300 2. MALE ATE 



these inhibitions. Peters and Wakelin (1946) first demonstrated that 

 pyruvate oxidation is much more readily inhibited than succinate oxidase, 

 the enzyme to which most attention had been previously paid. Further- 

 more, they demonstrated that the inhibition is not due to reaction of the 

 maleate with low molecular weight thiols, since the dialyzed enzyme is 

 also well inhibited. A 34% inhibition of pyruvate dehydrogenase by 0.04 

 mM and a 50% inhibition of the oxidase by 0.12 mM maleate are indicative 

 of a marked susceptibility of this system. The oxidation of pyruvate by 

 rat heart mitochondria is not as sensitive to maleate (see accompanying 

 tabulation), but is more sensitive than the oxidations of malate, fumarate, 



isocitrate, and succinate (Montgomery and Webb, 1956 b). If the reaction 

 of the maleate is with coenzyme A or lipoate, the a-ketoglutarate oxidase 

 should also be inhibited, and the results of Angielski and Rogulski (1962) 

 indicate about the same sensitivity (Table 2-2) as the heart pyruvate oxi- 

 dase. The fact that pyruvate decarboxylase seems to be insensitive to mal- 

 eate would substantiate such a site of action. 



The inhibition of pyruvate oxidation by heart mitochondria is always 

 greater when fumarate is used as the source of oxalacetate than when malate 

 is used; the results in the tabulation above are from experiments with mal- 

 ate, but when fumarate is used the inhibitions are some 10-20% higher. 

 This could mean that some action is exerted on fumarase. Massey (1953 b) 

 determined that K^ is 11 mM for the inhibition of pig heart fumarase by 

 maleate; this is presumably a competition with fumarate rather than a 

 reaction with SH groups. The inhibition of fumarase by 5 mM maleate in 

 the presence of 5 mM fumarate (the concentration used in the mitochondrial 

 work) can be calculated from the data of Massey to be around 11% at pH 

 6.35; at pH 6.8 the inhibition would be somewhat less since the affinity 

 of the enzyme for maleate decreases above pH 6.4. Thus the inhibition of 

 fumarase could contribute slightly at the highst concentration, assuming 

 the results on crystalline fumarase can be applied to mitochondria, but 

 could not be very important in the over-all affects on the cycle. 



If the inhibition of an enzyme in due to reactions of its SH groups with 

 maleate, the inhibition should be irreversible, but this has not been tested 



