ENZYMES OTHER THAN SUCCINATE DEHYDROGENASE 59 



by fiimarate. We shall then be in a position to evaluate the specificity of 

 malonate. There are other reasons, of course, for taking up these subjects; 

 for example, malonate is frequently used to block the cycle in living tissue 

 and in this connection it is essential to understand how malonate can alter 

 cycle activity under various conditions. 



Some effects of malonate on miscellaneous enzymes are presented in 

 Table 1-12. There are three major difficulties in the establishment of the 

 over all spectrum of action of malonate. First, there are many quite impor- 

 tant enzymes whose response to malonate has never been investigated 

 directly. Second, inspection of the table will show that in few cases has 

 more than one concentration been used, and often the single concentration 

 reported is either too high or too low to be of much value. Third, the same 

 enzyme from different species often shows widely varying susceptibility 

 to malonate (e.g. NADH oxidase, /5-glucuronidase, lactate dehydrogenase, 

 oxalacetate decarboxylase, and others in the table), making it clear that 

 there are many different spectra of malonate inhibition and that statements 

 on specificity must be qualifield by naming the source of the enzymes in 

 question. 



Ideally, the results on the inhibition of an enzyme by malonate should 

 be given for several concentrations, preferably convering the range from that 

 concentrations just sufficient to produce some inhibitions, through that 

 causing approximately 50% inhibition to higher inhibitions (unless these 

 latter concentrations are unreasonably high). Another way of looking at the 

 problem is to consider that range of malonate concentrations most likely 

 to give useful information when tested on enzymes. This will depend 

 on the source of the enzymes. For example, in mammalian tissues it usu- 

 ally requires malonate concentrations between 2 and 5 mM to inhibit suc- 

 cinate dehydrogenase around 90% in the presence of 5-10 mM succinate. 

 In the interests of establishing the degree of specificity, it would thus be 

 most important to test the effects of malonate at concentrations around 

 5 mM on enzymes from such sources. When the organism studied possesses a 

 succinate dehydrogenase less sensitive to malonate, correspondingly higher 

 concentrations must be applied to the other enzymes. 



Instances of Competitive Inhibition 



Until it is time to discuss the matter of specificity, there is little to say 

 about these inhibitions since the results in the table speak for themselves. 

 It is evident that several enzymes other than succinate dehydrogenase 

 are readily inhibited. One would not be surprised if enzymes attacking the 

 dicarboxylate anions, where the carboxylate groups are separated by two 

 carbon atoms, were inhibitable by malonate to some extent, since it is 

 likely that these enzymes also possess cationic groups appropriately spaced. 

 Actually, fumarase, malate dehydrogenase, the malic enzyme, oxalacetate 



