602 METABOLISM, NUTRITION AND DIETETICS 



lively and qualitatively in respect to their amino-acids raises the ques- 

 tion of the relative value of different proteins in nutrition. In this is 

 involved the further question, whether the body can itself synthesize 

 from other materials any of the amino-acids which may be deficient, 

 or change one amino-acid into another. That the ' peptones ' derived 

 from a protein which is itself capable of permanently supplying 

 the whole nitrogenous intake of the organism can be substituted 

 for the protein scarcely needs demonstration, since it is known that 

 the protein is converted into peptones in digestion. Nevertheless, 

 this has been proved conclusively by feeding experiments with 

 peptones. It was to be expected, too, leaving out of account all 

 consideration of the means of overcoming the repugnance of animals 

 to accepting such unnatural food substances, that the further 

 products of protein hydrolysis, the amino-acids, etc., could be 

 substituted for the original proteins when these were themselves 

 adequate. For it is in the form of amino-acids or at most of such 

 relatively simple polypeptide groups as may still hang together 

 after complete digestion and absorption, that the nitrogenous food 

 substances are normally offered to the tissues. Experimental 

 demonstration of the feasibility of this substitution has also been 

 obtained. The split products of meat, for example, will -keep an 

 animal in nitrogen equilibrium as well as the meat from which they 

 are derived. But what happens when one or more of the amino- 

 acids found in the proteins of the body are missing from the protein 

 of the food ? That the components of an amino-acid like arginin 

 (ornithin and urea), into which it can be split not only by the 

 possibly crude and violent methods of the chemical laboratory, but 

 also by the delicate and precisely-adapted ' biological ' action of a 

 special enzyme (arginase), should be able to replace the original 

 amino-acid is a fact which does not greatly help towards an answer. 

 For when these components, or ornithin alone, since urea is always, 

 present in the body, are given instead of arginin, the reversal of the 

 enzyme reaction by which arginin is decomposed is all that is neces- 

 sary for its synthesis, and the reversal of such a reaction is doubtless 

 a very commonplace affair in tissue metabolism. The formation 

 of one amino-acid from another, or from materials which do not 

 originate exclusively from protein, is a different thing, and the 

 answer to the question raised, so far as it can yet be given, is that 

 the way in which the body deals with a deficiency in the protein 

 ' building stones ' depends upon the nature of the missing amino- 

 acids. Thus, the phospho-protein casein does not yield glycin on 

 hydrolysis; yet it has been shown that casein is a perfectly adequate 

 or complete protein food capable of covering the whole nitrogen 

 requirement of the body over long periods. The same is true of the 

 cleavage products of casein which has been subjected to pancreatic 

 digestion. In an animal fed on no other protein than casein, with 



