title: Kinetics and mechanism of the oxidation of the glutathione dimer by hypochlorous Acid and catalytic reduction of the chloroamine product by glutathione reductase.
abstract: Oxidized glutathione (GSSG) reacts with two molar equivalents of HOCl/OCl- (a neutrophil-derived oxidant and a common biocide) to form the dichloro (bis-N-chloro-gamma-l-glutamyl) derivative (NDG). The reaction of less than two molar equivalents of HOCl with GSSG does not yield the unsymmetrical monochloro derivative (NCG) but rather a stoichiometric amount of NDG and GSSG. This result is explained by a faster reaction of the second equivalent of HOCl with NCG than that of the first equivalent of HOCl with GSSG. The rates of reaction of GSSG2-, GSSG3-, and GSSG4- (successive deprotonation of the ammonium groups) have been investigated, and it is clear that GSSG2- is unreactive, whereas GSSG4- is about twice as reactive as GSSG3-. Accordingly, the following mechanism is proposed (constants for 5 degrees C): H+ + OCl- = HOCl, pK1 = -7.47; GSSG2- = GSSG3- + H+, pK2 = 8.5; GSSG3- = GSSG4- + H+, pK3 = 9.5; GSSG3- + HOCl --> NCG3- + H2O, k4 = 2.7(2) x 106 M-1 s-1; GSSG4- + HOCl --> NCG4- + H2O, k5 = 3.5(3) x 107 M-1 s-1; NCG3- --> NDG4- + H+, k6 = fast; and NCG4- + HOCl --> NDG4- + H2O, k7 = fast. At physiologic pH, the k4 pathway dominates. NDG decomposes at pH 7.4 in a first-order process with kdec = 4.22(1) x 10-4 s-1 (t1/2 = 27 min). Glutathione reductase (EC 1.6.4.2) is capable of catalyzing the reduction of NDG by NADPH. The only NDG-derived product that is observed (by NMR) after the reduction by NADPH is GSH. Thus, in the presence of the GOR/NADPH system, GSH is capable of redox buffering a 3/2 mol equiv of HOCl rather than a 1/2 mol equiv as previously assumed.
authors.affiliation: Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
authors.forename: Péter
authors.initials: P
authors.lastname: Nagy
authors.forename: Michael T
authors.initials: MT
authors.lastname: Ashby
mesh_terms: D002384:Catalysis; D002700:Chloramines / Q000378:metabolism*; D019281:Dimerization; D005978:Glutathione / Q000737:chemistry / Q000378:metabolism*; D005980:Glutathione Reductase / Q000378:metabolism*; D006997:Hypochlorous Acid / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet
chemical_list: D002700:Chloramines; D006997:Hypochlorous Acid; D005980:Glutathione Reductase; D005978:Glutathione