title: High Km soluble 5'-nucleotidase from human placenta. Properties and allosteric regulation by IMP and ATP.
abstract: A human placental soluble "high Km" 5'-nucleotidase has been separated from "low Km" 5'-nucleotidase and nonspecific phosphatase by AMP-Sepharose affinity chromatography. The enzyme was purified 8000-fold to a specific activity of 25.6 mumol/min/mg. The subunit molecular mass is 53 kDa, and the native molecular mass is 210 kDa, suggesting a tetrameric structure. Soluble high Km 5'-nucleotidase is most active with IMP and GMP and their deoxy derivatives. IMP is hydrolyzed 15 times faster than AMP. The enzyme has a virtually absolute requirement for magnesium ions and is regulated by them. Purine nucleoside 5'-triphosphates strongly activate the enzyme with the potency order dATP greater than ATP greater than GTP. 2,3-Diphosphoglycerate activates the enzyme as potently as ATP. Three millimolar ATP decreased the Km for IMP from 0.33 to 0.09 mM and increased the Vmax 12-fold. ATP activation was modified by the IMP concentration. At 20 microM IMP the ATP-dependent activation curve was sigmoidal, while at 2 mM IMP it was hyperbolic. The A0.5 values for ATP were 2.26 and 0.70 mM, and the relative maximal velocities were 32.9 and 126.0 nmol/min, respectively. Inorganic phosphate shifts the hyperbolic substrate velocity relationship for IMP to a sigmoidal one. With physiological concentrations of cofactors (3 mM ATP, 1-4 mM Pi, 150 mM KCl) at pH 7.4, the enzyme is 25-35 times more active toward 100 microM IMP than 100 microM AMP. These data show that: (a) soluble human placental high Km 5'-nucleotidase coexists in human placenta with the low Km enzyme; (b) under physiological conditions the enzyme favors the hydrolysis of IMP and is critically regulated by IMP, ATP, and Pi levels; and (c) kinetic properties of ATP and IMP are each modified by the other compound suggesting complex interaction of the associated binding sites.
authors.affiliation: Department of Internal Medicine, University Hospital, Ann Arbor, Michigan 48109-0108.
authors.forename: J
authors.initials: J
authors.lastname: Spychała
authors.forename: V
authors.initials: V
authors.lastname: Madrid-Marina
authors.forename: I H
authors.initials: IH
authors.lastname: Fox
mesh_terms: D015720:5'-Nucleotidase; D000255:Adenosine Triphosphate / Q000494:pharmacology*; D002846:Chromatography, Affinity; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007291:Inosine Monophosphate / Q000494:pharmacology*; D007292:Inosine Nucleotides / Q000494:pharmacology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D015636:Magnesium Chloride; D009708:Nucleotidases / Q000378:metabolism*; D010710:Phosphates / Q000494:pharmacology; D010920:Placenta / Q000201:enzymology*
chemical_list: D007292:Inosine Nucleotides; D010710:Phosphates; D015636:Magnesium Chloride; D007291:Inosine Monophosphate; D000255:Adenosine Triphosphate; D009708:Nucleotidases; D015720:5'-Nucleotidase; D008274:Magnesium