Effect of Zn2+ and Mg2+ on alkaline phosphatase from human placenta and intestine.

Alkaline phosphatases (orthophosphoric monoester phosphohydrolase, E.C. 3. 1. 3. 1) from human placenta and intestine are activated by Mg2+, but inhibited by Zn2+. In these respects, the enzymes are different from the human biliary, bone, liver and kidney enzymes, which are much activated by Mg2+ and Zn2+. At pH 10.5, the Km of enzymes and enzymes inhibited by inorganic phosphate are not modified by Mg2+. These results suggest that the activation by Mg2+ proceeds through a binding of Mg2+ with other part of the active site of alkaline phosphatase. It was also found that the activity of alkaline phosphatases was remarkably influenced by binding metals, and Zn2+ and Mg2+ contents in both enzymes were 4 g-atoms/mole and 3-5 g-atoms/mole, respectively.