METABOLISM 165 



and the alleged formation of paraneuclein by the action of pep- 

 sin as reported by Robertson being apparently due to adsorp- 

 tion phenomena. 1 On the other hand, however, many authori- 

 ties 2 are inclined to regard reversibility as a general charac- 

 teristic of enzym action and mere negative evidence cannot, of 

 course, disprove this belief. At any rate the conception of 

 a reversible reaction between the amino acids of the blood and 

 lymph and the proteins of the cells affords a comparatively 

 simple and unforced explanation of the facts outlined in the 

 foregoing paragraphs, as well as of others relating to the in- 

 fluence of the supply of feed protein on metabolism which will 

 be considered later (402). In particular, it may be observed 

 that, according to this view, by no means all the amino acids 

 resorbed into the blood stream would undergo synthesis to 

 proteins but that, especially if the amino acid supply were 

 liberal, a large part of them might pass directly to the 'second 

 stage of protein katabolism, viz., deaminization. 



Finally, since the proportions of the single amino acids sup- 

 plied from the digestive tract vary, one must conceive, not of 

 a single reaction between protein and amino acid, but, speak- 

 ing broadly, of as many independent reversible reactions as 

 there are amino acids concerned. 



233. Deaminization. The second general stage of protein 

 katabolism seems to be the splitting off of the NH 2 group from 

 the amino acids, the products being the corresponding or closely 

 related non-nitrogenous organic acids on the one hand and 

 ammonia on the other. This also, it would appear, is a case 

 of enzym action, although the discovery of deaminizing enzyms 

 in various tissues is comparatively recent and its biological im- 

 portance is still to some extent speculative. 



The ammonia resulting from the deaminization of the amino 

 acids is believed to be the immediate antecedent of urea, into 

 which it is rapidly converted, chiefly although not exclusively 

 in the liver (228). In this way the nitrogen of any amino acids 

 resorbed in excess of the immediate demands of the body cells 

 for protein building material is promptly converted into ex- 

 cretory products and so disposed of, while the larger part of 

 their carbon and hydrogen remains in a series of substances 



1 Compare Rohonyi ; Biochem. Ztschr., 53 (1013), 179- 



2 Compare Bayliss; The Nature of Enzym Action (1908), Chapter V. 



