INHIBITION OF ENZYMES 301 



in most cases where such a mechanism is the most likely. Greenberg and 

 Winnick (1940) reported that the inhibitions of papain and asclepain by 

 maleate can be to a large extent reversed by addition of cyanide. Since 

 one might expect an SH reaction with these enzymes, the reversal is dif- 

 ficult to explain. They postulated that maleate oxidizes the enzymes, being 

 simultaneously reduced to succinate, and that cyanide reactivates by res- 

 toration of the normal SH groups. However, it is thermodynamically un- 

 likely that the reaction 



Maleate + 2 SH -> S — S + succinate 



would occur, and furthermore in the studies in which maleate and various 

 thiols have been allowed to react, no succinate has been reported. Since 

 the reactivation by cyanide was not complete, it is possible that when 

 maleate reacts with a certain SH group on the enzyme, secondary changes 

 involving oxidation of other SH groups occur, these latter changes being 

 reversible. One must admit that the concept of maleate as an SH reagent 

 in enzyme inhibition is in no case on a solid basis, and there is much need 

 for SH titration studies with pure enzymes exposed to this inhibitor. 



Turning to the competitive mechanisms of inhibition, we are on much 

 more solid, although often less interesting, ground. One would anticipate 

 maleate to have some affinity for certain enzyme active sites normally 

 reacting with other dicarboxylates. Competitive inhibition by maleate 

 has been established for fumarase (Massey, 1953 b), cysteine desulfurase 

 (Fromageot and Patino-Bun, 1961), transaminases (Jenkins et al., 1959; 

 Velick and Vavra, 1962), and possibly /5-glycerophosphatase (Nigam et 

 al., 1959). In the case of transaminase, maleate is competitive with respect 

 to both amino and keto acids, the inhibition is reversible by dialysis, and 

 reaction of maleate with the enzyme can be followed spectrophotometrically. 

 The results with succinate dehydrogenase have been so variable that it is 

 difficult to characterize the mechanism of inhibition. In the early work of 

 Hopkins et al. (1938) it was assumed that SH group reaction occurred 

 because of the slow rate of inhibition, and this may well be true under their 

 conditions. However, Dervartanian and Veeger (1962) found the inhibition 

 to be competitive with K^ = Q raM, and maleate to produce ultraviolet 

 absorption changes similar to those observed when the enzyme is treated 

 with fumarate or malonate. On the other hand, Hellerman et al. (1960) 

 could detect no inhibition of the soluble succinate dehydrogenase from pig 

 heart mitochondria, and in our work with rat heart mitochondria (Mont- 

 gomery and Webb, 1956 b) we invariably found a stimulation of succinate 

 oxidation by maleate at 1-5 raM. Certain enzymes which are inhibited by 

 most dicarboxylates, such as aspartase (Ellfolk, 1954) and L-glutamate 

 dehydrogenase (Caughey et al., 1957), are not inhibited by maleate, in- 

 dicating the importance of the steric arrangement of the carboxylate groups. 



