Mechanism of action of Zn2+ and Mg2+ on rat placenta alkaline phosphatase. II. Studies on membrane-bound phosphatase in tissue sections and in whole placenta
Alkaline phosphatase (EC 3.1.3.1) bound to trophoblastic cells in rat placenta is activated by Mg2+ and inhibited by Zn2+ in the same way as is found with partially purified soluble alkaline phosphatase in the same tissue. Studies done with tissue sections (6-10 .mu.m) show that alkaline phosphatase activity and labeling of active sites by Pi are lost during incubation with ethanolamine at pH 9.0. Addition of Mg2+ causes total recovery of catalytic activity and active sites labeling. Zn2+ displaces and replaces at the Mg2+ binding sites. The affinity for both ions is similar, and dissociation of Zn2+ from the enzyme is a very slow process, even in the presence of Mg2+. The Zn2+-alkaline phosphatase and Mg2+-alkaline phosphatase, which only differ by the ion bound to an apparent modulator site, have the same catalytic activity at pH < 7.0, but the Zn2+ species has little activity at alkaline pH. Phosphorylation of the enzyme by Pi indicates that with both enzyme species the phosphoryl intermediate does not accumulate at alkaline pH. With Pi, the phosphorylation step is apparently rate determining for both enzymes, with Zn2+ affecting this step to a much greater extent. Zn2+ and Mg2+ probably regulate alkaline phosphatase in rat placenta. The concentration of both ions in maternal serum and placenta suggest that such a mechanism could exist in vivo.