INHIBITION OF ENZYMES 



47 



genase binds NADH less well after treatment with iodoacetamide, but 

 NADH does not protect the enzyme (Grimm and Doherty, 1962), indicating 

 possibly that the concentration of NADH used was not sufficient. The pro- 

 tection afforded by reversible inhibitors against irreversible inhibitors is 

 illustrated by the results on succinate dehydrogenase in the tabulation 

 above, and it may be noted that efficient protection is given only by fluoride 

 and phosphate together, just as the inhibition by fluoride and phosphate 

 together is greater than with either alone. 



Quantitative studies on protection may have some value but most of the 

 reports simply demonstrate protection and the authors conclude that the 

 iodoacetate or other inhibitor must thus react with the active center, which 

 is not such an exciting revelation. Even this conclusion is not strictly valid 

 because protection can arise by several mechanisms (Fig. 1-5). The pro- 



^^^^ 



Fig. 1-5. Possible mechanisms for the 



protection of enzyme active sites from 



iodoacetate (lA) or other SH reagents. 



See text for explanation. 



tector could sterically ward off iodoacetate from a vicinal SH group (I), 

 electrostatically repel a negatively charged iodoacetate molecule (II), alter 

 the enzyme structure to sequester an SH group (III), or actually cover the 

 SH group at the active center (IV). In addition, a substrate might stabilize 



