INHIBITORY EFFECT OF MAGNESIUM-ION ON THE HUMAN PLACENTAL ALKALINE PHOSPHATASE-CATALYZED REACTION IN A REVERSE MICELLAR SYSTEM

Human placental alkaline phosphatase is a membrane-anchored protein. E ntrapping the enzyme into a reverse micellar vesicle mimics the in viv o conditions and allows examination of the properties of the enzyme. P lacental alkaline phosphatase is enzymatically active in Aerosol-OT/is ooctane reverse micelles. Substantially different kinetic behavior of the enzyme has been observed in aqueous or reverse micellar systems. I n aqueous solution, Mg2+ is a nonessential activator of the enzyme. In the experiments described in the present report Mg2+ was found to be an inhibitor for the enzyme in reverse micelles. This inhibition is pr esumably due to a time-dependent conformational change of the enzyme m olecule, which resulted in a curvature in the recorder tracings of the enzyme assays. The Mg2+-induced conformational change of the enzyme w as completely prevented by phosphate and partially reserved by EDTA. H igh concentrations of Zn2+ also strongly inhibited enzyme activity in both aqueous and reverse micellar solvent systems, presumably by occup ying the Mg2+ (M3) site of the enzyme. However, binding of Zn2+ at the M3 site did not cause conformational change of the enzyme and the enz yme assay tracing was linear. The. M3 site of the enzyme is proposed t o have a modulatory role in vivo using magnesium ion as the modulator.