570 



12. RATES OF INHIBITION 



kinetically than when the inhibitor is preincubated with the particular 

 component with which it reacts. Of course, if the inhibition is for practical 

 purposes instantaneous, the order of addition makes little difference, but 

 if the rate of inhibition can be measured and if one is interested in deter- 

 mining the rate constant, k^, or the energy of activation, the i^reincubation 

 technique is preferable. It is sometimes jiossible to localize the action of 

 the inhibitor to one component of the enzyme system in this way. For 



TIME ^ 



Fig. 12-22. Inhibition of cholinesterase by 

 0, - dimethyl - 0-2,2 - dichlorovinyl phos- 

 phate (DDVP). (From van Asperen and 

 Dekhuijzen, 1958.) Curve A, enzyme that 

 has been phosphorylated. Curve B, enzyme 

 that has been dephosphorylated. Curve 

 C, course of the inhibition showing the 

 achievement of a steady state. 



example, if we consider the simi^lest system, in which the enzyme reaction 

 involves only the enzyme and the substrate as components, the following 

 four exi)eriments should be performed if the nature of the inhibition is 

 unknown: 



From (a) and (c) one may be able to show whether the inhibitor reacts with 

 the enzyme or the substrate, while (b) and (d) serve as controls in case 



