Reaction of ascorbic acid with S-nitrosothiols: clear evidence for two distinct reaction pathways

Abstract

Ascorbate reacts with S-nitrosothiols generally, in the pH range 3–13 by way of two distinct pathways, (a) at low [ascorbate], typically below ≈1 × 10⁻⁴ 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.