INHIBITION OF ENZYMES 



45 



lOOOACETAMlOE 



lODOACETATE 



Fig. 1-4. Inhibitions of human erythrocyte tripeptidase at pH 8 



and 0°, incubation being carried out overnight. (From Tsuboi 



et al., 1957.) 



Effect of pH on Inhibitions by lodoacetate 



We have noted (page 12) that the rate of reaction of iodoacetate with 

 various thiols increases with increase in pH, and that the pH determines 

 the type of group reacting in ribonuclease (page 33). With respect to en- 

 zyme inhibition, surprisingly little on pH effects has been reported. Indeed, 

 no investigation of an iodoacetate-sensitive SH enzyme has ever been made. 

 Maschmann (1937 a, b) studied the effects of iodoacetate at pH 5 and 7 on 

 bacterial proteases, but these enzymes are inhibited so little that compar- 

 ison at the different pH's is impossible. Schneyer (1952) found salivary 

 amylase to be inhibited more at pH 3.5 than at 7, but this enzyme does 

 not contain SH groups, so that these results may be a reflection of the al- 

 kylation of methionine or histidine residues. Similarly, Ramasarma et al. 

 (1959) stated that ^-glycerophosphatase is inhibited slightly at pH 3.2 but 

 not at all at pH 5.4-9.6. Thus this early work did not support the expecta- 

 tion that inhibition would increase with the pH, but more recent reports 

 correlate better with the results obtained using simple thiols. The penicil- 

 linase from Aerobacter is inhibited by iodoacetamide much more strongly 

 at pH 7.4 than at 6.0 (J. T. Smith, 1963 b), the carboxydismutase catalyz- 

 ing the primary COg fixation in photosynthesis is inhibited by iodoaceta- 

 mide more rapidly as the pH is increased from 8 to 10 (Rabin and Trown, 

 1964), and horse liver alcohol dehydrogenase is inhibited faster at pH 9 



