64 PHYSIOLOGICAL CHEMISTRY 



acids. These amino acids 1 constitute a long list of important substances 

 which contain nuclei belonging either to the aliphatic, carbocyclic, or 

 heterocydic classes of compounds. The list includes glycocoll (glycine), 

 alanine, serine, phenylalanine, tyrosine, cystine, tryptophane, histidine, 

 valine, arginine, leucine, isoleucine, lysine, aspartic acid, glutamic acid, 

 proline, oxyproline, and norleucine. Of these amino acids, tyrosine 

 and phenylalanine contain carbocyclic nuclei; histidine, proline, and 

 tryptophane contain heterocyclic nuclei; and the remaining members 

 of the list, as given, contain aliphatic nuclei. The amino acids are 

 preeminently the most important class of protein decomposition prod- 

 ucts. These amino acids are all a-amino acids, and, with the exception 

 of glycocoll, are all optically active. Furthermore, they are amphoteric 

 substances and consequently are able to form salts with both bases and 

 acids. These properties are inherent in the NH 2 and COOH groups of 

 the amino acids. 



The decomposition products of protein may be grouped as pri- 

 mary and secondary decomposition products. By primary products are 

 meant those which exist as radicals within the protein molecule and 

 which are liberated, upon cleavage of this molecule, with their carbon 

 chains intact and the position of their nitrogen unaltered. The second- 

 ary products are those which result from the disintegration of the 

 primary cleavage products. No matter what method is used to de- 

 compose a given protein molecule, the primary products are largely the 

 same under all conditions. 2 



In the process of hydrolysis the protein molecule is gradually broken 

 down and less complicated aggregates than the original molecule are 

 formed, which are known as proteoses, peptones, and peptides, and which 

 still possess true protein characteristics. Further hydrolysis causes the 

 ultimate transformation of these substances, of a protein nature, into the 

 amino acids of known chemical structure. In this decomposition the 

 protein molecule is not broken down in a regular manner into J^, J4, 

 % portions and the amino acids formed in a group at the termination of 

 the hydrolysis. On the contrary, certain amino acids are formed very 

 early in the process, in fact while the main hydrolytic action has pro- 

 ceeded no further than the proteose stage. Gradually the complexity 

 of the protein portion undergoing decomposition is simplified by the 

 splitting off of the amino acids and finally it is so far decomposed 

 through previous cleavages that it yields only amino acids at the 

 succeeding cleavage. In shortj the general plan of the hydrolysis of 

 the protein molecule is similar to the hydrolysis of starch. In the case 



1 For a discussion of amino acids see Underbill's "Physiology of Amino Acids," Yale 

 University Press, Nov., 1915. 



2 Alkaline hydrolysis yields urea and ornithine which result from arginine. {he product of 

 acid hydrolysis. 



