Page:Popular Science Monthly Volume 72.djvu/398

 the necessity of confirmatory evidence on this important matter, for a hypothesis so far-reaching in its significance demands careful consideration 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 interesting 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 Rona, 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 completely 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 products 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 crystalline 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 specific proteins needed for the replacement of those used up in the processes of life. The hydrolytic cleavage induced by these digestive