TRANSAMINATION 213 



the substrates available for transamination can be considered as 

 consisting of two types: 



1. "Primary" or "active" substrates. These have a high affinity 

 for the enzyme and react with mono- or dibasic acids after adsorp- 

 tion. Primary substrates are the dibasic alpha-amino or alpha-keto 

 acids, and include compounds such as cysteic acid. 



2. "Secondary" or "passive" substrates. This group includes all the 

 alpha-amino and alpha-keto monobasic acids which have no direct 

 affinity for the enzyme and serve only as "reaction partners" for the 

 primary substrates. 



This idea of Braunstein can be extended to mean that only the 

 dibasic alpha-amino or -keto acids are activated by the enzyme. 

 This would suggest that the dibasic alpha-amino and -keto acids act 

 as prosthetic groups, which in the presence of the enzyine react 

 with the secondary substrates. That is, the secondary substrates have 

 an affinity for the enzyme only when the "primary" substrates have 

 become activated. This concept is not unreasonable, but it appears 

 unnecessary to classify the substrates as active or passive. From 

 Braunstein's point of view this is essential, since he has to account 

 for the activity of the large number of monobasic alpha-amino and 

 -keto acids which he reports to be active in transamination. From the 

 writer's point of view this concept is unnecessary, since his experi- 

 mental data lead to the conclusion that transamination is essentially 

 a limited reaction and concerned chiefly with the dibasic alpha- 

 amino and -keto acids. The activity of other compounds can be 

 explained on the basis of different affinities for the enzyme. Thus 

 with large amounts of tissue and long incubation periods other 

 amino acids show a small amount of activity (20). 



"Catalytic" Transamination.— Braunstein (1) and Braunstein and 

 Kritzmann (21) have reported that the addition of small amounts of 

 a dibasic alpha-amino or alpha-keto acid (as little as M/16000) 

 causes transamination to take place in pigeon breast muscle brei 

 between lysine and pyruvic acid. The latter system alone is inactive. 

 With glutamic acid, the reaction is pictured as proceeding in the 

 following manner: 



( 3 ) glutamic acid + pyruvic acid — > a-ketoglutaric acid + alanine 



( 4 ) a-ketoglutaric acid -f lysine —> glutamic acid -f 2-keto-6-aminocaproic acid 

 The net effect of this would be: 



( 5 ) lysine + pyruvic acid —> 2-keto-6-aminocaproic acid -|- alanine 



The effectiveness of so low a concentration as M/16000 is remark- 



