

PROTEIN METABOLISM 809 



of the corresponding amino-acid in the urine. Since keto-acids occur as the 

 ordinary products of the breakdown of amino-acids and also as the inter- 

 mediate products of oxidation of oxy-acids, e. g. lactic acid, it is evident that 

 the animal body can assimilate ammonia and form amino-acids, provided 

 only that it is supplied with the proper non-nitrogenous acids. These latter 

 need not be derived from proteins at all but, like lactic acid, be a result of 

 carbohydrate metabolism. Thus, if the fitting non-nitrogenous food be given 

 (e.g. oxy-fatty acids, or carbohydrates, from which these bodies may be 

 formed), part of the nitrogen set free by protein disintegration might be 

 recombined with the formation of amino-fatty acids without giving rise to 

 urea or appearing in any way in the nitrogen balance-sheet of the body. 

 This possibility enjoins the necessity of caution in interpreting the results of 

 metabolism experiments where the nitrogen excreted is taken to represent 

 the total protein metabolism of 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 amino-fatty acids, namely, glycine, can be easily manu- 

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

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

 be easily manufactured by the body, by the animation 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 

 unchanged, and so increases the proportion of ammonia in this fluid. If 

 however the ammonium chloride be administered at the same time as the 

 animal is receiving its ordinary vegetable diet, there is no increase in the 

 ammonia in the urine, the whole of the ammonium chloride being converted 



