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 Kmfor the substrate.