ACTIVATION AND INHIBITION OF THE BRUSH-BORDER MEMBRANE-BOUND ALKALINE-PHOSPHATASE ACTIVITY OF HYMENOLEPIS-DIMINUTA (CESTODA) BY DIVALENT-CATIONS

In the absence of exogenous divalent cations, the isolated brush-border plasma membrane of Hymenolepis diminuta possesses alkaline phosphatase activity (APA). APA is stimulated in the presence of exogenous Mg2+ and inhibited by low concentrations of Zn2+ or high concentrations of Ca2+, and inhibition of APA by Zn2+ is reversed by both Mg2+ and Ca2+. APA is inhibited by ethylenediamine tetraacetic acid (EDTA), ethyleneglycol-bis-(beta-aminoethyl ether) N,N'-tetraacetic acid, and 1,10-phenanthroline in time- and concentration-dependent fashions, with EDTA being the most effective inhibitor. Following treatment with EDTA, APA is restored by Mg2+ and, to a lesser extent, by Ca2+, but not by Zn2+. Thus, APA represents a Mg2+-dependent enzyme that can be partly activated by Ca2+ but only in the absence of Mg2+.