ENZYMES OTHER THAN SUCCINATE DEHYDROGENASE 



65 



decarboxylase, and the condensing enzyme are inhibited by malonate, al- 

 though usually not as potently as is succinate dehydrogenase. Enzymes 

 catalyzing reactions of the dicarboxylates in which the charges are farther 

 apart (e.g. a-ketoglutarate) would be expected to be less susceptible. It is 

 likely that these inhibitions are mosth^ competitive but sufficient data to 

 establish this are generally lacking. 



The inhibition oifumarase by malonate has been shown to be competitive 

 in the thorough study of Massey (1953 b) but the affinity of the enzyme for 

 malonate is not very high {K^ = 40 n\M). This implies either a very dif- 

 ferent intercationic distance in this enzyme from that in succinate dehydro- 

 genase or a different configuration of the enzyme surface surrounding these 

 cationic groups, probably the latter. Both directions of the reaction cata- 

 lyzed by the malic enzyme are inhibited by malonate, which competes 

 with either malate (Stickland, 1959 b) or p\Tuvate (Stickland, 1959 a) (see 

 accompanying tabulation). The inhibition of this enzyme might well alter 



the operation of the tricarboxylic acid cycle under certain conditions. The 

 situation is different for lactate dehydrogenase, since malonate is competitive 

 with respect to lactate but noncompetitive with respect to pyruvate (Ot- 

 tolenghi and Denstedt, 1958), leading to the suggestion that these two sub- 

 strates react with different sites on the enzyme. The X/s are 6.4 m.M for 

 the oxidation of lactate and 27 mM for the reduction of pyruvate. Oxalate 

 and tartronate are much better inhibitors of this enzyme. The D-a-hydroxy 

 acid dehydrogenase of yeast, which oxidizes lactate, malate, a-hydroxybuty- 

 rate, and glycerate, is competitively inhibited by malonate with a K^ of 

 0.9 mil/ (Boeri et al., 1960). In this case, oxalate is a very potent inhibitor 

 {Ki = 0.0025 mM) while tartronate is of similar potency to malonate. 

 Finally, in reducing systems in which succinate can serve as an electron 

 donor, malonate maj^ inhibit competitively. This is the case for the particu- 

 late nitrate reductase of soybean root nodules, the K^ for malonate being 

 0.017 milf (Cheniae and Evans, 1959). The nitrate reductase is not inhibited 

 by malonate directly but the results on the over aU system might make it 

 appear to be the case. In all cases where malonate inhibits competitively, 

 the susceptibility of the reaction in complex systems will depend on the 

 concentration of the substrate, and thus may be quite high in living systems 

 where the concentrations of intermediates are frequently low. 



