In Chapter IV (The Nutritive Value Of The Proteins From Various Sources. 63. Protein The Most Prominent Organic Component Of The Body) a brief account was given of the experimental studies which established the fact that the nutritive value of a protein depends upon the number and amounts of the amino-acids it yields on artificial hydrolysis or on digestion. It was shown that a protein such as gelatin, which is lacking in the amino-acids, tyrosin, cystin and tryptophan, or that a protein such as zein of maize kernel, which is lacking in lysin, is incomplete for all physiological purposes since is cannot supply all the "building stones" necessary for the construction of body proteins. No matter how much of such a protein is present in a diet, which may be entirely adequate with respect to all other food factors, nutritive disaster will speedily overtake the animal confined to it. The nutritive value of a protein or mixture of proteins depends upon the presence in its molecules of all the essential amino-acids, and upon the extent to which their proportions correspond to those existing in the body proteins they are to be transformed into. It is obvious, therefore, that mixtures of proteins unlike in their constitution may yield amounts of the several amino-acids which will tend to make them more efficient than either or any individual protein in the mixture. One protein may supplement the deficiency of the other. On these facts was based a criticism of certain of the experimental work designed to yield data showing in a quantitative way, the comparative values of different purified proteins. These purified proteins were not fed as the sole source of nitrogen (amino-acids) but were supplemented with fairly liberal additions of amino-acids in the "protein-free milk" or yeast which was included in the diet.

In order to illustrate further the differences in the constitution of individual proteins from different sources a few tables are presented which contain in summarized form the best analytical data which it has been possible to obtain by chemical methods.

Table I shows the composition of a series of animal and vegetable proteins and the sources of origin. On the left side is shown the percentage composition of each in carbon, hydrogen, nitrogen, sulphur and oxygen, which are the only elements entering into the formation of the simple proteins. These elements are designated by the symbols, C, H, N, S and 0 respectively. On the right-hand side in their respective columns are tabulated the number of atoms of each of these elements which a single molecule of the protein is believed to contain, and in the column headed "Molecular Weight" are the numbers which indicate approximately the weight of a single protein molecule as compared with an atom of hydrogen which is taken as unity.

100. Elementary Composition Of All Proteins Is Of The Same Order

The most interesting information to be gained from this table, is the remarkable similarity of the proteins from various sources in respect to their content of carbon, hydrogen and oxygen, and their relatively wide differences in nitrogen and sulphur content. Ovalbumin of the hen's egg contains but 15.51 per cent of nitrogen, whereas amandin from the almond contains 19.32 per cent. Legumin from the bean contains but 0.385 per cent of sulphur, while serumalbumin from human blood contains 2.25 per cent of this element, or nearly six times as much for equal weights of the two substances.