MUTUAL DEPLETION SYSTEMS 



185 



much as in zone C„ competitive and noncompetitive inhibitions are ex- 

 pressed by identical equations (3-36 and 3-53) — i = (I,)/(E,) — it is evident 

 that one cannot distinguish between them; as we have seen, however, it 

 is unlikely that a competitive system can exist in zone C,. Zone C, inhi- 

 bition is plotted in Fig. 5-24 and these curves may be compared to those 

 in Fig. 5-3 to illustrate the differences between zone A^ and zone Cj be- 



lO.OmM 



(1) 



01 



OOmM 



Fig. 5-22. Competitive inhibition isobologram (Eq. 5-35) plotted on 

 logarithmic scales. K^ = \ niiV and X,- = 0.1 mM. 



havior. The curves plotted by methods A, B, C, and F are similar for both 

 zones but the slopes and intercepts have different values, dependent on 

 the relative concentrations of enzyme and inhibitor and not on K^. If 

 zone C; behavior is not recognized (and it may not be in plots of types 

 A-C and F), calculation of K^ in the usual manner gives a constant without 

 physical meaning. Zone Cj kinetics may be detected by plots of type D 

 and E; particularly if a plot of Iji against l/(/^) results in a straight line 

 whose extension passes through the origin, zone C^ behavior is evident, 

 and from curves of this type an estimate of the enzyme concentration may 

 be made (see Chapter 3). The rate equation for noncompetitive inhibition 

 in zone C ^ is given by: 



Vi 



F„ 



(S) 

 (S) + K, 



1 + 



(h)' 

 (E,) 



(5-36) 



