624 



13. EEVERSAL OF INHIBITION 



the inhibitor led to less and less reactivation from added cysteine (Sols 

 and Crane, 1954). The higher the inhibitor concentration and thus the great- 

 er the inhibition, the more likely is secondary inactivation to occur and 

 reversibility be limited. Sometimes this may involve a structural change 

 in the enzyme due to an increased instability in the presence of the inhi- 

 bitor, or it may be the result of the reaction of the inhibitor with additional 

 less reactive groups on the enzyme, the latter reactions being either irre- 



0.5 



(340mu)0 



CONTROL 

 (t cysteine) 



p-CMB = 

 O.OI280mM 



p-CMB= 

 } /0.0336mM 



CYSTEINE 



CYSTEINE 



TIME (Mmutes)- 



FiG. 13-8. Reactivation of phosphoglyceraldehyde 

 dehydrogenase inhibited by 53-chloromercuriben- 

 zoate (39-CMB) with cysteine added at the time 

 indicated by the arrows. The cysteine concentra- 

 tion was 3 mM. (From Vehck, 1953, 1954.) 



versible or slowly reversible. In any event, if the reversiljility of the pri- 

 mary inhibition is being tested, it is advisable to add the reversor as soon 

 as possible, and it is generally preferable to inhibit the enzyme to the extent 

 of only around 50% instead of completely. It is quite possible that the par- 

 tial and total irreversibilities mentioned above were to some degree due 

 to these secondary reactions. 



Information Obtained from Protection and Reversal Studies 



Since the majority of the studies with certain inhibitors, especially 

 those reacting with sulfhydryl groups, include data on the reversibility of 

 the inhibition following the addition of a sulfhydryl compound to remove 



