864 PHYSIOLOGY 



results is not so great as that following a large protein meal. It 

 seems therefore that the CHO moiety of the protein undergoes oxida- 

 tion more rapidly than either dextrose or the ordinary fats of the diet, 

 and that, as we concluded in an earlier chapter, the metabolism of 

 these substances is really to a considerable extent dependent on the 

 quantity presented to the organism rather than on the actual needs 

 of the cells of the body. It is this rise in metabolism and respiratory 

 exchanges after protein ingestion which justifies to a certain extent 

 the idea that the proteins, more than any other food-stuff, have a 

 stimulant action on metabolism. The reason why the CHO remainder 

 of the protein molecule is so prone to oxidation and does not, like an 

 excess of carbohydrates, undergo conversion into fats in the body, we 

 shall have to consider in greater detail in dealing with the fate of 

 this latter class of substances. We need, however, considerably 

 more evidence as to the extent to which deamination occurs 

 and as to its conditions and end-products before we can hope to 

 determine the cause for the rapid breakdown of these end-products 

 in the body. 



ARE THE AMINO- ACIDS INTERCONVERTIBLE ? 



Although the animal organism is apparently capable of synthetising 

 amino-acids from ammonia and the corresponding keto- or oxy-fatty 

 acid, it is unable to convert one amino-acid into another. On this 

 account many proteins are inadequate as food substances since they 

 do not contain the necessary amino-acid groups. Life cannot be 

 supported on such bodies as zein or gelatin, which are lacking in the 

 tryptophane and tyrosine groups. The failure in these cases is not, 

 as has been generally supposed, owing to an inability to assimilate, 

 i.e. synthetise, nitrogen as ammonia, but to the fact that in the animal 

 the apparatus is wanting for the manufacture of some of the oxy-fatty 

 acids and other radicals which form the non-nitrogenous part of the 

 amino-acids. This view receives confirmation from the fact that the 

 simplest of the fatty acids, namely, glycine, can be easily manufactured 

 in the body, acetic acid being one of the latest stages in the oxidation 

 of most carbohydrates and fats. It is probable that alanine too could 

 be easily manufactured by the body, but definite evidence on this 

 point is not yet forthcoming. 



THE EXCRETION OF AMMONIA 



A large proportion of the urea appearing in the urine after a 

 protein meal is exogenous and is derived by a rapid separation of 

 ammonia from the proteins or their disintegration products almost 

 immediately after their absorption. The greater part of the ammonia 

 passes to the liver, and is there converted into urea, which is excreted 



