The reaction of S-nitrosothiols with thiols at high thiol concentration

Abstract

Reactions of S-nitrosothiols (RSNO) with their corresponding thiols (RSH) present in a large excess (>20-fold) proceed readily to give the disulfide. Ammonia is formed together with some nitrite anion, and these constitute >90% of the "nitrogen" products. This is in marked contrast with the reaction at low thiol concentration, where nitric oxide is the major initial "nitrogen" product, which is rapidly converted in the presence of oxygen in water to nitrite anion. Also in marked contrast to the "low thiol concentration" reaction, the reaction at high thiol concentration is not affected by added Cu2+, nor by the metal-ion scavenger EDTA. Kinetically all reactions were excellent first-order processes, and the reactions were also strictly first order in thiol concentration. A large range of nitrosothiols were studied and the generality of the reaction established. Some reactions of RSNO with other thiols (R'SH) were examined and the results readily interpreted in terms of a prior rapid equilibrium transnitrosation. The pH dependence for the reaction of S-nitrosocysteine with cysteine clearly showed that the reactive species is the cysteine thiolate anion. The results are discussed along with those of two other recent reports of these reactions, in terms of thiolate attack initially at the nitroso nitrogen atom, and subsequently at sulfur atoms, eliminating RSSR and yielding hydroxylamine, which is rapidly reduced by thiolate ion to ammonia. The results are also discussed in connection with the release of NO from nitrosothiols and with the important biological consequences, both for the in vivo reactions of NO and for the potential of nitrosothiols as NO-releasing drugs for medical use.Key words: S-nitrosothiols, thiols, nitric oxide, ammonia, nitrous oxide.