RATKS OF INHIBITION OF PUEE ENZYMES 



557 



for this discrepancy is the assumption discussed above, namely that tlie 

 enzyme is either in the form of ES or EI. When this assumption is valid, 

 as it is in the inhibition of cholinesterase under the conditions imposed by 

 Goldstein, (S') must be quite large and much greater than unity, so that the 

 error will be insignificant. However, it is worth pointing out again that 

 when (I') + (S') is not much larger than unity, these equations will be 

 inaccurate with respect to both the rate of inhibition and the final inhibition 

 that is reached. 



Fig. 12-18. Rate of inhibition of serum cholinesterase by physostigniine. The points 

 represent the data of Goldstein (1944) and the curves are plotted from Eq. 12-34: 

 Ki = 1.9 X 10-8 M, K, = 1.23 X 10-3 M, (S) = 8.05 x 10-^ if, and k\ = 8.3 x 10* 

 liters mole-i min-i. Curve I, (I) = 1.1 X 10-= M: curve II, (I) = 1.1 X lO-s M. 



Kinetic studies of this type can also provide evidence of competitive 

 inhibition and this, as pointed out by Goldstein (1944), is j^erhaps the most 

 direct method available. The other two methods, which have been discussed 

 in previous chapters, are (1) the plotting of equilibrium inhibition data in 

 various ways (see Chapter 5) and (2) the demonstration that it requires a 

 greater concentration of inhibitor in the presence of substrate to produce 

 a certain degree of inhibition than in the absence of substrate. The kinetic 

 method would be particularly useful in those situations where, for one 



