124 



4. SUBSTRATE INHIBITION AND PRODUCT INHIBITION 



to an equation such as 4-7 with K,,^ = 0.018 mM and a = 139 at pH 6.8. 

 The active complex is probably E-DPNH-pyruvate and ^,„ for pyruvate 

 is a complex function of several rate constants, involving the binding of 

 DPNH, so it is doubtful if a simple interpretation of K„^ can be made (Wi- 

 ner and Schwert, 1958). However, ^,„ is equal to or greater than the disso- 

 ciation constant for pyruvate in the active complex so that the second in- 

 hibiting pyruvate molecule is bound less tightly by at least 3 kcal/mole. 



I 



(XANTHINE)- 



4mM 



Fig. 4-9. Inhibition of xanthine oxidase by substrate 

 and isoxanthopterin. (From Hofstee, 1955.) Curve 1: 

 inhibition by substrate (xanthine) alone; curve 2: 

 isoxanthopterin (0.01 mM); curve 3: isoxanthopterin 

 (0.1 miH). 



The binding of both pyruvate molecules depends on pH. The inhibition 

 by pyruvate decreases with increase in pH as shown in Fig. 4-10; the 

 constants used to construct these curves are given in the tabulation below. 



