Human Alkaline Phosphatases. II. Metalloenzyme Properties of the Enzyme from Human Liver

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Abstract
Human liver alkaline phosphatase [EC 3.1.3.1] is a metalloenzyme requiring Zn2+ and Mg2+ for full activity. Zn2+ cannot be replaced by Mn, Co or Ca, whereas Mg2+ can be replaced by Mn or Ca. The binding constants of the enzyme for different divalent cations were determined by the use of complexing agents. The enzyme is inhibited by a number of reducing and complexing agents such as 2-mercaptoethanol, cyanide, nitrilotriacetic acid and EDTA. From studies using these inhibitors it is suggested that there are different mechanisms of inhibition. Reversible inhibition occurs if the free Zn2+ concentration is not significantly lower than 10-12 M. Inhibition is irreversible at lower Zn2+ concentrations. Evidence is given, that the human liver alkaline phosphatase possesses different Zn binding sites, which are responsible for the catalytic function and for the integrity of the enzyme structure.