Alkaline phosphatase activity at physiological pH: kinetic properties and biological significance

The activity of rat liver alkaline phosphatase (ALP) was studied at physiological pH, using para-nitrophenyl phosphate (pNPP) as substrate. At this pH, the purified enzyme had optimal catalytic efficiency and its activity was maximal for the very low substrate concentrations. During thermal inactivation of rat liver plasma membranes activities, the ratio of the measured residual activities (pH 10.5/pH 7.5) varied, showing that ALP was not the only plasma membranes pNPP hydrolase. Indeed, the proportion of pNPP hydrolase activities attributable to ALP in plasma membranes at pH 7.5 was relatively low. Effectively, it was shown using bromolevamisole, a potent and specific inhibitor of ALP, that contrary to what it was previously reported, ALP was not the major pNPP hydrolase of liver plasma membrane.