Reaction of ascorbic acid with S-nitrosothiols:: clear evidence for two distinct reaction pathways
Reaction of ascorbic acid with S-nitrosothiols:: clear evidence for two distinct reaction pathways
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DOI：
10.1039/b004028m
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发表时间：
2000-01-01
期刊：
影响因子：
--
通讯作者：
Williams, DLH
中科院分区：
文献类型：
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作者：
Holmes, AJ;Williams, DLH
在pH值为3-13的范围内，抗坏血酸与S-亚硫醇的反应一般通过两条不同的途径进行：(A)在低[抗坏血酸]时，通常低于1×10(-4)摩尔·dm(-3)，导致NO和二硫化物的生成；(B)在较高[抗坏血酸]时，产物是硫醇和NO。反应(A)依赖于Cu2+，在EDTA存在下完全被切断，而反应(B)完全独立于[Cu2+]，无论有没有EDTA都很容易发生。对于S-亚硝基谷胱甘肽(GSnO)，这两个反应可以完全分开，尽管对于某些反应物，这两个反应是重叠的。在反应(A)中，抗坏血酸充当还原剂，从Cu2+生成Cu2+，而Cu2+又与RSNO反应，最初生成NO、Cu2+和RS-。后者可以起到还原Cu2+的作用，导致二硫化物的形成。当游离的硫酸盐最初被大量过量的亚硝酸还原到非常低的水平时，抗坏血酸就会启动反应。反应(B)被解释为抗坏血酸对亚硝基-氮原子的亲核攻击，导致硫醇和O-亚硝基抗坏血酸通过自由基途径分解，得到脱氢抗坏血酸和一氧化氮。类似的途径是文献中公认的亚硝酸和烷基亚硝酸盐亚硝化抗坏血酸的机理。Cu2+非依赖途径的速率常数随着pH的变化而急剧增加，对速率常数随pH的变化的分析表明，抗坏血酸的反应途径既有单阴离子形式的，也有双阴离子形式的，后者更具活性。不出所料，活化的熵很大，而且是负的。讨论了结构-活性趋势的某些方面。
Ascorbate reacts with S-nitrosothiols generally, in the pH range 3-13 by way of two distinct pathways, (a) at low [ascorbate], typically below approximate to 1x10(-4) mol dm(-3) which leads to the formation of NO and the disulfide, and (b) at higher [ascorbate] when the products are the thiol and NO. Reaction (a) is Cu2+-dependent, and is completely cut out in the presence of EDTA, whereas reaction (b) is totally independent of [Cu2+] and takes place readily whether EDTA is present or not. For S-nitrosoglutathione (GSNO) the two reactions can be made quite separate, although for some reactants the two reactions overlap. In reaction (a), ascorbate acts as a reducing agent, generating Cu+ from Cu2+, which in turn reacts with RSNO forming initially NO, Cu2+ and RS-. The latter can then play the role of reducing agent for Cu2+, leading to disulfide formation. Ascorbate will initiate reaction when the free thiolate has initially been reduced to a very low level by the synthesis of RSNO from a large excess of nitrous acid over the thiol. Reaction (b) is interpreted in terms of nucleophilic attack by ascorbate at the nitroso-nitrogen atom, leading to thiol and O-nitrosoascorbate which breaks up, by a free-radical pathway, to give dehydroascorbic acid and NO. A similar pathway is the accepted mechanism in the literature for the nitrosation of ascorbate by nitrous acid and alkyl nitrites. The rate constant for the Cu2+ -independent pathway increases sharply with pH and analysis of the variation of the rate constant with pH identifies a reaction pathway via both the mono- and di-anion forms of ascorbate, with the latter being the more reactive. As expected the entropy of activation is large and negative. Some aspects of structure-reactivity trends are discussed.