around which all the chemical reactions of the body seemed to centre. Lavoisier, however, was not a physiologist, and he was, quite naturally perhaps, led into some errors. For example, he considered that the process of combustion or oxidation took place in the lungs, certain fluids rich in carbon and hydrogen formed in the different organs of the body being brought there for exposure to the inspired oxygen. Further, his views implied a simple and complete combustion, in which complex substances rich in carbon were directly and completely oxidized to carbon dioxide and water, in much the same manner as combustion occurs outside the body. Again, he assumed that the amount of oxygen taken into the lungs determined the extent of oxidation, just as the use of the bellows, by increasing the draft of air, causes the fire to burn more brightly.
To Liebig (1842) the next great advance was due. This phenomenally clear-minded man, while recognizing at their full value the fundamental theories advanced by Lavoisier, saw and fully appreciated their incompleteness, and he likewise understood their failure to explain many of the phenomena of life more familiar to the physiological mind than to that of a simple chemist like Lavoisier. Liebig had made a special study of the chemical composition of foodstuffs, and likewise of the tissues and organs of the body. He had, moreover, given great attention to the decomposition products formed in the body, especially the nitrogenous substances excreted through the kidneys, as well as the carbon dioxide and water passed out through the lungs and skin. It was not strange, therefore, that he should take exception to Lavoisier’s view that oxidation in the body consisted in the combustion of a fluid, rich in carbon and hydrogen, which was brought to the lungs. On the contrary, Liebig contended that it was the organic compounds, proteids, fats, and carbohydrates, that underwent oxidation, and not necessarily in the lungs, but all over the body, wherever organs and tissues were active. Especially noteworthy was the view advanced by Liebig, and upheld for many years, that of these three classes of compounds the proteids alone served for the construction of organized tissues, like muscle, and that in the activity of this tissue, as in muscle contraction or muscle work, the energy for the work was derived solely from the breaking down or oxidation of this organized proteid. On this ground he termed the proteid foodstuffs “plastic,” or tissue-building foods. Liebig further pointed out that the substances of the body have the power of combining with and holding on to the inspired oxygen, and that fats and carbohydrates, i. e., the non-nitrogenous compounds, easily undergo oxidation or combustion, and thereby furnish the heat of the body. For this reason he termed the corresponding foodstuffs “respiratory” foods. Proteids, on the other hand, according to Liebig’s view, are capable of combustion only in slight degree. The cause of the decomposition of proteid substances in the body was to be traced solely to muscle work, i. e., the energy of muscle contraction, or muscle work, was derived from the breaking down of the proteids of the muscle tissue, and work was the stimulus which brought about proteid decomposition. Non-nitrogenous substances played no part in these reactions; muscle work was without influence on these compounds, oxygen being the sole stimulus which led to their combustion, and heat was the sole product of the combustion.
If Liebig’s theory is correct, that the proteids of the body are decomposed only as the result or the accompaniment of muscle work, and the proteids of the food are used up only as they take the place of the organized proteid so metabolized, it follows that with a like degree of muscular activity a given body will always decompose the same amount of proteid. If excess of proteid food is taken, the surplus will be stored in the tissues, or, in other words, the excretion of nitrogen will not be influenced by the amount of proteid consumed in the food. This was the line of argument made use of by various physiologists[18] who were disposed to criticise Liebig’s view, and quite naturally the question was soon made the subject of many experiments. It will suffice here merely to say that many concordant results were obtained, showing that an abundance of proteid food leads to an increase in the excretion of nitrogen, muscle activity remaining at a constant level. Hence, as Voit states, some other ground than muscle work must be sought as the true cause of proteid katabolism. Consequently, we find this hypothesis of Liebig replaced by the theory of “luxus consumption,” in which it is maintained that while whatever proteid is used up by the work of the muscle must be made good from the proteid of the food, any excess of proteid absorbed from the intestinal canal is to be considered as “luxus,” and like the non-nitrogenous foods may be burned up in the blood, by the oxygen therein, without being previously organized. Hence, we see suggested two causes for the decomposition of proteid in the body, viz., the work of the muscle and the oxygen of the blood. Further, as stated by C Voit,[19] the nitrogen excretion of the hungry or fasting animal affords, according to these views, a measure of the extent to which tissue proteid must be broken down in the maintenance of life, and of the amount of proteid food necessary to be consumed in order to make good the loss; viz., the minimum proteid requirement. Again, since any excess of proteid food beyond this minimal requirement, according to the theory, is destined to be burned up in the blood, or elsewhere, to furnish heat the same as non-nitrogenous foods, it follows that the excess of proteid food can be replaced by non-nitrogenous aliment.
Oxidation, however, is the keynote in any explanation of the processes of metabolism, whether nitrogenous or non-nitrogenous matter is involved. Both alike undergo oxidation, but it is not simple oxidation or combustion that we have to deal with. In the time of Lavoisier, as already stated, it was thought that oxygen alone was the cause of the decomposition going on in the body, but simply increasing the intake of air or oxygen, as in quickened breathing or deeper inspiration, does not increase correspondingly the rate of oxidation. In other words, it is not a direct combination of oxygen with the carbon and hydrogen of the foodstuffs, or tissue elements, that takes place in the body, but rather a gradual, progressive decomposition of complex organic compounds into simpler products; made possible, however, by the agency of the oxygen carried from the lungs by the circulating blood. It was demonstrated years ago that animals breathing pure oxygen do not consume any more of the gas than when breathing ordinary air, and likewise no more carbon dioxide is produced in the one case than in the other. Fifty years ago, Liebig and other physiologists showed that frogs’ muscle placed in an atmosphere free of oxygen could be made to contract or do work for some considerable time, and with liberation of heat. This fact implies a breaking down of muscle substance into simpler bodies, but there is here no free oxygen to act as the inciting cause; indeed, what actually occurs is a cleavage or splitting up of substances in the muscle tissue, but at the expense of oxygen in some form of combination in the muscle. This oxygen must have been taken from the blood at some previous time and stored in the tissue for future use. Again, as C Voit has expressed it, if oxygen were really the immediate cause of the decompositions taking place in the organism, we should expect combustion to occur in harmony with the well-known relationship of the three classes of organic foodstuffs to oxygen. In other words, fats would undergo combustion most readily, carbohydrates next, and lastly the nitrogenous or albuminous compounds. In reality, however, proteid matter is decomposed in largest quantity; a generous addition of proteid food is always accompanied by an increased consumption of oxygen. Yet oxygen is not the inciting cause of the proteid decomposition, as is seen from the fact that in muscle work, where the intake of oxygen is greatly increased, there is no noticeable change in the amount of proteid material broken down. Plainly, in the body we have to deal not with a direct oxidation of the complex compounds of the tissues or of the food, but rather with a gradual cleavage of these higher compounds into simpler substances, these latter undergoing progressively a still further breaking down with intake of oxygen. To repeat, oxygen is not the cause of the decompositions within the body, but the extent of the breaking down of the tissue or food material is the determining factor in the amount of oxygen taken on and used up. The products of decomposition contain more oxygen than the original substances undergoing the breaking down process, which means that oxygen is taken from the blood and used in the physiological combustion that is going on. It is not, however, strictly a combustion process; it is more complicated and more gradual than ordinary combustion, involving first of all a series of what may be termed oxidative cleavages, in which large molecules are gradually, step by step, broken down into simpler molecules, and these latter then oxidized to still simpler forms. Hence, we find the oxidative changes preceded by a variety of alterations in which oxygen may take no part whatever; such as hydrolytic cleavage, where the elements of water are taken on as a necessary step in the cleavage process; dissociation of a simple sort, as when a large molecule breaks up directly into smaller molecules, etc.
These statements by no means detract from the importance of oxygen in the katabolic processes of the body, but it is physiological oxidation that we have to do with, and not simple combustion. Oxygen is not the direct cause of the transformations taking place in the body. As one looks over the history of progress in our knowledge of nutrition from the time of Lavoisier to the present, it is easy to note the gradual change of view regarding oxidation in the living organism. Step by step, it has been demonstrated that there are many factors involved in this breaking down of complex substances; that while oxygen is an ever present requirement, there are other equally important factors to be taken into account. The contrast between the older views and those now current is clearly shown by the difference in attitude regarding the place in the body where oxidation occurs. Thus, in the earlier days, when the view was gradually gaining ground that nutritional changes were mainly the result of oxidation, and that the oxygen drawn into the lungs in inspiration was a primary factor, then, as we have seen, the lungs were considered as the laboratory where the transformation takes place. This view, however, was soon exploded, and next we find the blood, the lymph, and other fluids, but especially the blood, looked on as the locality where oxidation occurs. This was indeed quite a natural view to hold, since the blood is the carrier of oxygen, but we now know, in harmony with the fact that the breaking down of complex food material is a complicated process, involving various kinds of chemical change, that these katabolic processes are not located in any one place, but occur all over the body wherever there are active tissues. As has been previously stated, the human body is a “nation” of cells, all of which are more or less active, and it is in these miniature laboratories mainly that oxidation and all the other nutritional changes coincident to life take place. Muscle tissue and nerve tissue, the large secreting glands, such as the liver, stomach, and pancreas, all are the seat of oxidative and other changes which we class under the broad term of nutritional. To these cells, therefore, we must look for an explanation of the causes of oxidation, and the other transformations of a kindred nature that take place in the body.
In our brief survey of digestion, and of the methods there followed for the proper utilization of the organic foodstuffs, it was seen that the unorganized ferments or enzymes are the active agents in accomplishing the breaking down of proteids, and the less profound alteration of fats and carbohydrates. Is it not possible that the tissues of the body are likewise supplied with enzymes of various types, and that upon these powerful agents rests the responsibility for the different kinds of decomposition, oxidation and other changes, that take place in the body? Some years ago much interest was aroused by the observation that certain glands in the body, if simply warmed at body temperature with water, in the presence of some germicidal agent sufficient to prevent putrefactive changes, underwent what is now termed autodigestion, i. e., a process of self-digestion, with formation of various products, notably such as would naturally result from the breaking down of proteid material by ordinary proteolytic enzymes. This would seem to imply the presence in the glands of a proteid-splitting enzyme, the products formed being proteoses, peptones, amino-acids, etc., just such products as result from the action of trypsin. To-day, we know that practically all tissues and organs can, under suitable conditions, undergo autolysis, and in many instances the enzymes themselves can be separated from the tissues by appropriate treatment. Liver, muscle, lymph glands, spleen, kidneys, lungs, thymus, etc., all contain what are very appropriately called intracellular enzymes. These enzymes are of various kinds. Especially conspicuous are the hydrolytic, proteid-splitting enzymes, which behave in a manner quite similar to, if not identical with, that of the digestive enzymes of the gastro-intestinal tract, i. e., pepsin, trypsin, and erepsin. Further, there are other hydrolytic cleavages taking place in tissue cells, such as the cleavage of fats, due as we now know to intracellular enzymes of the lipase type, and by which neutral fats are split apart into glycerin and fatty acid. Again, there are in many organs intracellular enzymes which act upon the complex nucleoproteids of the tissue, causing them to break apart into proteid and nucleic acid, the latter being further broken down by other enzymes with liberation of the contained nuclein or purin bases. Many other chemical reactions are brought about by specific enzymes of various kinds, present in the cells of particular glandular organs. Thus, intracellular enzymes have been found, as in the liver, which are able to transform amino-acids into amides, and still others capable of splitting up amides.
Equally important, and even more suggestive, are the data which have been collected recently regarding oxidative processes in the tissues of the body. Specific ferments, known as oxidases, are found widely distributed in many organs and tissues, and it is difficult to escape the conclusion that as intracellular enzymes they have an important part to play in some, at least, of the transformations characteristic of tissue katabolism.[20] As a single example, mention may be made of aldehydase, which accomplishes the oxidation of substances having the structure of aldehydes into corresponding acids. Ferments or enzymes of this class are found in the liver, spleen, salivary glands, lungs, brain, kidneys, etc., and they may well be considered as important agents in the chemical transformations going on in the tissues of the body. It would take us too far afield to enter into a detailed consideration of these intracellular enzymes; it must suffice to emphasize the general fact that in all the tissues and organs of the body there are present a large number of enzymes of different types, endowed with different lines of activity, and consequently capable of accomplishing a great variety of results in metabolism. Oxidation may still be a dominant feature in nutrition, oxidative changes may characterize more or less every tissue and organ in the body, but the processes are subtle and are not to be defined in harmony with simple chemical or physical laws. The living cell, with its intracellular enzymes, is the guiding and controlling power by which the processes of katabolism are regulated in harmony with the needs of the body. Complex organic matter is broken down step by step in the various tissues, with gradual liberation of the contained energy; processes of hydrolytic cleavage alternate with processes of oxidation, the molecules acted upon growing smaller with each downward step, until at last the final end-products are reached, viz., carbon dioxide, water, and urea, which the body eliminates through various channels as true physiological waste-products.
It will be advisable for us to consider briefly some of these intermediary products of tissue metabolism, since in any discussion of nutritive changes it is quite essential to have some understanding of the chemical relationship existing between the various products which result from the breaking down of proteid and other materials in tissue katabolism. This is especially true of proteid material, since in the gradual disintegration of this substance in tissue metabolism many intermediary bodies are formed, which undoubtedly exercise some physiological influence prior to their transformation into simpler bodies, with ultimate formation of the final product, urea. As has been pointed out so many times, the proteid foods are peculiar in that they alone contain the necessary nitrogen, and in the peculiar form able to meet the physiological requirements of the body. Variations in the proteid intake are of necessity accompanied by variations in the formation of nitrogenous intermediary products, and both quality and quantity of these substances must be given due attention in any study of nutrition. Further, it is only by an understanding of the general or ground structure of proteids that we can hope to attain knowledge of the processes going on in the different tissues and organs in connection with metabolism, while a true appreciation of the chemical peculiarities of the individual proteids will help to explain the different nutritional value of vegetable as contrasted with animal proteids.
Our understanding of the chemical structure of any organic substance is based primarily upon a study of the decomposition products which result from its breaking down, under the influence of various chemical agencies. Simple proteid substances when acted upon by pancreatic juice reinforced by the enzyme erepsin, or when boiled with dilute acids, undergo hydrolytic cleavage with ultimate formation of a large number of relatively simple bodies, mostly amino-acids, the chemical structure of which throws some light upon the nature of the proteid. Thus, in the pancreatic digestion of proteid in the intestine we may adopt the following scheme as showing in a general way the progressive transformation that occurs, understanding at the same time that like transformations may be accomplished by corresponding intracellular enzymes in the tissues and organs of the body; and further, that by the long-continued action of hydrolytic agents there is a complete breaking down into amino-acids and other simple products.
Native proteid, Protoproteose, Deuteroproteose, Heteroproteose, Primary proteoses, Secondary proteoses, Peptone, Amino-acids
Among these end-products, or amino-acids, are leucin, tyrosin, aspartic acid, glutaminic acid, glycocoll, arginin, lysin, histidin, and likewise the peculiar aromatic body tryptophan. The chemical make-up of these substances may be indicated by the following structural formulæ, which, if even only partially understood, will suggest to the non-chemical mind some idea of close chemical relationship:
Glutaminic acid Aspartic acid
Glycocoll Leucin
Tyrosin
Tryptophan
Arginin Lysin Histidin
In these various decomposition products there is apparent certain definite lines of resemblance, on which is based one or more suggestions regarding possible ways in which these chemical groups are linked, or bound together, in the proteid molecule. Thus, there is apparently present a complex or nucleus which may be indicated as
The proteid molecule is presumably built up of amino-acids variously joined together, this synthesis being accomplished, doubtless, by the condensation of different types of amino-acids, in which the first of the above groups represents the more common method of union. We may indeed conjecture that such methods of condensation take place in the human body, in the epithelial cells of the intestine, and in the tissues in general; and that by such methods, construction of proteid is accomplished out of the various fragments split off by digestion, etc. In a tentative way, the principle may be illustrated by the fusion of leucin and glutaminic acid,—following Hofmeister’s suggestion,—in which a still larger complex is formed:
Leucin Glutaminic acid
In this way, step by step, the proteid molecule is built up, and naturally in katabolism the proteid breaks down along certain definite lines of cleavage, with formation of katabolic products containing those groups, or chemical nuclei, which characterize the different proteid molecules. For it is to be clearly understood that there are many different forms of proteid, perhaps superficially alike, but possessed of physiological individuality. This is well illustrated by the two primary proteoses formed in digestion. As will be recalled, there are at first two proteoses produced, protoproteose and heteroproteose. These are, superficially at least, not radically unlike; they possess essentially the same percentage composition, but when broken down by vigorous chemical methods they show a totally different make-up. In other words, at the very beginning of digestion there is a splitting up of the proteid into two parts, which have quite a different chemical structure, as is clearly indicated by the difference in the character and amount of the decomposition products yielded by hydrolytic cleavage. Thus, heteroalbumose as derived from blood-fibrin contains 39 per cent of its total nitrogen in basic form, i. e., in a form which goes over into the basic bodies, arginin, lysin, and histidin, etc. On the other hand, protoalbumose from the same source yields hardly 25 per cent of basic nitrogen. Further, heteroalbumose yields only a very small amount of tyrosin, while protoalbumose gives on decomposition a large amount of this substance. Again, heteroalbumose furnishes a large yield of leucin and glycocoll, while protoalbumose gives no glycocoll and only a little leucin. Obviously, these two proteoses have an inner structure quite divergent one from the other, and owing to this fact they must play a quite different rôle in metabolism.
Even greater differences in inner chemical structure are found among native proteids. By way of illustration, we may take egg-albumin, the casein of cow’s milk, gliadin of wheat, and the edestin of hemp seed. These are all typical proteids; they are all useful as food, but they are radically different in their inner chemical structure, as is clearly indicated by the following data,[21] which show the percentage yield of the different amino-acids and ammonia:
| Leucin. | Tyrosin. | Glutam- inic Acid. | Arginin. | Lysin. | Histidin. | Ammonia. | |
|---|---|---|---|---|---|---|---|
| Egg-albumin | 6.1 | 1.1 | 9.0 | . . . | . . . | . . . | 1.6 |
| Casein | 10.5 | 4.5 | 10.7 | 4.8 | 5.8 | 2.6 | 1.9 |
| Gliadin | 5.7 | 1.2 | 37.3 | 3.2 | 0 | 0.6 | 5.1 |
| Edestin | 19.9 | 2.7 | 14.0 | 14.2 | 1.6 | 2.2 | 2.3 |
These are not mere technical differences, but they represent divergences of structure which cannot help counting as material factors in nutritional processes. Especially noticeable is the large yield of glutaminic acid from wheat proteid, as contrasted with the proteid (casein) of animal origin. As a rule, glutaminic acid forms a larger proportion of the decomposition products of vegetable than of animal proteids. Similarly, arginin is present in much larger proportion in most vegetable proteids than in most animal proteids. While many other data more or less trustworthy might be added, these figures will suffice to emphasize the main point under discussion, viz., that individual proteids show marked variation in the amount of the several amino-acids which serve as corner-stones or nuclei in the building up of the molecule, and consequently they must yield correspondingly different katabolic products when serving the body as food.
Turning now to another phase of tissue metabolism, we may consider briefly the nucleoproteids and their characteristic decomposition products; bodies which are widely distributed as cleavage products formed in the disintegration of most cell protoplasm, and having special interest in nutrition because of their chemical relationship to that well-known substance, uric acid. Nucleoproteids of some type are found in all cells; consequently they are present in all tissues, in all glandular organs, and their widespread distribution constitutes evidence of their great physiological importance. Nucleoproteids are compound substances made up of some form of proteid and nucleic acid. By simple hydrolysis with dilute mineral acids they are broken down into proteid, phosphoric acid, and one or more bodies known as nuclein bases. Of these latter substances, there are four well-defined bodies, viz., adenin, hypoxanthin, guanin, and xanthin, which from their peculiar chemical constitution are known as “purin bases.” In the body, there is present in many cells a peculiar intracellular enzyme termed nuclease, which has the power of liberating these purin bases from their combination as a component part of tissue nucleoproteids, or of the contained nucleic acid. In autolysis or self-digestion of many glands, such as the spleen, thymus, etc., this chemical reaction is easily induced by action of the contained nuclease. Further, the liberated purin bases then undergo change because of the presence of certain deamidizing enzymes, and as a result guanin is transformed into xanthin, and adenin is converted into hypoxanthin. These ferments are true intracellular enzymes, and are termed respectively guanase and adenase. The real essence of the reaction they accomplish is clearly indicated by the following formulæ, which likewise show the chemical nature and relationship of the four substances:
Guanin Xanthin
Adenin Hypoxanthin
These two enzymes are typical hydrolyzing enzymes, but it is to be noted that there is not only a taking on of water with a retention of the oxygen, but there is also a giving off of ammonia, by which the transformation is made possible. Adenin is known as an amino-purin and guanin as an amino-oxypurin, while hypoxanthin is an oxypurin and xanthin a dioxypurin. In other words, the two intracellular enzymes are able to transform the two amino-purins into the corresponding oxypurins; i. e., the enzymes are deamidizing ferments, liberating the NH2 group of the adenin and guanin and thus forming two new compounds. These reactions, though more or less technical, are emphasized in this way not merely because they illustrate the action of intracellular enzymes in intermediary metabolism, thus affording a striking example of the gradual changes that take place in ordinary katabolic processes, but especially because they throw light upon the production of another substance common in body metabolism, viz., uric acid. It has long been known that nucleoproteids, nucleins, and other compounds containing these purin radicles, when taken as food, cause at once an increased output of uric acid, and it has been clearly recognized that in some way this latter substance, as a product of metabolism, must come from the transformation of nuclein bases. To-day, we understand that in many tissues, as in the liver, spleen, lungs, and muscle, there is present a peculiar oxidizing ferment, an oxidase, by the action of which hypoxanthin can be converted into xanthin, and the latter directly oxidized to uric acid. This conversion into uric acid is purely a process of oxidation, brought about by a typical intracellular oxidase, known specifically as “xanthin oxidase,” the reaction involved being as follows:
Xanthin Uric acid
From these several reactions, it is clear how various intracellular enzymes working one after the other are able gradually to evolve uric acid from tissue nucleoproteids. Further, it is to be noted that there is another tissue oxidase—contained principally in the kidneys, muscle, and liver—which has the power of oxidizing and thus destroying uric acid, with formation, among other substances, of urea. Remembering that urea has the following chemical constitution
it is easy to see, by comparison of the formulæ, how uric acid might easily yield two molecules of urea through simple oxidation. In this way, excess of uric acid produced in the body can be converted into urea, and in this harmless form be excreted from the system.
Finally, reference should be made here to several other products of tissue metabolism, products of the breaking down of proteid matter in the body, since they are liable to prove of interest to us in other connections. Thus creatin, abundant in the muscle and other places; the related substance creatinin, present in the urine; methyl guanidin, a decomposition product of creatin; and urea, all call for a word of description. The chemical relationship of these bodies is clearly indicated by the following formulæ:
Creatin Creatinin
Methyl guanidin Urea
Creatinin is chemically the anhydride of creatin, i. e., it can be formed from creatin by the simple extraction of one molecule of water, H2O. Creatin, by hydrolytic cleavage, will break down into one molecule of urea and one molecule of sarcosin or methyl glycocoll, as shown in the following equation:
Creatin Sarcosin Urea
Methyl guanidin is a decomposition product of creatin, while guanidin, as can be seen from the formula, is like urea, excepting that the group NH replaces the oxygen of urea. These simple statements will suffice for our present purpose, viz., to indicate the more or less close chemical relationships existing between many of these nitrogenous decomposition products resulting from proteid katabolism; also to suggest how by slight chemical alteration one decomposition product may be resolved into another related substance in the processes of katabolism. Our conception of the processes involved in proteid katabolism is that of a series of progressive chemical decompositions, in which intracellular enzymes play the all-important part. The intermediary products formed are definite bodies because of the specific nature of the active enzymes, and, secondly, because of the chemical nature of the substances acted upon. In other words, oxidation in the animal body takes the shape of a series of well-defined chemical reactions, in which chemical constitution and specific enzyme action are the predetermining cause. In the absence of the particular chemical groups, the oxidase is unable to bring about oxidation, or, given the proper compound or mother substance in the absence of the specific oxidase, there is no oxidation. Hence, oxidation in the animal body is not the result of simple combustion, but, on the contrary, it consists of a series of orderly chemical processes, each one of which is presided over by an intracellular enzyme, specific in its nature, in that it is capable of acting only upon substances having a certain definite constitution, and leading invariably to a certain definite result. The processes which years ago were considered as due to the peculiar vital properties of the tissue cells, and which were supposed to be entirely dependent upon their morphological and functional integrity, are now seen to be due primarily to a great variety of enzymes, manufactured indeed by the living cells, but capable of manifesting their activity even when free from the influence of the living protoplasm. The varied processes of tissue katabolism are the result of orderly and progressive chemical changes, in which cleavage, hydrolysis, reduction, oxidation, deamidization, etc., alternate with each other under the influence of specific enzymes, where chemical constitution and the structural make-up of the various molecules are determining factors in the changes produced.