652 14. EFFECTS OF pH ON ENZYME INHIBITION 



VARIATION OF ENZYME ACTIVITY WITH pH 



The effects of pH variation on the rates of enzyme reactions will be 

 briefly discussed to provide a basis for the understanding of the depen- 

 dency of inhibitions on the pH. Many excellent theoretical treatments 

 of pH effects are available and the following are particularly recommended: 

 Laidler (1955b, c,d, 1958, p. 117), Alberty (1956a, b), Friedenwald and 

 Maengwyn-Davies (1954), Dixon and Webb (1958, p. 120), and Reiner 

 (1959, p. 229). The experimental data can, in most cases, be adequately 

 interpreted at the present time in terms of various ionizing groups 

 assigned to the active center, the substrate or the ES complex, or to any 

 coenzymes or activators that may be involved in the reaction. One aim 

 of this chapter is to apply and extend the results and theories to the prob- 

 lems of enzyme inhibition. It is evident that the experimental determi- 

 nation of the dependence of inhibition on pH must involve simultaneous 

 measurements of the variations of the uninhibited rates in order that con- 

 trol values may be obtained and in many cases knowledge of the behavior 

 of the enzyme in the absence of the inhibitor is essential to interpret the 

 mechanism by which the inhibition is modified by pH. In even the most 

 simple enzyme preparations used for the determination of activity, there 

 is often a variety of interacting substances altering the relationships of 

 the enzyme and substrate — activators, coenzymes, inhibitors, physiolog- 

 ical and buffer ions — and the mass-action principles of interaction are 

 applicable in all cases. One may consider all such substances as modifiers 

 and very general treatments may be formulated on this basis. However, 

 for practical purposes it is usually more convenient to consider only those 

 substances that are varied in the experimental procedure; thus the effects 

 of the hydrogen ion can be treated separately. Although the interactions 

 of the enzyme with the hydrogen ion are basically quite comparable to 

 the interactions with other modifiers, the results of variation of the pH 

 are usually more complex because so many of these other modifiers are 

 themselves affected by pH changes. Therefore, in each case in which the 

 pH is altered, one must take into account all the possible effects on the 

 dissociable components of the system. The pH-dependent behavior of an 

 enzyme may thus be very complex and although every effort should be 

 made to simplify and isolate the individual effects, this inherent complexity 

 should never be ignored. 



Mechanisms by Which pH Effects May Be Exerted 



The rates of enzyme reactions may depend on the pH for a variety of 

 reasons. The following classification includes most of the important 

 mechanisms. 



