622 13. REVERSAL OF INHIBITION 



ured to either one of the possibly limiting reactions. The rates of reactions 

 13-31 and 13-32 must be determined independently in order to interpret 

 the data obtained when they are proceeding simultaneously. 



The most general situation in which the rates of the two reactions are 

 comparable is more difficult to treat rigorously. Two simultaneous dif- 

 ferential equations must be solved. One will be either Eq. 13-33 or 13-35, 

 depending on whether the reversor is depleted or not, and the other will 

 be the usual equation for the rate of inhibition change: 



diidt = k_,a') - A:_i[(r) + i]i (13-36) 



The decrease in the inhibition will be slower than in either of the extreme 

 cases discussed above (assuming the same rate constants and conditions) 

 but will follow the same type of curve. It would be most difficult to distin- 

 guish between these various possibilites from the inhibition reversal data 

 alone. 



Examples of Inhibition Reversal 



Most of the studies on reversal have been made with inhibitors that com- 

 bine with enzyme sulfhydryl groups. It will suffice for the present purpose 

 to mention some of the results obtained on the reversal of inhibitions pro- 

 duced by the mercurials. More detailed discussion will be given in the chap- 

 ters on the various inhibitors. One may find instances of complete reversal, 

 partial reversal, or no reversal when sulfhydryl compounds are added to 

 remove the mercurial from the enzymes. For example, complete restoration 

 of activity has been reported in inhibitions of the following enzymes after 

 addition of either cysteine or glutathione: yeast glyceraldehyde-3-phosphate 

 dehydrogenase (Velick, 1954), brain hexokinase (Sols and Crane, 1954), 

 heart lactic dehydrogenase (Neilands, 1954), xanthine oxidase (Harris and 

 Hellerman, 1953), and corn leaf glutamic dehydrogenase (Bulen, 1956). 

 All of these inliibitions had been produced by jy-chloromercuribenzoate 

 (p-CMB). Partial reversal occurred when dimercaprol was added to succin- 

 oxidase inhibited with HgClj (Barron and Kalnitsky, 1947) or when 

 glutathione was added to yeast alcohol dehydrogenase inhibited by p- 

 CMB (Barron and Levine, 1952). No reversal was observed with the fol- 

 lowing enzymes inhibited by p-CMB: muscle pyruvate oxidase — cysteine 

 or dimercaprol (Onrust et al., 1954); heart malic dehydrogenase — cysteine 

 or glutathione (Wolfe and Neilands, 1956); EleciropJwrus acetylcholines- 

 terase—cysteine (Hargreaves, 1955); and crotonase — cysteine or glutathione 

 (Wakil and Mahler, 1954) to mention only a few. It is likely that these 

 different behaviors do not necessarily reflect different mechanisms of in- 

 hibition. It has often been assumed that a lack of reversibility indicates 

 that sulfhyfdryl groups are not involved in the inhibition. Such statements 

 as: "Reactivation is an obligatory condition for testing the specificity 



