[Phosphoethanolamine--a substrate of alkaline phosphatase isolated from rat calvaria]

Alkaline phosphatase from calvaria of 8 to 12-day-old Wistar rats was purified to electrophoretic homogeneity by a simple procedure (homogenisation, solubilisation by Triton X-100, DEAE-Sephacel ion exchange chromatography). For the holoenzyme, a Mr of about 160 kDa was determined, and it seems to consist of two identical subunits. The pH optima for the hydrolysis of phosphoethanolamine and p-nitrophenylphosphate are 10.0 and pH 9.0-10.5, respectively. The rate constants for the hydrolysis of phosphoethanolamine, p-nitrophenylphosphate and other phosphomonoesters at pH 10.0 are comparable, but the Km values differ by one to two orders of magnitude. At physiological pH (7.5) the maximum hydrolysis rate of the substrates phosphoethanolamine and p-nitrophenylphosphate was only 8% and 5%, respectively, of that determined at the pH optimum. On the basis of the kinetic data an in vivo function of alkaline phosphatase in bones as a monophosphate ester hydrolyzing enzyme seems unlikely.