40 1. MALONATE 



state of respiratory control (Kunz, 1963). Malonate does not stimulate 

 under these conditions and thus the effects of oxalacetate on intact mito- 

 chondrial succinate oxidation differ in some manner from those of malonate. 

 This, however, is probably explained by other reactions of oxalacetate, e.g., 

 the oxidation of NADH to NAD, which would release the mitochondria 

 from respiratory control. 



{d) Substituted malonates and succinates. Franke (1944 a) found that alkyl- 

 malonates are inactive on heart succinate dehydrogenase, confirming the 

 weak inhibitions reported by Thunberg (1933). Alkylsuccinates likewise are 

 inactive until the length of the alkyl chain is greater than eight carbon 

 atoms. These higher alkylsuccinates may exhibit some degree of competitive 

 inhibition but there is also inhibition of the oxidation of p-phenylenediamine 

 and, hence, of the cytochrome system. Therefore, it is clear that additions 

 of even small alkyl groups to malonate and succinate depress or abolish 

 the normal inhibitory activity. Hydroxymalonate (tartronate) also is es- 

 sentially inactive. These results indicate the importance of that part of the 

 molecule between the two carboxylate groups and would seem to argue 

 against a simple interaction of the molecules with cationic groups on a 

 flat surface, the substituted groups protruding outward. Of course, it is 

 necessary that the — CHgCHo — grouping of succinate interact with the 

 enzyme in order for dehydrogenation to take place. The failure of short- 

 chain alkylmalonates to inhibit appreciably must be attributed to some 

 manner of steric interference by the alkyl groups. 



(e) Cyclic dicarhoxylate ions. Cyclobutane and cyclopentane dicarboxylates 

 are weak inhibitors of succinate dehydrogenase. Since the intercarboxylate 

 distance in cyclobutane- 1,1-dicarboxy late and CT's-cyclopentane-l,2-dicar- 

 boxylate are not too far from that in malonate (Table 1-1), the poor inhibi- 

 tion may be due to the bulkiness of the rings interfering sterically, as do the 

 alkyl groiips discussed in the previous section. It is strange, however, that 

 there is no inhibitory difference between the cis and trans isomers of cyclo- 

 pentane- 1,2-dicarboxylate, since the intercarboxylate distances differ by 

 1.39 A. Thus the inhibition may not be by the same mechanism as for mal- 

 onate; indeed, cw-cyclohexane-l,2-dicarboxylate inhibits succinate dehy- 

 drogenase noncompetitively (Hellerman et al., 1960). 



Nature of the Active Center and the Binding of Malonate 



The evidence indicates the presence at the active center of two cationic 

 groups and a nearby SH group. The cationic groups, perhaps 3-4 A apart, 

 are suggested by the very weak inhibitions exerted by monocarboxylates 

 (Quastel and Wooldridge, 1928; Dietrich et al., 1952) and the complete 

 lack of a competitive inhibition by compounds in which the negative 

 charges on the carboxylate groups are eliminated. Malondialdehyde (Holt- 



