The inhibition of yeast enolase by Li+ and Na+1

The activity of yeast enolase is inhibited by Li+ and Na+. At pH 7.1, inhibition by Li+ is "mixed" with respect to Mg2+; both Vmax and Km (Mg2+) are increased by Li+. The inhibition by Li+ appears to be partial, indicating that enzyme with Li+ bound is active. The step inhibited by Li+ cannot be proton abstraction since Li+ decreases the kinetic isotope effect on Vmax. At pH 9.2, where proton abstraction is no longer partially rate-limiting, inhibiton by Li+ is competitive with respect to Mg2+. The rate of enzyme-catalyzed exchange of the C-2 hydrogen with solvent is not affected by Li+. We interpret these results as follows: Li+ (and Na+) binds to enolase and decreases the rate of at least one step in the mechanism. At pH 7.1, this step is partially rate-limiting; at pH 9.2, this step is a fast step in the reaction. The step inhibited by Li+ cannot be proton abstraction but may be release of product (phosphoenol pyruvate) or Mg2+.