Effect of metal ion on alp enzyme activity
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We examined the effects of various metal ions upon alkaline phosphatase (EC 3.1.3.1) from human liver and placenta, individual patient sera, calf intestine, and commercial quality-control materials. Of the metals investigated, Be2+ was the most effective inhibitor of enzyme activity: >50% inhibition at concentrations ≥5 μmol/L. This inhibition was neither time-dependent (over 30 min) nor reversible by addition of Mg2+ or Zn2+, or both. The inhibition due to Be2+ was purely competitive with 4-nitrophenyl phosphate for alkaline phosphatase from both human liver and placenta, with K(i) of 0.45 and 0.26 μmol/L, respectively. These data suggest that the [Be(OH)4]2-anion present at alkaline pH is responsible for this inhbition. Fe3+ inhibited enzymes from all sources other than placenta; addition of Mg2+ protected against this inhibition. Mn2+ inhibited the liver enzyme; addition of Mg2+ strengthened this inhibitory effect. Other metals inhibitory at ≤10 mg/L were Co2+, Ni2+, Cd2+ Cr3+, Al3+, Mn2+ and Sn2+, Cu2+ was stimulatory only for the quality-control serum. Zn2+ was stimulatory at ≤15 μmol/L for all enzyme materials, but at 150 μmol/L it was inhibitory for all except quality-control materials. These effects are interpreted according to the known metal requirement of the enzyme. Some quality-control materials behave significantly differently from purified human liver or serum enzymes in the presence of trace amounts of metal ions. The inhibition due to Ni2+ is time dependent; 4-nitrophenyl phosphate blocks that inhibition. The presence of substrate retards re-association of the apoenzyme with metal ions.
The effect of Mg2+ concentrations on the Michaelis constant (Km) suggests that Mg2+ activates kidney ALP by increasing the affinity of the enzyme for pNPP. ... Pre-incubation of ALP activity with the metal ion cofactors led to increase in the hydrolysis of ALP.