| Gliadin (whaet). | Hordein (barley). | Zein (corn). | Legumin (vetch). | Edestin (hemp). | Globulin (squash seed). | Amandin (almonds). | |
|---|---|---|---|---|---|---|---|
| 71.46 | 78.16 | 85.27 | 62.65 | 82.38 | 55.77 | 59.00 | |
| Glycine | 0.02 | 0.00 | 0.00 | 0.39 | 3.80 | 0.57 | 0.51 |
| Alanine | 2.00 | 0.43 | 9.79 | 1.15 | 3.60 | 1.92 | 1.40 |
| Valine | 0.21 | 0.13 | 1.88 | 1.36 | 6.20 | 0.26 | 0.16 |
| Leicine | 5.61 | 5.67 | 19.55 | 8.80 | 14.50 | 7.32 | 4.45 |
| Proline | 7.06 | 13.73 | 9.04 | 4.04 | 4.10 | 2.82 | 2.44 |
| Phenylalanine | 2.35 | 5.03 | 6.55 | 2.87 | 3.09 | 3.32 | 2.53 |
| Aspartic acid | 0.58 | ..... | 1.71 | 3.21 | 4.50 | 3.30 | 5.42 |
| Glutamic acid | 42.98 | 43.19 | 26.17 | 18.30 | 18.84 | 12.35 | 23.14 |
| Serine | 0.13 | ? | 1.02 | ? | 0.33 | ? | ? |
| Cystine | 0.45 | ? | ? | ? | 1.00 | 0.23 | ? |
| Tyrosine | 1.20 | 1.67 | 3.55 | 2.42 | 2.13 | 3.07 | 1.12 |
| Arginine | 3.16 | 2.16 | 1.55 | 11.06 | 14.17 | 14.44 | 11.85 |
| Histidine | 0.61 | 1.28 | 0.43 | 2.94 | 2.19 | 2.63 | 1.58 |
| Lysine | ..... | ..... | ..... | 3.99 | 1.65 | 1.99 | 0.70 |
| Tryptophane | present | present | present | present | present | present | present |
| Ammonia | 5.11 | 4.87 | 3.64 | 2.12 | 2.28 | 1.55 | 3.70 |
At the time when these analyses were made, a method for the quantitative estimation of tryptophane had not been devised, although one is now available. The addition of the percentages of tryptophane and of other amino-acids for which methods of determination are not yet known, would bring the total, in each case, more nearly up to the full 100 per cent. These data will serve to show how widely the different plant proteins vary in the proportions of the different amino-acids which they contain. Animal proteins have been found to be still more variable in composition.
In the use of the proteins as food for animals, it appears that the different amino-acids are in some way connected with the different physiological functions which the proteins have to perform in the animal body: thus, tryptophane is absolutely essential to the maintenance of life, but does not promote growth; lysine, on the other hand, definitely promotes growth, so that animals which have been maintained without any increase in weight for many months immediately begin to grow when furnished with a diet in which lysine is a constituent; while arginine seems to be definitely associated with the reproductive function; and cystine, with the growth of hair, feathers, etc. It is not known whether there is any similar relation of amino-acids to the functions of different proteins in plant metabolism.
The separation of the individual amino-acids from the mixture which results from the hydrolysis of any given protein is a long and tedious process and, at best, yields only moderately satisfactory results. For that reason, it has recently been almost entirely abandoned in favor of the separation devised by Van Slyke, which divides the total nitrogenous matter in the mixture resulting from the hydrolysis of a protein into the following groups; ammonia N, humin (or melanin) N, cystine N, arginine N, histidine N, lysine N, amino N of the filtrate, and non-amino N of the filtrate. These groups can be conveniently and fairly accurately separated out of the hydrolysis mixture, by means of various precipitating agents, and the quantity of N in the several precipitates determined by the usual Kjeldahl method. The actual process for these separations need not be discussed here, as it is given in detail in all standard text-books dealing with the methods of biochemical analysis. The distribution of the nitrogen in any given protein into these various groups is characteristic for that particular protein, and the process serves both as a means of identification of individual proteins and a method for tracing their changes through various vital, or biochemical, transformations.
GENERAL PROPERTIES OF THE PROTEINS
Individual proteins differ slightly in their characteristics, but in general they are all alike in the following physical and chemical properties.[5]
Physical Properties.—(1) The proteins are all colloidal in character, that is, they form solutions in water, out of which they cannot be dialyzed through parchment, or other similar membranes. (2) All natural proteins, when in colloidal solution, may be coagulated, forming a semi-solid gel, which cannot again be rendered soluble except by decomposition. The most familiar example of this type of coagulation is that of egg-albumin, when eggs are cooked. This coagulation may be produced by heat, by the action of certain enzymes, or by the addition of alcohol to the solution. (3) All solutions of plant proteins are optically active, rotating the plane of polarized light to the left, in every case. (4) Proteins are precipitated out of their solutions, without change in the composition of the protein, by saturating the solution with various neutral salts of the alkali, or alkaline earth, metals, such as sodium chloride, ammonium sulfate, magnesium sulfate, etc. This is only another way of saying that the proteins are insoluble in strong salt solutions. Separation from solution by the addition of salts is different from coagulation by heat, etc., as in this case simple dilution of the salt solution will cause the protein to redissolve, whereas a coagulated protein cannot be redissolved without some change in its composition.
Chemical Properties. (1) Precipitation reactions.—The proteins have both acid and basic properties (due to the presence in their molecules of both free NH2 groups and free COOH groups). Bodies of this kind are known as "amphoteric electrolytes," since they yield both positive and negative ions, if dissociated. The proteins readily form salts, which are generally insoluble in water, with strong acids. For this reason, they are generally precipitated out of solution by the addition of the common mineral acids. They are also precipitated by many of the "alkaloidal reagents," to which reference has been made in the preceding chapter, namely, phosphotungstic, phosphomolybdic, tannic, picric, ferrocyanic, and trichloracetic acids, the double iodide of potassium, mercuric iodide, etc. The precipitates produced by strong mineral acids are often soluble in excess of the acid, with the formation of certain so-called "derived proteins," which are probably products of the partial hydrolysis of the protein.
The proteins are also precipitated out of solution by the addition of small amounts of salts of various heavy metals, such as the chlorides, sulfates, and acetates of iron, copper, mercury, lead, etc. This precipitation is different than that caused by the saturation of the solution with the salts of the alkali metals, as in this case the metal unites with the protein to form definite, insoluble salts, which cannot be redissolved except by treatment with some reagent which removes the metal from its combination with the protein (hydrogen sulfide is commonly used for this purpose).
(2) Color reactions.—Certain specific groups which are present in most proteins give definite color reactions with various reagents. It is apparent that any individual protein will respond to a particular color reaction, or will not do so, depending upon whether the particular group which is responsible for the color in question is present in that particular protein. Color reactions to which most of the common plant proteins respond are the following ones: