pH dependence and solvent isotope effects in the hydrolysis of phosphomonoesters by human prostatic acid phosphatase.

The pH dependence of the human prostatic acid phosphatase-catalyzed hydrolysis of p-nitrophenyl phosphate and beta-glyceryl phosphate has been studied over a wide range of pH and the values of Km and V calculated with the aid of the Cleland HYPER program. The pH dependence of Km shows the effect of substrate ionization: pK values of 5.6 and 6.4 are observed as for the respective values of free substrates. The pH dependence of both Km and V for each substrate reveals the involvement of an ionizable group in the ES complex which is ascribed to a phosphohistidine-enzyme intermediate. The small deuterium solvent isotope effects which are observed on V are consistent with values observed for solvolysis of phosphoramidates. The measured data for Km indicates limits on burst-titration experiments of prostatic acid phosphatase (orthophosphoric-monoester phosphohydrolase, EC 3.1.3.2).