title: Modulation of activity of human alkaline phosphatases by Mg2+ and thiol compounds.
abstract: Interaction of purified human liver and placental alkaline phosphatases (orthophosphoric-monoester phosphohydrolase (alkaline optimum), EC 3.1.3.1) with sulfhydryl groups, sulfhydryl reagents, and Mg2+ were studied. L-Cysteine (0.1 mmol/l) or Mg2+ activated the liver enzyme 4-5-fold and the placental enzyme 2-3-fold, with optimal pH 7.5-8.0; these activations were not additive. L-Cysteine (2 mmol/l) inhibited both enzymes maximally at pH greater than 9.0; phosphate protected the enzymes. S-Methylcysteine had little effect, with or without Mg2+. Inhibition by sulfur-containing compounds paralleled their ability to bind Zn2+. Fluoresceine mercury acetate (specific for sulfhydryl groups) inhibited the isoenzymes, whereas iodoacetic acid, iodoacetamide, dithionitrobenzoic acid, and p-chloromercuribenzoate had little effect. The inhibition was reversed by L-cysteine and only slightly protected by inorganic phosphate. Thus, there are two sites on human liver and placental alkaline phosphatase that interact with L-cysteine; a Mg2+-binding site, which results in activation, and a site that involves one or both of the bound Zn2+ ions and results in inactivation. Both enzymes have a protected essential thiol group.
mesh_terms: D000469:Alkaline Phosphatase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D005452:Fluoresceins / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000201:enzymology; D008274:Magnesium / Q000494:pharmacology*; D009941:Organomercury Compounds / Q000494:pharmacology; D010920:Placenta / Q000201:enzymology; D013438:Sulfhydryl Compounds / Q000494:pharmacology*; D013439:Sulfhydryl Reagents / Q000494:pharmacology; D015032:Zinc / Q000378:metabolism
chemical_list: D005452:Fluoresceins; D009941:Organomercury Compounds; D013438:Sulfhydryl Compounds; D013439:Sulfhydryl Reagents; C001673:fluorescein mercuric acetate; D000469:Alkaline Phosphatase; D008274:Magnesium; D015032:Zinc