The various proteins differ from one another in the relative quantity of the different amino-acids which they contain, and also undoubtedly in the manner of chemical linkage of those acids. Thus Abderhalden has called attention to the fact that if the seventeen different chemical units be joined together in different ways, 350,000,000 times 1,000,000 different combinations are possible even though only a single representative of each unit is used. In this manner the amino.acids may form combinations the possible multiplicity of which recalls the number of words in the dictionary formed from the letters of the alphabet.

1 For further details see Plimmer: "The Chemical Constitution of the Proteins," 1908.

The following table presents analytic data showing the approximate amounts of the different amino-acids contained in well-known varieties of vegetable and animal proteins:

Comparative Composition Of Proteins

Concerning the crystalline vegetable proteins which he has investigated Osborne3 writes: "It is possible to establish a constancy of properties and ultimate composition between successive fractional precipitations which give no reason for believing the substance to be a mixture of two or more individuals. On chemical grounds there is no more reason for dividing the proteins into two groups of animal and vegetable proteins than there is in making a similar distinction between the carbohydrates. Of twenty-three seed proteins which have been hydrolized, all have yielded leucin, prolin, phenylalanin, aspartic acid, glutamic acid, tyrosin, histidin, arginin, and ammonia. Glycocoll, lysin, and tryptophan are the only amino-acids which have been proved lacking in any one of these proteins".

1 Osborne, T. B., and Mendel, L. B.: "Journal Biological Chemistry," 1914, xvii, 336, modified as to the arginin, histidin, and lysin content of gliadin and lactalbumin to accord with Osborne, Van Slyke, Leavenworth, and Vino. grad, Ibid., 1915, xxii, 259.

2 Osborne and Jones: "American Journal Physiology," 1909, xxiv, 437, modified by the findings of Osborne and Jones, Ibid., 1910, xxvi, 305.

3 Osborne: "Science," 1908, xxviii, 417.

Osborne and Abderhalden are agreed that the chemical constituents of protein are probably all known, and that the usual deficit found on their analysis is due to the inadequacy of the methods employed. Thus Osborne and Jones found that they recovered varying percentages of different amino-acids when a mixture of known quantities was analyzed. If one computes their analysis of ox muscle protein on the basis of analytic losses similar to those found when the mixture of known quantities of amino-acids was analyzed, one obtains nearly 103 per cent, of the value of the original ox protein. This value includes the water added by hydrolysis in the break up of the molecule.

Osborne finds that the quantity of ammonia liberated in hydrolysis bears a constant relation to the amount of glutamic and aspartic acids recovered. He concludes that one of the carboxyl groups (COOH) exists as an amid (CONH)2, and that in reality glutamin and asparagin are present in the molecule, and become the sources of ammonia when the molecule is broken.

The physiology of protein metabolism has become in late years the physiology of the amino-acids. When once so regarded, the problem is one of the study of the behavior within the body of chemical entities which can be prepared in pure crystalline form and the formulae of which are definitely known. The fate of these individual amino-acids will be considered at another time (see p. 184). It is sufficient to state here that the cleavage of protein into amino-acids through digestion hydrolysis is accomplished without the liberation of an appreciable quantity of heat,1 that the resulting amino-acids are absorbed directly into the bloodstream, and that in so far as they are reconstructed into new protein within the organism the process takes place without any measurable thermodynamic reaction (see p. 245). Since the protein content of blood-plasma is nearly the same in fasting as after large ingestion of meat, it is evident that the storage of such ingested protein must be effected elsewhere than in the blood.

A preliminary survey of the more recently discovered information regarding the interplay between the proteins and the amino-acids of the organism may be of service at this juncture. The absorption of amino-acids by the blood was first indicated by the work of Howell,2 who dialyzed dog's blood both before and after giving meat, and in the latter instance recovered more material on adding naphthylsulpho-chlorid to the diffusate than in the former. The precipitate, however, was an oil and its quantity could not be measured accurately. Folin and Denis3 introduced glycocoll or alanin into the small intestines of cats, and on analyzing the blood and muscle tissue noticed a large increase in the quantity of "residual nitrogen" which was obtained by subtracting "urea nitrogen" from "total non-protein nitrogen." The increase was so great that it could only have been caused by the influx of the amino-acids themselves. An hour after the introduction of the amino-acids urea appeared in increased quantity in the blood. Their results indicate that absorbed amino-acids circulate in the blood, are retained in the muscle tissue, and that after an hour urea rises in the blood in response to the increased production of urea in the tissues. They found no increase in the quantity of urea or of ammonia in the blood of the portal vein after introducing glycocoll or alanin into a loop of the intestine, and by this experiment demonstrated that the amino-acids were absorbed unchanged without deamination, which would have involved ammonia or urea production.

1Hari: "Pfluger's Archiv," 1906, cxv, II.

2 Howell, W. H.: "American Journal of Physiology," 1906, xvii, 273. 3 Folin and Denis: "Journal of Biological Chemistry," 1912, xii, 141, and previous papers.