Nagy Péter, Ashby Michael T
Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
Chem Res Toxicol. 2007 Jan;20(1):79-87. doi: 10.1021/tx060184g.
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.
氧化型谷胱甘肽（GSSG）与两摩尔当量的次氯酸/次氯酸根离子（一种中性粒细胞衍生的氧化剂和常见的杀菌剂）反应，形成二氯（双 - N - 氯 - γ - L - 谷氨酰）衍生物（NDG）。次氯酸与GSSG反应的摩尔当量小于两摩尔时，不会生成不对称的一氯衍生物（NCG），而是生成化学计量的NDG和GSSG。这一结果的解释是，次氯酸的第二个当量与NCG的反应速度比第一个当量与GSSG的反应速度快。研究了GSSG2 -、GSSG3 -和GSSG4 -（铵基团的连续去质子化）的反应速率，很明显GSSG2 -没有反应活性，而GSSG4 -的反应活性约为GSSG3 -的两倍。因此，提出了以下反应机理（5℃时的常数）：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)×10⁶ M⁻¹ s⁻¹；GSSG4 - + HOCl → NCG4 - + H₂O，k5 = 3.5(3)×10⁷ M⁻¹ s⁻¹；NCG3 - → NDG4 - + H⁺，k6 = 快速；NCG4 - + HOCl → NDG4 - + H₂O，k7 = 快速。在生理pH值下，k4途径占主导。NDG在pH 7.4时以一级反应过程分解，kdec = 4.22(1)×10⁻⁴ s⁻¹（半衰期 = 27分钟）。谷胱甘肽还原酶（EC 1.6.4.2）能够催化NADPH还原NDG。NADPH还原后（通过NMR观察）唯一观察到的NDG衍生产物是GSH。因此，在GOR/NADPH系统存在的情况下，GSH能够进行氧化还原缓冲3/2摩尔当量的次氯酸，而不是像之前假设的1/2摩尔当量。
