PH-DEPENDENT SALTING-OUT EFFECT IN ENZYME-CATALYZED REACTION-KINETICS

The influence of KCl on the second-order rate constant kII = k2/Ks of acetylcholinesterase-catalyzed hydrolysis of butyl acetate is quantitatively described by the linear equation log kII = log kII + Δκc, where c is the concentration and Δκ = κE - κX* + κS is the difference between the salting-out coefficients of the initial reagents (E and S) and of the transition state (X*. The salting-out parameter Δκ increases with increasing pH (pH range 4.8-6.0) up to the constant value at pH 6.0-8.0, where Δκ = κS. The pH-dependent Δκ is discussed as an indication of the change in the volume and/or solvation of the enzyme upon the formation of the transition state. The data show that while the pKa1 = 5.6 ± 0.1 and pKa2 = 8.0 ± 0.1 of kII are independent of the KCl concentration, the apparent pKa shift can be observed at high salt concentrations due to a pH-dependent salt effect. © 1990.