120 4. SUBSTRATE INHIBITION AND PRODUCT INHIBITION 



towards zero but levels off at a finite value as the concentration of sub- 

 strate is increased, this implies that the SES complex can break down to 

 form the product and that the inhibition is partially noncompetitive. It 

 may be seen from Eq. 4-12 that at high substrate concentrations, the rate 

 is given by v = /5F,„. This situation occurs, for example, in the case of car- 

 boxypeptidase (Lumry et al. 1951). Such systems have been discussed by 

 Laidler (1958, p. 71) and Reiner (1959, p. 67). 



Very few attempts have been made to distinguish between type A and 

 type B mechanisms. The work of Hofstee (1955) is notable in this respect. 



Fig. 4-5. Variation of the rate with the substrate concentration for completely 

 noncompetitive type B substrate inhibition (Eq. 4-11). /C, = 1 vaM and F^ = 100. 

 Curve 1: y = 1; curve 2: y = 3; curve 3: y = 10; curve 4: y = 30; curve 5: y = 100. 



He showed for xanthine oxidase that kinetic data using substrate alone 

 are insufficient to determine the nature of the inhibition. However, by 

 the use of the competitive inhibitor isoxanthopterin he was able to distin- 

 guish between the two mechanisms (see p. 137). It was concluded that a 

 noncompetitive type B inhibition was most likely, with J^^ = 2 X 10"^ M, 

 y = 50, F,„ = 6.5, and v^ — 5.1. The binding of two xanthine molecules 

 was believed to occur independently at distinct groups on the enzyme; 

 one group was responsible for the binding and activation of the xanthine, 

 while the other was necessary in the oxidation but not involved in the bind- 

 ing. If this is correct, y does not represent an interaction constant but 

 merely the difference in binding between the two groups. Isoxanthopterin 

 competes with xanthine for the active group but not for the auxiliary group; 

 if the substrate inhibition had been of type A, isoxanthopterin would prob- 

 ably compete with both substrate molecules. 



