title: Inhibitory effect of magnesium ion on the human placental alkaline phosphatase-catalyzed reaction in a reverse micellar system.
abstract: Human placental alkaline phosphatase is a membrane-anchored protein. Entrapping the enzyme into a reverse micellar vesicle mimics the in vivo conditions and allows examination of the properties of the enzyme. Placental alkaline phosphatase is enzymatically active in Aerosol-OT/isooctane reverse micelles. Substantially different kinetic behavior of the enzyme has been observed in aqueous or reverse micellar systems. In aqueous solution, Mg2+ is a nonessential activator of the enzyme. In the experiments described in the present report Mg2+ was found to be an inhibitor for the enzyme in reverse micelles. This inhibition is presumably due to a time-dependent conformational change of the enzyme molecule, which resulted in a curvature in the recorder tracings of the enzyme assays. The Mg2+-induced conformational change of the enzyme was completely prevented by phosphate and partially reserved by EDTA. High concentrations of Zn2+ also strongly inhibited enzyme activity in both aqueous and reverse micellar solvent systems, presumably by occupying the Mg2+ (M3) site of the enzyme. However, binding of Zn2+ at the M3 site did not cause conformational change of the enzyme and the enzyme assay tracing was linear. The M3 site of the enzyme is proposed to have a modulatory role in vivo using magnesium ion as the modulator.
authors.affiliation: Department of Biochemistry, National Defense Medical Center, Taipei, Taiwan, Republic of China.
authors.forename: H C
authors.initials: HC
authors.lastname: Hung
authors.forename: T M
authors.initials: TM
authors.lastname: Huang
authors.forename: G G
authors.initials: GG
authors.lastname: Chang
mesh_terms: D000469:Alkaline Phosphatase / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D002384:Catalysis; D004143:Dioctyl Sulfosuccinic Acid; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008823:Micelles; D009795:Octanes; D010710:Phosphates / Q000494:pharmacology; D010920:Placenta / Q000187:drug effects* / Q000201:enzymology; D011487:Protein Conformation; D012996:Solutions; D013050:Spectrometry, Fluorescence; D015032:Zinc / Q000494:pharmacology
chemical_list: D008823:Micelles; D009795:Octanes; D010710:Phosphates; D012996:Solutions; D004143:Dioctyl Sulfosuccinic Acid; D000469:Alkaline Phosphatase; D008274:Magnesium; D015032:Zinc; C045798:2,2,4-trimethylpentane