Regulatory effect of divalent cations on rat liver alkaline phosphatase activity: How Mg2+ activates (and inhibits) the hydrolysis of p-nitrophenylphosphate

The concentration-dependent stimulation of rat liver alkaline phosphatase (ALP) catalyzed hydrolysis of para- nitrophenylphosphate (pNPP) was studied. ALP displayed some activity even in the absence of exogenous Mg2+. Kinetic analyses show that activation by Mg2+ is exerted at the Vmax level without necessarily enhancing the affinity of the enzyme for the ion. However, the hyperbolic activation operates only within the optimal level of 0 to 5mM concentrations of the metal ion. Higher concentrations were actually inhibitory in a pure non-competitive manner. Mg2+, either as an activator (optimal concentrations) or inhibitor (supra-optimal levels) exerts its action via a Vmax effect with only negligible effect on Km for the substrate. Keywords: magnesium ion, alkaline phosphatase, supra optimal regulation Biokemistri Vol. 17(2) 2005: 129-136