SUBSTRATE INHIBITION 



137 



binding of the second substrate molecule is approximately as tight as in 

 the active complex, the inhibition may be reduced quite markedly. Com- 

 petitive inhibition constants determined in the usual manner on such sys- 

 tems will be in error by an appreciable amount unless precautions are tak- 

 en as discussed in the following chapter. The effects of increasing concen- 

 tration of inhibitor on the rate-pS curves are evident from Fig. 4-14 — the 



30 — 



20 



4 / \ 



/ y \\ 



/ / 2 / \\ 



ps' 



Fig. 4-14. Effects of a competitive inhibitor on the rate-pS' curves for substrate 

 inhibition (Eq. 4-26). or = 1. Curve 1: (I') = 0, (So') = 1; curve 2: (I') = 1; 

 (So') = 1.41; curve 3: (I') = 10, (So') = 3.32; curve 4: (I') = 100, (So') = 10, 



decrease in Vq, the shift of (Sq) to higher concentrations, and the " pushing 

 in " of the left side of the curves as noted by Murray (1930). 



It was mentioned that Hofstee (1955) used the competitive inhibitor 

 isoxanthopterin to determine the mechanism of substrate inhibition for 

 xanthine oxidase. Inasmuch as this is a generally useful method of distin- 

 guishing between type A and type B noncompetitive mechanisms, the 

 procedure will be outlined briefly. The equations Hofstee derived for the 

 two situations are: 



Type A substrate inhibition (Eqs. 4-7 and 4-26) 



I',. = F 



1 +1/(S') +(S')/a 

 1 



1 + [1/(S')][1 +(1')] +(S')/a 



