SUBSTRATE INHIBITION 133 



Thiirlow, 1924) since the affinity of the methylene bkie for the enzyme 

 was shown to be markedly reduced. The oxidation of leucine by the L-amino 

 acid oxidase of snake venom with methylene blue as an acceptor also shows 

 substrate inhibition of this type (Dixon and Webb, 1958, p. 86). The rate-(S) 

 curves for this mechanism may show a fairly broad plateau, instead of a 

 peak, if the affinity of the substrate for the acceptor site is much less than 

 for the substrate site, since inhibition may not occur until the substrate 

 concentration is substantially above that required to saturate the enzyme 

 with respect to activity. By determining the course of substrate inhibition 

 at different concentrations of acceptor, it is possible to identify this type 

 of inhibition and obtain the various dissociation constants, as illustrated 

 in the following chapter. Alberty (1958) has given a complete theoretical 

 analysis of substrate inhibition for situations in which a coenzyme is 

 involved in the reaction and he has included cases where the excess sub- 

 strate may interfere with the binding of the coenzyme. 



The Substrate Reduces the Concentration of Water (Type F) 



When water is one of the reactants, reduction in its concentration may 

 slow the rate, and this seems to be the mechanism for the inhibition by 

 high sucrose concentrations of yeast /?-fructofuranosidase (Nelson and 

 Schubert, 1928). Inhibition was determined over a range of sucrose con- 

 centration from 0.17-2.0 M (6-70%) and the rate was proportional to the 

 water concentration throughout. This type of inhibition is uncommon 

 because concentrations of substrate sufficiently high to reduce water con- 

 centration appreciably are seldom used. 



The Substrate Inhibits by an Ionic Strength Effect (Type G) 



This type of substrate inhibition has not been demonstrated conclusive- 

 ly but certainly is a possibility that cannot be neglected. Substrate inhi- 

 bition of enolase depends on the ionic strength and disappears when 0.6 M 

 KCl is present (AVesthead and Malmstrom, 1957). Since at this concen- 

 tration of KCl, changes in ionic strength due to substrate would be negli- 

 gible, this might be interpreted to mean that the inhibition is an ionic 

 strength effect. However, as has been discussed, the inhibition has been 

 shown to be due to removal of the activator Mg++ by the 2-phosphogly- 

 cerate. The effect of increasing the ionic strength with KCl is probably 

 to decrease the interaction between substrate and activator so that free 

 activator is not reduced so readily. These results serve to illustrate the 

 complexities that may be encountered by varying the ionic strength in 

 enzyme systems with several components. 



