776 7. MERCURIALS 



ited even more strongly; such work points out the importance of enzyme 

 purity for accurate studies of mercurial inhibition. The elevation of the 

 plgo from 0.0002 mM to 0.014 uiM by serum for the inhibition of 3-phos- 

 phoglyceraldehyde dehydrogenase by p-MB is a further example (Weitzel 

 and Schaeg, 1959). 



When an enzyme is reported to be inhibited to a specified degree, say 

 50%, by a certain concentration of mercurial, exactly how is this to be 

 interpreted? Is 50% of the enzyme combined with the mercurial in a com- 

 pletely inactive EI complex, or is all the enzyme combined with the mer- 

 curial and the EI complex possesses 50% of the original activity? If the 

 ordinary equilibrium formulation is followed and it is assumed that the 

 fractional activity of the EI complex is r, noncompetitive inhibition will 

 be given by 



.^ (l-r)(I) 

 (I) + Ki 

 and 



(7-4) 



1 Ki 



+ -r. -^;vr (7-5) 



i (1-r) (l-r)(I) 



so that a plot of \ji against 1/(1) will give a straight line intersecting the 

 1/i axis at 1/(1 — r), or \jimax- If mutual depletion occurs (zone C), a sim- 

 ilar result is obtained, although the slope will be different. A simple plot 

 of this type may help to decide between the two possibilities above. If the 

 plot is not linear near the \ji axis, one might suspect that another type of 

 inhibition is occurring at higher inhibitor concentrations, or that secondary 

 inactivation of the enzyme is a factor. 



One example of the deviations from classic inhibition kinetics that may 

 be seen with the mercurials is the inhibition of human plasma cholinesterase 

 by Hg++ as analyzed by Goldstein and Doherty (1951 ). This slowly develop- 

 ing, pH- and temperature-dependent inhibition presents some interesting 

 but often uninterpretable results. The l/v-l/(S) plots exhibit two sorts of 

 deviation (Fig. 7-11). The results from long incubation with low concentra- 

 tions of Hg++ fall on reasonably straight lines (A and B), but the slopes are 

 a good deal greater than expected for pure noncompetitive inhibition, as 

 for mixed inhibition (Fig. I-5-6A) the interaction constant a being some 

 finite value > 1. Of course, it may not actually be true mixed inhibition, 

 the deviation being due to some other factor. The results from short in- 

 cubations with high concentrations of Hg++ differ so much from any sort 

 of classic behavior that it is impossible to interpret them (C and D). It was 

 suggested that low and high concentrations of Hg++ inhibit by different 

 mechanisms, possibly with different SH groups, the former with groups 

 outside the active center causing secondary irreversible inactivation and 

 the latter directly with groups in the active center. This would to some 



