INHIBITION OF ENZYMES 



791 



The effects of pH on enzyme inhibition by the mercurials are even more 

 complex and one would not anticipate consistent behavior, a prediction 

 that is borne out in the following discussion. 



Some pH effects on mercurial inhibition are shown in Table 7-10. In 7 

 cases the inhibition is greatest at low pH and in 6 cases at high pH, and 

 this certainly indicates that more than one factor must be involved. Indeed, 

 many of the inhibition-pH curves are complex (Figs. 7-13 and 7-14) and 



80 



60 



20 



Fig. 7-13. Effects of pH on the inhibitions of /J-fruc- 



tofuranosidase by HgClj and Hg(N03)2. (Data from 



Myrback, 1926.) 



often biphasic (/^-glucuronidase, /J-fructofuranosidase, ATPase, and ribo- 

 nuclease). In some cases marked stimulation is found within a certain pH 

 range, inhibition occurring outside this range. This implies that the activity- 

 pH curves and the pH^p^ are shifted by the mercurials, usually to lower 

 pH's, as for yeast proteinase (Lenney, 1956) and ascites cell ribonuclease 

 (EUem and Colter, 1961; Colter et al., 1961). It is difficult to interpret such 

 shifts in pH^p^, but if the pH^p^ is related to the ionization of two or more 

 groups on enzyme and substrate (see page 1-660), a shift implies some mod- 

 ification of the enzyme groups in or near the active center with a resultant 

 alteration of the interaction of the substrate with the enzyme. Dixon's 

 method of plotting K,„ and K^ against pH (page 1-683) was applied to ^- 

 glucuronidase by Fernley (1962), and the curves are shown in Fig. 7-15. 

 The pK^ curve is suggestive of two ionizing groups on the enzyme with 

 pi^L^'s around 4.4 and 6.3 if it could be simply interpreted, or possibly the 



