The deuterium solvent isotope effect on hydrolysis of acetylamino acids catalyzed by porcine kidney aminoacylase I (EC 3.5.1.14) was studied. With Ac-L-Met, a ‘standard’ aminoacylase substrate, the effect was normal at low pH (k cat(D)/k cat(H) = 0.7 at pH 6), virtually absent at neutrality, and distinctly inverse (k cat(D)/K cat(H) = 1.4) at pH 9. Km was not significantly affected. The rates of Ac-L-Phe hydrolysis in D2O considerably exceeded those in H2O at any pH between 6.5 and 9. We explain this unusual effect of D2O on aminoacylase I catalysis by an inverse equilibrium effect partially cancelling or, at high pH, reversing a normal isotope effect on k cat.
