Mechanism of action of Mg2+ and Zn2+ on rat placental alkaline phosphatase. I. Studies on the soluble Zn2+ and Mg2+ alkaline phosphatases.

Rat placental alkaline phosphatase (EC 3.1.3.1), a dimer of 135,000 daltons, is strongly activated by Mg2+. However, Zn2+ has to be present on the apoenzyme to obtain this activation. Mg2+ alone is unable to reconstitute functional active sites. Excess Zn2+ which competes for the Mg2+ site leads to a phosphatase with little catalytic activity at alkaline pH but with normal active sites at acidic pH as shown by covalent incorporation of ortho-[32P]phosphate. Two enzyme species with identical functional active sites have been reconstituted that only differ by the presence of Zn2+ or Mg2+ at the effector site. A mechanism is presented by which alkaline phosphatase activity of rat placenta would be controlled by a molecular process involving the interaction of Mg2+ and Zn2+ with the dimeric enzyme molecule.