INHIBITION OF ENZYMES 



777 



extent explain why the inhibition is more competitive at high Hg++ and 

 more noncompetitive at low Hg++, but it does not explain the deviations 

 discussed above. Curves C and D presumably do not represent equilibrium 

 inhibitions and are more illustrative of protection of the enzyme by the 

 substrate; it would seem that acetylcholine above 50 mM protects the en- 

 zyme almost completely against very short exposures to Hg++, which is 

 not too unreasonable considering the relatively high affinity of the enzyme 

 for acetylcholine. Although the kinetics of protection and the application 



2000- 



1500- 



1000- 



500 



20 



60 



Fig. 7-11. Double reciprocal plots for the inhi- 

 bition of human plasma cholinesterase by Hg++, 

 showing deviations from linearity at high Hg++ 

 concentrations. (Modified from Goldstein and 

 Doherty, 1951.) 



to plotting procedures have never been worked out as far as I know — and 

 it would be difficult to treat the phenomenon rigorously one might predict 

 that curves with rather steep slopes in the l/v-l/(S) plot would be found, and 

 that such curves would occasionally intersect the control curve to the right 

 of the \\v axis, i.e., the longer the incubation, the closer to equilibrium 

 would the inhibition come, and the less competition or protection would 

 be exerted by the substrate. One also wonders if the increased tilt of curves 

 A and B might be due to the fact that these relatively high substrate con- 

 centrations protect the active center against structural changes brought 

 about by reaction of the Hg++ at vicinal sites, since there are many examples 



