762 PHYSIOLOGY 



these results of Lang, and Folin, effect an entire revolution in our views 

 of protein metabolism. Instead of regarding the urea which appears in the 

 urine after protein ingestion as produced by the total disintegration of the 

 protein molecule, we see now that it represents merely the throwing off 

 of the nitrogenous part of this molecule. This deamination may be a 

 purely hydrolytic change or it may be associated with oxidation or reduction. 

 Deamination of alanine, for instance, by simple hydrolysis would result in the 

 formation of lactic acid (an oxy- fatty acid). 



CH 3 CH 3 



CH.NH 2 + H 2 = NH 3 + CHOH 



COOH COOH 



If the deamination were accompanied with oxidation the corresponding 

 keto-fatty acid would be formed, thus 



CH 3 .CHNH 2 .COOH + = NH 3 + CH 3 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 all three cases may occur. 

 The investigation of the stages in deamination, and indeed in the disintegra- 

 tion 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 oxidation 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 



i 



is therefore an a-keto-acid. Further oxidation affects the CO group, so that 



i 



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 .COOH, 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 quantities in the urine as an ether- soluble 

 acetyl derivative, which was easily isolated in a state of purity. If then this 



