REACTION OF NITRIC-OXIDE WITH THE FREE SULFHYDRYL-GROUP OF HUMAN SERUM-ALBUMIN YIELDS A SULFENIC ACID AND NITROUS-OXIDE

Nitric oxide (NO) generated by diethylamine nonoate (DEA/NO), an NO do nor, readily oxidized the free sulfhydryl group of human serum albumin (HSA) as well as the sulfhydryl groups of reduced glutathione (GSH) a nd dithiothreitol (DTT) at pH 7.4 and 37 degrees C. Under anaerobic co nditions, the major products of the oxidation of HSA thiol by NO were the sulfenic acid (RSOH) of HSA and nitrous oxide (N2O). The stoichiom etry for this reaction, viz., 1 mol of HSA sulfhydryl oxidized to 1 mo l of N2O produced, is consistent with a net two-electron oxidation of the protein thiol to a sulfenic acid. The sulfenic acid product of HSA was shown to react with dimedone and GSH, two known reactions of sulf enic acids. In contrast, anaerobic oxidation of GSH and DTT by NO gave a stoichiometry close to the expected ratio of 2:1 (sulfhydryl oxidiz ed to N2O produced) for the oxidation of these thiols to their disulfi des and N2O. Under aerobic conditions, significant fractions of the su lfhydryl groups of HSA, GSH, and DTT were oxidized to their respective thionitrites, presumably by N2O3. Thionitrite formation was not obser ved in the absence of oxygen. The production of HSA-sulfenic acid by N O, as well as by other oxidizing agents such as H2O2 and peroxynitrite , followed by its reaction with circulating GSH or L-Cys may account f or the mixed disulfides of HSA observed in plasma.