INHIBITION OF ENZYMES 



809 



tically it is unlikely that in all the dehydrogenases such a distribution would 

 occur. On the other hand, Jecsai and Elodi (1963) claimed that pig muscle 

 lactate dehydrogenase in the native state does not react with p-MB, but 

 that at pH 10 the blocking of 20 SH groups leads to 50% inactivation. They 

 concluded that in this particular enzyme the SH groups are not at all in- 

 volved in the catalysis. These examples only illustrate some of the problems 

 which arise in enzyme titrations and emphasize that a program for relating 

 SH groups to enzyme activity cannot be undertaken lightly. 



100 



^ 20 



Fig. 7-25. Titration of microsomal cytochrome c 

 reductase with p-MB. The sohd curve gives the 

 development of the inhibition as equivalents of 

 mercurial are added. The dashed line continuing 

 from the initial linear portion of the curve shows 

 that one SH group is required for activity; the 

 other dashed curve shows the assumed reaction 

 with SH groups (it was not experimentally de- 

 termined). It may be estimated that K^IK^ is 

 near 25-50 if there are two SH groups. (From P. 

 Strittmatter, 1959.) 



Kinetics of Mercaptide Formation and Development of Inhibition 



Apparently SH groups range in reactivity all the way from those which 

 combine with mercurials so rapidly that the rates are difficult to measure, 

 to those which are completely blocked and do not react at all. It is thus 



