Similarly, proenzymes have been reported as occurring in numerous plant tissues. These proenzymes are believed to be present in the plant cells in the form of definite characteristic granules, which may be observed under the microscope, and which disappear when the enzyme becomes active. Thus, "proinulase" has been reported as occurring in artichoke tubers: "prolipase," in castor beans; "proinvertase," in several species of fungi; and, probably, "prooxidase," in tobacco leaves. In the case of the last-named zymogen, it has been observed that after the zymogen has been once activated, as in response to the need for increased activity due to the entrance of the germs of certain leaf-diseases, it can once again produce a second supply of the enzyme, but the process cannot again be repeated.
Calcium salts, or very dilute acids, are usually energetic activators of proenzymes.
PHYSIOLOGICAL USES OF ENZYMES
There can be no doubt that enzymes exert a tremendously important influence in vital phenomena, by determining the rate at which the chemical changes which are involved in these phenomena shall proceed. Since they do not initiate reactions, and since they may catalyze reversible reactions in either direction, it cannot be said that they determine the type of reactions which will take place in any given mass of protoplasm; but, undoubtedly, they do exert a determining influence upon the rate at which the reaction will proceed, after the protoplasmic activity has determined the direction in which it shall go.
Without the intervention of these catalyzing agents, it would be impossible for reactions between these non-ionized organic components of the cell contents to come to completion with anything like the marvelous rapidity with which these changes must take place in order to permit the organism to grow, to perform its necessary vital functions, or to adjust itself to the changes in its environmental conditions.
Since the number of different reactions which take place within a living cell is very great, and since these chemical changes are extremely variable in type, it follows that the number of different enzymes which must exist in either a plant or an animal organism is likewise very large. For example, fourteen different enzymes have been isolated from the digestive system, and at least sixteen from the liver, of animals. They are universally present in living protoplasm of every kind, from the most minute bacterium to the largest forest trees, in the plant kingdom; and from the amœba to the whale, in animals.
While there is a great variety of enzymes which may be produced by a single individual organism, the same enzyme may be found in the greatest variety of organisms; as, for example, the protease trypsin, which has been found in several species of bacteria, in the carnivorous plant known as "Venus' Fly Trap," and in the human pancreas, as well as that of all other animals.
FURTHER STUDIES NEEDED
From the discussions which have been presented in this chapter, it is apparent that the enzymes play a tremendously important part in vital phenomena, by controlling the rate at which the biochemical reactions take place in the cells of the living organism.
The means by which the protoplasm elaborates these all-important chemical compounds are as yet absolutely unknown. Even the nature of the enzymes themselves is still a matter of speculation and study. Much intensive study is needed and should be given to these matters, for the purpose of elucidating the methods by which the enzymes accomplish their remarkable catalytic effects, and, if possible, the actual chemical nature of the enzymes themselves. It is conceivable, of course, that if the latter object of these studies should ever be reached, it might be possible to synthetize enzymes artificially, and so to develop a means for the artificial duplication of the synthesis of organic compounds with the same velocity that this is done in the plant cells. Such a result would have a scientific interest fully as great as did Wöhler's artificial synthesis of urea, which proved that there is no essential difference in character between the compounds which are the products of living organisms and those which are produced in the laboratory; and, at the same time, might have an immensely more important practical bearing, since it would lead the way to the artificial production of the carbohydrates, proteins, fats, etc., for which we are now dependent upon plant growth as the source of these materials for use as human food.