RATES OF INHIBITION OF PURE ENZYMES 



545 



less kinetically and in the construction of inhibition-time curves it is 

 important that the inhibitions at specific times be determined. 



Several enzyme inhibitions following an approximately exponential course 

 have been reported. In these cases, the reaction is bimolecular but the 

 kinetics are first order because the inhibitor is much in excess of the en- 

 zyme and its concentration does not change appreciably during the reaction. 



Fig. 12-7. Time courses of inhibition for different concentrations of the inliibitor 



(Eq. 12-11). Ki = 1 mil/ and k^^ = 10 min-i. Curve A, (I) = 50 mM; curve B, (I) = 



20 ml/; curve C, (I) = 5 m3/; curve D. (I) = 2 mil/; curve E. (I) = 1 mM. 



The inhibition of plasma cholinesterase by HgClg was studied kinetically 

 by Goldstein and Doherty (1951) and curves such as the one shown in 

 Fig. 12-8 were obtained. A logarithmic plot of this curve gives a reasonably 

 straight line over the initial 60 min (Fig. 12-9) from which the following 

 values may be estimated: slope = 0.007 min~^, a = 0.016 min"^, and k^ = 

 1.15 liters mole~^ sec~^. At higher concentrations of HgCl, (over 1 mM), 

 the behavior of the inhibition deviates from this simple mechanism and 

 the logarithmic plots are no longer straight lines. Another example that 

 may be mentioned is the inhibition of monoamine oxidase by iproniazid 

 for which Davison (1957) found an apparent first-order reaction as indi- 

 cat d by the logarithmic plots in Fig. 12-10 for three concentrations of the 



