444 



9. INHIBITION IN CELLS AND TISSUES 



The reciprocal equation for case 1 is: 



1 K 



+ 



F + F(S) 1 + (SJ/(S) 



(9-2) 



the curve for which is given in Fig. 9-2. At low substrate concentrations 

 the curve falls off from linearity, as pointed out by Reiner (1959, p. 89). 



Fig. 9-2. Effect of endogenous metabolism 

 on the determination of the enzyme con- 

 stants in the double reciprocal plot (Eq. 

 9-2). (Sg) is the concentration of the en- 

 dogenous substrate. 



From Eq. 9-1, it is seen that case 2 will also not provide a straight line. 

 Finally, in cases 3 and 4, where the total rate mav be written in the form: 



« =: C + 



F(S) 



(S) + K, 



(9-3) 



the constant endogenous rate being represented by C, Ijv is also not lin- 

 ear with 1/(S). 



When an inhibitor is used, the interpretation of the usual plots must 

 also be made with caution if there is an endogenous factor. This can best 

 be illustrated by discussing certain aspects for each of the four possible 

 cases described above. There are basically two questions: (1) what is the 

 relation of the total inhibition to the individual inhibitions, and (2) how 

 does endogenous metabolism affect the characteristics of the common ana- 

 lytical plots? 



