490 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



presence of specific atomic groupings in the dietary is, however, afforded 

 by the investigations of Hopkins and Willcock and of Osborne and 

 Mendel upon the ability of various pure Proteins to supply the nitrogen- 

 requirements of growth and maintenance. 



We have seen that the various protein constituents of the tissues and 

 of the diet are built up out of varying permutations and combinations 

 of a limited number (nineteen in all) .of Amino-acid radicals which are 

 linked together in long chains. Now certain of these nineteen radicals 

 are lacking in some of the proteins, and the administration of such 

 proteins to growing animals as the sole source of nitrogen in the diet 

 enables us to ascertain whether the amino-acid which is lacking is 

 synthesizable by animal tissues, for synthesized it must be if normal 

 tissues are to be produced by the animal and it is not procurable 

 preformed in the diet. 



From the investigations cited it appears very probable that the 

 only amino-acid radical which is synthesizable by animal tissues is 

 Glycocoll, or Amino-acetic Acid. Of the remainder, it is probable that 

 all must be present preformed in the diet in order to permit the accre- 

 tion of living tissue; at all events this has been positively established 

 for several of the amino-acid radicals, for example Lysine, Tryptophane, 

 Tyrosine, and Cystine. 



The alcohol-soluble protein of maize, Zein, is lacking in glycocoll, 

 tryptophane and lysine, and the investigations of Hopkins and Willcock 

 and of Osborne and Mendel have shown that if Zein be the sole source 

 of nitrogen in the diet, not only is accretion of fresh tissue impossible, 

 but the maintenance of that already formed is also impossible, so that 

 when supplied with abundance of nitrogen, carbon and salts in correct 

 proportion, water and calories, the animal nevertheless dies of inani- 

 tion. If tryptophane be added maintenance becomes possible, but not 

 growth. On such a diet, or if supplied with Gliadin which lacks only 

 glycocoll and lysine, a young animal lives but ceases to grow and 

 maintains an infantile appearance, and full capacity to grow upon 

 readmission of the lacking constituent to the diet, until what would 

 normally be a "ripe old age" (Fig. 33). Upon addition of lysine as well 

 as tryptophane, normal growth and maintenance are at once rendered 

 possible, the glycocoll being synthesized by the animal itself. Evi- 

 dently the Endogenous Metabolism, or waste incidental to and an essen- 

 tial consequence of life, of the amino-acid lysine is reducible to zero, 

 possibly because a limited supply of lysine may be utilized over and over 

 again in the processes of waste and repair, while, on the contrary, the 

 endogenous metabolism of tryptophane is not reducible to zero, possibly 

 because it is employed, not only in the manufacture of tissue, but also 

 of constituents of the body which undergo irreversible consumption. 

 The result is, at all events, that an inevitable waste of tryptophane 

 attends the maintenance of life, and in its absence from the diet, the 

 tissues of animals being 'unable to synthesize it from other nitrogenous 

 constituents of the diet, tissue-waste can no longer be accurately 



