title: Kinetics of the reaction of prostaglandin H synthase compound II with ascorbic acid.
abstract: The reduction of prostaglandin H synthase compound II by ascorbic acid in the presence of diethyldithiocarbamate was studied in 0.1 M phosphate buffer (pH 8.0) at 4.0 +/- 0.5 degrees C, by rapid scan spectrometry and transient state kinetics. A saturation effect and nonzero intercept were observed in the plot of pseudo-first-order rate constant versus ascorbic acid concentration. The saturation behavior suggests formation of a complex between prostaglandin H synthase compound II and ascorbic acid, whereas the nonzero intercept is attributable to the reaction of compound II of prostaglandin H synthase with diethyldithiocarbamate present in the system as a stabilizing agent. A rate equation has been derived which includes all pathways for the conversion of prostaglandin H synthase compound II back to native enzyme. Kinetic parameters for the reduction of compound II by ascorbic acid were obtained. They are the second-order rate constant of (1.4 +/- 0.5) X 10(5) M-1, S-1, for the formation of the compound II-ascorbic acid complex, the first-order rate constant of (14 +/- 4) S-1 for the oxidation-reduction reaction of the complex and its dissociation, and a parameter, Km of 92 +/- 10 microM analogous to the Michaelis-Menten constant. Thus we demonstrate that a quantitative kinetic study on the prostaglandin H synthase reactions can be performed in the presence of diethyldithiocarbamate.
authors.affiliation: Department of Chemistry, University of Alberta, Edmonton, Canada.
authors.forename: Y
authors.initials: Y
authors.lastname: Hsuanyu
authors.forename: H B
authors.initials: HB
authors.lastname: Dunford
mesh_terms: D000475:Alkenes / Q000494:pharmacology; D000818:Animals; D001205:Ascorbic Acid / Q000378:metabolism*; D004050:Ditiocarb / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D010545:Peroxides / Q000494:pharmacology; D011451:Prostaglandin-Endoperoxide Synthases / Q000378:metabolism*; D012756:Sheep
chemical_list: D000475:Alkenes; D010545:Peroxides; C047254:5-phenylpent-4-enyl-1-hydroperoxide; D004050:Ditiocarb; D011451:Prostaglandin-Endoperoxide Synthases; D001205:Ascorbic Acid