622 3. DEHYDROACETATE 



on dehydroacetate. might be lower than on malonate. The succinate dehy- 

 drogenase from calf thymus nuclei is inhibited somewhat more potently by 

 dehydroacetate at pH 6.6 than at pH 7.6 (see accompanying tabulation) 



Yo Inhibition 



(McEwen et al., 1963 a). This does not fit the dianion inhibition theory very 

 well, since at pH 7.6 there should be more of the dianion than at pH 6.6. 

 One cannot attribute the change in inhibition to the ionization of enzyme 

 groups because malonate inhibits better at the higher pH from the limited 

 data provided. Unfortunately the succinate concentration was unvaried 

 from 20 mM and the formal nature of the inhibition remains unknown. 

 The mechanism of the inhibition of succinate dehydrogenase is thus at 

 present unsolved. 



One further experiment deserves brief mention. It was claimed that al- 

 though malonate protects succinate oxidase from SH reagents, it does not 

 protect against dehydroacetate (see accompanying tabulation). First, one 



Dehydroacetate (milf ) Malonate {mM) 



% Inhibition 

 of O2 uptake 



9.3 

 9.3 



would not expect malonate at a concentration inhibiting only 33% to pro- 

 tect very much. Second, the inhibition given by both inhibitors is exactly 

 what would be predicted if both acted at the same site on the enzyme. No 

 conclusions as to the mechanism of inhibition can be deduced from this ex- 

 periment. 



Other Enzymes 



Dehydroacetate has no effect on cholinesterase up to 20 mM (Seevers et 

 al., 1950), or on pepsin, amylase, and trypsin at 8.5 mM (Bauer and La Sala, 

 1956), while urease is stimulated by concentrations up to 93 mM and pan- 



