Formation of nanomolar concentrations of S-nitrosoalbumin in human plasma by nitric oxide

S-Nitrosothiols are potentially important mediators of biological processes including vascular function, apoptosis, and thrombosis. Recent studies ind icate that the concentrations of S-nitrosothiols in the plasma from healthy individuals are lower than previously reported and in the range of 30-120 nM. The mechanisms of formation and metabolism of these low nM concentratio ns, capable of exerting biological effects, remain unknown. An important is sue that remains unresolved is the significance of the reactions of low flu xes of nitric oxide (NO) with oxygen to form S-nitrosothiols in a complex b iological medium such as plasma, and the impact of red blood cells on the f ormation of S-nitrosothiols in blood. These issues were addressed by exposi ng plasma to varying fluxes of NO and measuring the net formation of S-nitr osothiols. In the presence of oxygen and physiological fluxes of NO, the pr edominant S-nitrosothiol formed is S-nitroso-albumin at concentrations in t he high nM range (similar to 400-1000 nM). Although the formation of S-nitr osothiols by NO was attenuated in whole blood, presumably by erythrocytic h emoglobin, significant amounts of S-nitrosothiols within the physiological range of S-nitrosothiol concentrations (similar to 80 nM) were still formed at physiological fluxes of NO. Little is known about the stability of S-ni troso-albumin in plasma, and this is central to our understanding of the bi ological effectiveness of S-nitrosothiols. Low molecular weight thiols decr eased the half-life of S-nitroso-albumin in plasma, and the stability of S- nitroso-albumin is enhanced by the alkylation or free thiols. Our data sugg ests that physiologically relevant concentrations of S-nitrosothiols can be formed in blood through the reaction of NO with oxygen and proteins, despi te the low rates of reaction of oxygen with NO and the presence of erythroc ytes. (C) 2001 Elsevier Science Inc.