ALKALINE-PHOSPHATASE ACTIVITY AT PHYSIOLOGICAL PH - KINETIC-PROPERTIES AND BIOLOGICAL SIGNIFICANCE

The activity of rat liver alkaline phosphatase (ALP) was studied at ph ysiological pH, using paranitrophenyl phosphate (pNPP) as substrate. A t this pH, the purified enzyme had optimal catalytic efficiency and it s activity was maximal for the very low substrate concentrations. Duri ng thermal inactivation of rat liver plasma membranes activities, the ratio of the measured residual activities (pH 10.5 /pH 7.5) varied, sh owing that ALP was not the only plasma membranes pNPP hydrolase. Indee d, the proportion of pNPP hydrolase activities attributable to ALP in plasma membranes at pH 7.5 was relatively low. Effectively, it was sho wn using bromolevamisole, a potent and specific inhibitor of ALP, that contrary to what it was previously reported, ALP was not the major pN PP hydrolase of liver plasma membrane.