4 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



17. Tryptophan, CjjH^N^Og, or 0-indole-a-aminopropionic acid. 

 C CH . CH(NH 2 ) . COOH 



/ \ 

 C 6 H 4 CH 



\ / 

 NH 



E. Ammonia. 



These seventeen amino acids, together with ammonia, which is 

 present in the form of acid amide groups, are the basis of the com- 

 position of the protein molecule. They are generally referred to as 

 " Bau-steine," the bricks, or foundation stones ; but as the English 

 translation of the German word is not entirely expressive of its mean- 

 ing, it is preferable to use the term unit or element for these compounds 

 in their relation to the proteins. 



Most proteins contain all these amino acids or units in various 

 proportions, but some proteins, such as gelatin, contain only fourteen 

 or fifteen, and some, such as salmine, are built up of three or four units, 

 these units being di-amino acids and histidine. Full data of the pro- 

 portions, so far as is known, are given in the tables on pp. 111-130, 

 and further reference to details is made on pp. 63-84. 



Our purely chemical knowledge of the proteins has led to a greater 

 knowledge of the digestion of proteins. Proteins are completely con- 

 verted into their constituent units in the alimentary canal of animals ; 

 in this form they reach the bloodstream and circulate to the various 

 organs of the body. Each organ rebuilds its tissue from the circulat- 

 ing amino acids, as well as giving rise to them during its catabolism. 

 In plants, too, protein is transferred from one region to another in the 

 form of amino acids. We think of and work with proteins in terms 

 of amino acids. All considerations and deductions as to the dietetic 

 value of proteins can no longer be based upon their total nitrogen 

 content, but must be based upon the amounts of the various amino 

 acids in their molecule. Nitrogen equivalence is not amino acid equi- 

 valence. Proteins devoid of certain units are of no value for the 

 maintenance of life ; proteins deficient in certain amino acids are of 

 little or of no value for ensuring the growth of young animals. These 

 particulars are given in Cathcart's Physiology of Protein Metabolism, 

 second edition. The analysis of proteins with special reference to 

 those units of metabolic value is therefore of the greatest importance. 



The long list of amino acids is sufficient evidence of the complexity 

 of the protein molecule ; and, as yet, it seems to be incomplete, for 

 several other products have been described : 



