394 POPULAR SCIENCE MONTHLY 



the necessity of confirmatory evidence on this important matter, for a 

 hypothesis so far-reaching in its significance demands careful consid- 

 eration before it can be given much credence. We understand full well 

 that protein is an essential foodstuff, without which life can not be 

 maintained. We have been accustomed to consider that no other form 

 of nitrogen than protein-nitrogen can supply the physiological needs of 

 the body. If, however, it is true that in normal digestion the protein 

 molecule is completely broken down into relatively simple fragments, 

 non-protein in nature, i. e., into amino-acids and polypeptides, and that 

 from these fragments specific proteins are reconstructed, then it is plain 

 that animals fed on a diet free from protein, but with a proper amount 

 of these nitrogenous cleavage products, should live and thrive, assuming, 

 of course, a sufficient addition of non-nitrogenous food. Experiments 

 after this order have been tried by various investigators, with very inter- 

 esting results. Dogs, for example, fed on a mixture of protein cleavage 

 products with suitable addition of fat and carbohydrate, maintained 

 nitrogen equilibrium and even stored up nitrogen, presumably in the 

 form of protein, thus indicating that the animals were able to utilize 

 these end products of protein decomposition in much the same way as 

 protein food would be utilized. The only logical supposition is that 

 the dogs were able to manufacture body protein out of this composite 

 of protein fragments; i. e., a synthesis of protein took place; otherwise 

 it would have been impossible to maintain a condition of nitrogenous 

 equilibrium even for a day. Similarly, white rats gained in weight 

 and stored up nitrogen on a diet in which the protein of their food was 

 entirely replaced by the digestion products formed by trypsin and 

 erepsin. Abderhalden and Bona, 5 using a dog as subject and feeding 

 the products formed by pancreatic digestion of casein, viz., the amino- 

 acids and other biuret-free products, found it possible to prevent com- 

 pletely the loss of body protein, a further proof of the power of the 

 animal organism to synthesize protein from its final cleavage products. 

 Experimental evidence of this character forces us, whatever our 

 preconceived notions, to admit the power of the animal body to build 

 up its needed protein out of the relatively simple decomposition prod- 

 ucts into which the various forms of food protein are broken down by 

 the processes of digestion. Oxidation does not appear — on the surface, 

 at least — but progressive hydrolytic cleavage is the key-note. The food 

 protein, like a crystalline geode, is split apart into numerous crystal- 

 line fragments by action of the several digestive enzymes to which it 

 is exposed in the gastro-intestinal tract, and from these fragments the 

 body cells apparently select such as are required to construct the spe- 

 cific proteins needed for the replacement of those used up in the proc- 

 esses of life. The hydrolytic cleavage induced by these digestive 

 8 Zeitschrift f. physiologischen Chemie, Band 44, p. 200. 



