D-AMINO ACID OXIDASE 347 



Although the concept that these inhibitors bind to the enzyme and com- 

 pete with the substrate is very reasonable, some recent evidence may 

 indicate that the situation is a little more complex. The values of K^ for 

 benzoate should be the same for every substrate, according to the classic 

 treatment, but Klein (1956, 1960) has found that they are not, although 

 all the 1/v — 1/(S) plots are definitely competitive. The A;/s may vary as 

 much as almost 3-fold (see tabulation below). One possibility is that in 



the aqueous extracts of pig kidney there are different oxidases for each 

 substrate but Klein prefers to assume that the inhibitors may react with 

 the ES complex to release the substrate: 



ES + I ^ EI 4- S 



The equilibrium constant K = (ES) (I)/(EI) (S) depends on the substrate 

 used and in the usual rate equation for competitive inhibition, {l)KJK^ 

 would be replaced by {l)IK. Since K = KJK„ the determined values of K 

 should be inversely proportional to K,. They are not inversely proportional 

 to K„„ but is K„, = K, in this case? It may be noted that this mechanism is 

 not quite the same as uncoupling inhibition, since there an ESI complex is 

 formed and the substrate is not forced off. I must admit that I cannot 

 easily visualize how a competitive inhibitor can actively displace an enzyme- 

 bound substrate molecule. 



Although Hellerman et al. (1946) reported the inhibition of D-amino acid 

 oxidase by benzoate to be independent of FAD concentration, there is 

 more recent evidence that certain aromatic carboxylates and phenols not 

 only can compete with substrate, but can also either compete with FAD 

 or complex directly with FAD (Yagi et al., 1957, 1959, 1960). The constants 

 for each of these reactions are given for a few of these inhibitors in the fol- 

 lowing tabulation. Only the carboxylates compete with substrate, while 

 the phenols act by the other two mechanisms; the substances with both 

 COO- and OH groups react in all three ways, although competition with 

 the substrate is the most important. Yagi and his group have recently 



