PROTEIN METABOLISM 861 



If the deamination were accompanied with oxidation the corresponding 

 keto-fatty acid would be formed, thus 



CH 3 .CHNH 2 .COOH + = = NH 3 + CH 8 .CO.COOH. 



If reduction took place at the same time, the result would be the 

 production of a saturated fatty acid. Knoop has shown that al] 

 three cases may occur. The investigation of the stages in deamination, 

 and indeed in the disintegration of fatty derivatives generally, is 

 rendered difficult by the fact that all the intermediate products 

 undergo further change and leave the body in a state of complete 

 oxidation, as carbon dioxide and water. If, however, an amino-acid 

 group be administered as part of an aromatic compound, i.e. forming 

 a side-chain of the benzene ring, it is protected from complete oxida- 

 tion by the stability of this ring. The oxidation of the fatty side-chain 

 may proceed to a certain degree, so that intermediate products of 

 metabolism may be excreted still attached to the benzene nucleus. 

 In the a-amino-acids the point where disintegration first occurs is 

 the a-group. Deamination Knoop finds most usually associated with 

 oxidation. The primary product is therefore an a-keto-acid. Further 



oxidation affects the CO group, so that carbon dioxide is eliminated 



and the next lower acid in the fatty acid series is produced. Thus 

 from alanine the body would produce pyruvic acid, CH 3 .CO.COOH, 

 and this on further oxidation would form acetic acid, CH 3 .COQH, and 

 carbon dioxide. On the other hand, these keto-acids may undergo 

 reduction to an oxy-acid, or even a step further, to a fatty acid, though 

 the conditions which determine whether oxidation or reduction shall 

 take place have not yet been fully studied. 



Of very great importance is Knoop's discovery that deamination 

 is a reversible process, so that, given a right molecular grouping, a 

 fatty acid residue may react with ammonia to form an amino-acid. 

 The proof of this fact was facilitated by the discovery that the next 

 higher homologue of phenylalanine, namely, phenyl-a-amino-butyric 

 acid, when administered to an animal, was excreted in large quanti- 

 ties in the urine as an ether-soluble acetyl derivative, which was 

 easily isolated in a state of purity. If then this amino-acid were 

 formed in the body, one might expect to find it without difficulty in 

 the urine. Knoop found that the administration of either phenyl- 

 a-keto-butyric acid or phenyl- a -oxy butyric acid led to the excretion 

 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 intermediate products of oxidation of oxy-acids, e.g. lactic acid, 

 it is evident that the animal body can assimilate ammonia and form 



