INHIBITION OF ENZYMES 



469 



(Pfleiderer et al., 1959). Although 50% inhibition is reached in around 

 10 min, full inhibition does not occur until nearly 180 min. During the 

 progress of the inhibition there is a progressive loss of free SH groups as 

 determined by titration with p-MB. The completely inhibited enzyme is 

 found to have had 14 SH groups reacted, but if the inactivated enzyme is 

 allowed to stand at pH 10, 3 SH groups are recovered, so that 3 SH groups 

 appear to react differently from the others with the quinone. The basic 

 mechanism is not a simple oxidation of SH groups to S — S groups. It is 

 interesting that the treated enzyme shows an altered absorption spectrum; 

 this might be used to determine the extent of reaction of quinones with 

 this enzyme and perhaps others. Inhibition rate curves are shown in 

 Fig. 5-2 for pancreatic a-amylase (Owens, 1953 a). The rate of inhibition 

 by 2,3-dichloro-l,4-naphthoquinone is seen to be almost double that by 

 1,4-naphthoquinone, but is still quite slow. 



Fig. 5-2. Rates of inhibition of pancreatic amylase by naph- 

 thoquinones at 1 mM. (From Owens, 1953 a.) 



Very little work on the effects of pH on enzyme inhibition by the qui- 

 nones has been reported. Curves for catalase and urease are given in Fig. 

 5-3 (Hoffmann-Ostenhof and Biach, 1948 a) and are so different in con- 

 figuration that it was postulated the mechanisms of inhibition cannot 

 be the same. Alteration of the pH would not only change Eq' but 

 might modify the rate of reaction with SH groups. The increase in the 

 inhibition of urease from pH 5 to 7 in the face of a decreasing Eq indicates 

 that factors other than the redox potential are involved, although the 

 curve for catalase might support this mechanism in part. The redox po- 



