作者:D. Lyn H. Williams
DOI:10.1039/cc9960001085
日期:——
S-Nitrosothiols (RSNO) are easily made by electrophilic nitrosation of thiols and are a convenient source of nitric oxide. Reaction occurs readily (in many cases) in aqueous buffer at pH 7.4 to give in addition the corresponding disulfide RSSR. If oxygen is not rigorously excluded from the solution, then the nitric oxide is converted quantitatively to nitrite ion, whereas in the absence of oxygen nitric oxide can be detected using a commercial NO-probe. Reaction, however, only occurs (apart from the photochemical pathway) if Cu2+ is present. There is often enough Cu2+ in the distilled water–buffer components to bring about reaction, but decomposition is halted if Cu2+ is complexed with EDTA. Experiments with the specific Cu+ chelator neocuproine however show that the true effective reagent is Cu+, formed by reduction of Cu2+ with thiolate ion. Kinetic experiments show that the most reactive nitrosothiols are those which can coordinate bidentately with Cu+, and there is a wide range of reactivity amongst the structures studied. Reactivity is crucially dependent on the concentrations of Cu2+ and RS–.Reaction also occurs, although somewhat more slowly, if the source of copper is the CuII complex with the tripeptide diglycyl-L-histidine (GGH) or as the CuII complex with human serum albumin (HSA). This allows the possibility that nitrosothiols could in principle generate nitric oxide in vivo using the naturally occurring sources of CuII.Rapid exchange of the NO-group in RSNO with thiols occurs, again in aqueous buffer at pH 7.4. This reaction has been established as a nucleophilic substitution reaction by the thiolate ion at the nitroso nitrogen atom.The implications of these results with regard to possible involvement of nitrosothiols in vivo are discussed.
S-亚硝基硫醇 (RSNO) 通过硫醇的电亲核亚硝化反应容易制备,并且是氮氧化物的便捷来源。在 pH 7.4 的水缓冲液中(在许多情况下),反应会迅速发生,从而生成相应的二硫化物 RSSR。如果溶液中没有严格排除氧气,则一氧化氮会定量转化为亚硝酸盐离子,而在缺氧的情况下,可以通过商业一氧化氮探针检测到一氧化氮。然而,反应仅在存在 Cu2+ 的情况下发生(除了光化学途径)。通常在蒸馏水-缓冲液成分中有足够的 Cu2+ 促使反应发生,但如果 Cu2+ 与 EDTA 形成络合物,则分解会被阻止。然而,使用特定的 Cu+ 螯合剂新铜碱(neocuproine)进行的实验表明,真正的有效试剂是 Cu+,它是通过硫醇阴离子还原 Cu2+ 形成的。动力学实验表明,反应性最强的亚硝基硫醇是那些能够与 Cu+ 形成二配位的分子,而且在所研究的结构中反应性存在广泛的变异。反应性在很大程度上依赖于 Cu2+ 和 RS– 的浓度。当铜的来源是与三肽二甘氨基-L-组氨酸 (GGH) 的 CuII 络合物或与人血清白蛋白 (HSA) 的 CuII 络合物时,反应也会发生,虽然速度较慢。这使得亚硝基硫醇在原则上能够利用天然存在的 CuII 产生体内的一氧化氮。RSNO 中 NO 基团与硫醇之间的快速交换同样发生,在 pH 7.4 的水缓冲液中进行。此反应已被确定为硫醇阴离子在亚硝基氮原子上的亲核取代反应。讨论了这些结果对于亚硝基硫醇在体内可能参与的影响。