Hydrolysis of S-Substituted Monoesters of Phosphorothioic Acid by Alkaline Phosphatase from Escherichia coli

Abstract

Abstract Alkaline phosphatase from Escherichia coli catalyzes the hydrolysis of S-substituted monoesters of phosphorothioic acid of the type RSPO3Na2 (R = —CH2CH2NH2,—CH2CH2NHCOCH3,—CH2COOH, or—CH2CH2COOC2H5), at the S—P bond, to yield orthophosphate and the corresponding thioalcohols. The rate of enzymic hydrolysis of cysteamine S-phosphate was measured at different pH values and substrate concentrations. The pH profile of the rate of hydrolysis, as well as the Km and Vmax values obtained, are similar to the corresponding values obtained for p-nitrophenyl phosphate. Cysteamine S-phosphate competitively inhibits the hydrolysis of p-nitrophenyl phosphate. Inorganic phosphate and phosphorothioate competitively inhibit the enzymic hydrolysis of both cysteamine S-phosphate and p-nitrophenyl phosphate. Phosphorothioate is hydrolyzed by alkaline phosphatase, in a manner analogous to inorganic orthophosphate, to yield orthophosphate and hydrogen sulfide. It is suggested that cysteamine S-phosphate and p-nitrophenyl phosphate are bound to the same catalytic site of alkaline phosphatase.