34 1. MALONATE 



differences in Table 1-6. If one is to compare the inhibitory potencies of 

 malonate on succinate oxidases from different tissues or species, equivalent 

 rates of succinate oxidation should be used and similar methods of deter- 

 mining the rate should be employed. 



Inhibition of Succinate Dehydrogenase by Other Dicarboxylates 



Before discussing the more intimate nature of the inhibition and the 

 possible ways by which malonate is bound to the active center, it will be 

 useful to consider the inhibitory potencies of other dicarboxylate ions. 

 The configuration of an active center may often be approached by comparing 

 the relative affinities of analogous compounds for the enzyme. At the end of 

 this chapter a more complete discussion of inhibitors related to malonate 

 will be given; for the present we shall be interested only in the inhibition 

 of succinate dehydrogenase. Inhibitions and inhibitor constants are summa- 

 rized in Tables 1-7 and 1-8. 



(a) U nsubstituted dicarboxylate ions. It was stated by Quastel and Whet- 

 ham (1925) that oxalate, glutarate, and adipate do not interfere with 

 succinate oxidation, in contrast to malonate, and, although in later work 

 they found some inhibition, this has generally been confirmed. Accurate 

 comparisons must be made using inhibitor constants, and these are not 

 available, but there is no doubt that in the series ~00C — (CHa)^ — C00~ 

 the inhibition reaches a sharp maximum at w = 1. One must assume that 

 malonate best fits the intercationic distance on the active center of suc- 

 cinate dehydrogenase, and this includes succinate itself since K^ is generally 

 larger than K, for malonate. 



(6) Unsaturated dicarboxylate ions. It is interesting to compare the two 

 isomers, fumarate and maleate, since they differ in the intercarboxylate 

 distance (Table 1-1). For mammalian succinate dehydrogenase it would 

 appear that fumarate is bound much less tightly than either succinate or 

 malonate (Table 1-8), but this does not necessarily hold for the bacterial 

 enzymes, providing evidence that the active center configurations may be 

 quite different in different dehydrogenases. One might expect a rather low 

 affinity for fumarate because of the fairly long intercarboxylate distance, 

 but it is surprising that maleate is such a poor inhibitor inasmuch as its inter- 

 carboxylate distance in only 0.38 A greater than in malonate. A complicating 

 factor is the ability of maleate to react with SH groups, and actually the 

 inhibitions observed by Hopkins et al. (1938) and Morgan and Friedmann 

 (1938 b) were obtained only after prolonged incubation. The possible 

 significance of these observations will be discussed in the next section. 

 On the other hand, acetylene-dicarboxylate, with a distance of 5.16 A 

 between carboxylate groups and a restriction to linearity, does inhibit and 

 this is completely competitive (Thomson, 1959). 



