INHIBITION OF ENZYMES 621 



by dehydroacetate was observed. It was concluded that dehydroacetate acts 

 on succinate dehydrogenase. Although this is probably true it is evident 

 that, according to the modern elaboration of succinate oxidase, other sites 

 are possible. 



Evidence that dehydroacetate does not inhibit by reacting with the SH 

 groups of succinate dehydrogenase was obtained. First, the inhibition is al- 

 most instantaneous and readily reversible, unlike inhibitions with most SH 

 reagents. Second, cysteine, glutathione, and dimercaprol are unable to pro- 

 tect the enzyme against dehydroacetate.* Third, no inhibition of urease 

 was observed, this enzyme being sensitive to most SH reagents; indeed 

 stimulation was observed. All in all, one must conclude that the possibility 

 of the reaction of dehydroacetate with SH groups has not been eliminated, 

 although there is little positive evidence for such a mechanism. 



The possibility of competitive inhibition of succinate dehydrogenase can- 

 not be eliminated since no data were given for the statement, "Increasing 

 the substrate concentration does not appreciably alter the degree of inhi- 

 bition." If, for example, a succinate concentration of 50 mM was used 

 (which is the only concentration mentioned in the paper) with dehydro- 

 acetate at 9.3 mM, increasing the succinate to 100 or 200 m.M would not 

 be expected to reduce the inhibition markedly. It is interesting to speculate 

 that the dianionic forms of dehydroacetate have a basically similar charge 

 distribution to malonate. However, due to resonance the charge magnitude 



H H 

 ^ ^C^ CH, O^ X ^^ 



c c c c 



o~ o" o' o" 



Dehydroacetate Malonate 



* Unfortunately the cysteine and glutathione were used at only about one fifth the 

 dehydroacetate concentration, so that even total reaction of the inhibitor would have 

 reduced the inhibition relatively little, actually about 5%. Cysteine reduced the inhi- 

 bition around 5% from the predicted value but glutathione did not. Cavallito and 

 Haskell (1945) mentioned that dehydroacetate does not react with cysteine. 



