Inhibition of the orthophosphatase and pyrophosphatase activities of human alkaline-phosphatase preparations

Abstract

Inhibition of the pyrophosphatase and orthophos phatase activities of human liver and small-intestinal alkaline-phosphatase preparations by different classes of inhibitors has been studied. Each type of substrate, pyro-phosphate or orthophosphate, is a competitive inhibitor of hydrolysis of the other type. L-Phenylalanine is a non-competitive inhibitor of both types of activity of the intestinal preparation, but inhibits neither activity of the liver enzyme. Arsenate is a competitive inhibitor of both activities of both preparations. For a given inhibitor, the values of Ki are independent of the type of substrate used when measurements are made at the same pH. Mg2+ ions activate orthophos phatase but inhibit pyrophosphatase, except in very low concentrations. These results are compatible with the presence in each tissue preparation of a single enzyme with one type of active center, possessing both orthophosphatase and pyrophosphatase activities.