title: Kinetic mechanism of the Zn-dependent aryl-phosphatase activity of myo-inositol-1-phosphatase.
abstract: Myo-inositol-1-phosphatase (EC 3.1.3.25) is able to hydrolyze myo-inositol-1-phosphate in the presence of Mg(2+) ions at neutral pH, and also p-nitrophenyl phosphate in the presence of Zn(2+)-ions at acidic pH. This enzyme plays a role in phosphatidylinositol cell signalling and is a putative target of lithium therapy in manic depression. We elucidate here the kinetic mechanism of the Zn-dependent activity of myo-inositol-1-phosphatase. As part of this analysis it was necessary to determine the basicity constants of p-nitrophenyl phosphate and the stability constant of its metal-complex in the presence of zinc chloride. We find that the Zn-dependent reaction may be described either by a rapid-equilibrium random mechanism or an ordered steady-state mechanism in which the substrate binds to the free enzyme prior to the metal ion. In both models the Zn-substrate complex acts as a high affinity inhibitor, yielding a dead-end species through its binding to the enzyme-Zn-substrate in rapid-equilibrium or to the enzyme-phosphate complexes in a steady-state model. Phosphate is a competitive inhibitor of the enzyme with respect to the substrate and an uncompetitive inhibitor with respect to zinc ions.
authors.affiliation: Department of Biochemical Sciences, University of Firenze, Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Caselli
authors.forename: Mario
authors.initials: M
authors.lastname: Casolaro
authors.forename: Francesco
authors.initials: F
authors.lastname: Ranaldi
authors.forename: Giampaolo
authors.initials: G
authors.lastname: Manao
authors.forename: Guido
authors.initials: G
authors.lastname: Camici
authors.forename: Eugenio
authors.initials: E
authors.lastname: Giachetti
mesh_terms: D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009596:Nitrophenols / Q000378:metabolism; D009943:Organophosphorus Compounds / Q000378:metabolism; D010710:Phosphates / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D013379:Substrate Specificity; D015032:Zinc
chemical_list: D009596:Nitrophenols; D009943:Organophosphorus Compounds; D010710:Phosphates; C008644:nitrophenylphosphate; D010744:Phosphoric Monoester Hydrolases; C021822:myo-inositol-1 (or 4)-monophosphatase; D015032:Zinc