Mechanism of Action of Mg²⁺ and Zn²⁺ on Rat Placental Alkaline Phosphatase. I. Studies on the Soluble Zn²⁺ and Mg²⁺ Alkaline Phosphatases

Abstract

Rat placental alkaline phosphatase (EC 3.1.3.1), a dimer of 135 000 daltons, is strongly activated by Mg 2+ . However, Zn 2+ has to be present on the apoenzyme to obtain this activation. Mg 2+ alone is unable to reconstitute functional active sites. Excess Zn 2+ which competes for the Mg 2+ 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-[ 32 P]phosphate.Two enzyme species with identical functional active sites have been reconstituted that only differ by the presence of Zn 2+ or Mg 2+ 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 Mg 2+ and Zn 2+ with the dimeric enzyme molecule.