title: Mechanism of nitrite-stimulated catalysis by lactoperoxidase.
abstract: The reactions of lactoperoxidase (LPO) intermediates compound I, compound II and compound III, with nitrite (NO2(-)) were investigated. Reduction of compound I by NO2(-) was rapid (k2 = 2.3 x 10(7) M(-1) x s(-1); pH = 7.2) and compound II was not an intermediate, indicating that NO2* radicals are not produced when NO2(-) reacts with compound I. The second-order rate constant for the reaction of compound II with NO2(-) at pH = 7.2 was 3.5 x 10(5) M(-1) x s(-1). The reaction of compound III with NO2(-) exhibited saturation behaviour when the observed pseudo first-order rate constants were plotted against NO2(-) concentrations and could be quantitatively explained by the formation of a 1 : 1 ratio compound III/NO2(-) complex. The Km of compound III for NO2(-) was 1.7 x 10(-4) M and the first-order decay constant of the compound III/ NO2(-) complex was 12.5 +/- 0.6 s(-1). The second-order rate constant for the reaction of the complex with NO2(-) was 3.3 x 10(3) M(-1) x s(-1). Rate enhancement by NO2(-) does not require NO2* as a redox intermediate. NO2(-) accelerates the overall rate of catalysis by reducing compound II to the ferric state. With increasing levels of H2O2, there is an increased tendency for the catalytically dead-end intermediate compound III to form. Under these conditions, the 'rescue' reaction of NO2(-) with compound III to form compound II will maintain the peroxidatic cycle of the enzyme.
authors.affiliation: Department of Chemical and Life Sciences, University of Greenwich, London, UK.
authors.forename: T B
authors.initials: TB
authors.lastname: Brück
authors.forename: R J
authors.initials: RJ
authors.lastname: Fielding
authors.forename: M C
authors.initials: MC
authors.lastname: Symons
authors.forename: P J
authors.initials: PJ
authors.lastname: Harvey
mesh_terms: D052160:Benzothiazoles; D002384:Catalysis / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D006861:Hydrogen Peroxide / Q000737:chemistry; D007700:Kinetics; D007784:Lactoperoxidase / Q000737:chemistry*; D009573:Nitrites / Q000494:pharmacology*; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013451:Sulfonic Acids / Q000737:chemistry
chemical_list: D052160:Benzothiazoles; D009573:Nitrites; D013451:Sulfonic Acids; C002502:2,2'-azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid; D006861:Hydrogen Peroxide; D007784:Lactoperoxidase