160 5. DETERMINATION OF MECHANISMS AND CONSTANTS 



If we make the reasonable assumption that the slope of the inhibited curve 

 is twice that of the uninhibited (R = 2), we find that K- = 0.6 K^. When 

 a has some value between 2 and 50, carefully plotted type D curves will 

 be seen to be curved concavely downward (Fig. 5-4 D) and this may pos- 

 sibly give a hint of partially competitive inhibition. Nevertheless this is 

 a modification of simple kinetics that is difficult to detect experimentally. 

 A rather cumbersome method may be suggested. If type A plots are made 

 for different concentrations of inhibitor and the values of R obtained from 

 each are plotted against K/, the true K^ is indicated by the /il /-intercept. 

 Perhaps a simpler procedure is to examine the 1/i intercepts of type E 

 plots; almost all types of inhibition give intercepts of unity but partial 

 inhibitions are characterized by higher intercepts (Fig. 5-4 E and 5-5 E). 

 The values of a or /? may be calculated if the type (competitive or non- 

 competitive) of inhibition is established by other plots; the 1/?' intercepts 

 are: 



a + {S)IK, 



Partially competitive: 



1 



Partially noncompetitive: . 



A situation which has seldom been considered, but yet may be of consid- 

 erable importance, is mixed inhibition. In this case the presence of the 

 inhibitor on the enzyme prevents the breakdown of the active complex 

 but also interferes to some extent with the binding of substrate. The in- 

 tersections of type A-D curves do not fall on either axis but at points from 

 which the extent of deviation from noncompetitive inhibition may be 

 determined. The literature contains many examples of type A plots where 

 the curves, although linear, do not meet at the axes; usually the inhibition 

 is stated to be competitive or noncompetitive depending upon the axis to 

 which the intersection is closer. It is quite possible that these represent 

 mixed inhibition. The inhibition in such cases will be intermediate be- 

 tween competitive and noncompetitive for any specific concentration of 

 inhibitor. From Fig. 5-6 A it may be observed that the intersection is at 

 — IjaKg on the abscissa and thus the value of a may be easily calculated. 

 It is interesting, however, that the slope does not depend on a and hence 

 the usual method of calculating K^ from the slope ratio leads to no error. 

 The other methods of plotting can yield similar results. If a is not too high, 

 a plot of type F can be indicative of mixed inhibition due to the character- 

 istic turning downward of the curve (Fig. 5-6 F). 



Coupling Inhibition (Uncompetitive or Anticompetitive) 



Although this type of inhibition may be relatively uncommon, there 

 should certainly be no difficulty in detecting it inasmuch as the various 



