PROTEIN METABOLISM 765 



and even if the capacity of these organs be strained to their utmost by 

 administration of large quantities of bones, the increase in the carbon dioxide 

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

 seems therefore that the CHO moiety of the protein undergoes oxidation 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 organ- 

 ism 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 

 ammo-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 amino-fatty acids, namely, glycine, can be easily manu- 

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

 .of most carbohydrates and fats. It has been shown that alanine too can 

 be easily manufactured by the body, by the amination of the three carbon 

 acids or oxy-acids derived from the breakdown of glucose or glycogen. 



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 is converted in the liver into 

 urea, which is excreted by the kidney. A certain small proportion of the 

 nitrogen in the urine is generally turned out in the form of ammonia. This 

 proportion is not increased by the administration of ammonium carbonate. 

 If ammonium chloride be given to a starving rabbit it appears in the urine 



