Mechanism of Nitric Oxide Release. I. Two-electron Reduction of Sodium Nitroprusside by l -cysteine in Aqueous Solution

Abstract Sodium nitroprusside (SNP) is a well-known vasodilator, which activates the cytosolic isozyme guanylate cyclase. It is quite stable by itself in aqueous solution and in the dark, but produces nitric oxide spontaneously under the appropriate reducing conditions. Both the neutral nitric oxide (NO) and the nitroxyl anion (NO m ) can be produced from SNP depending on the condition employed. NO release is favored at lower pH and cysteine concentration while NO m formation is more likely to occur at higher pH (>7) and cysteine concentration. NO was measured by electrochemical method and the chemical detection of NH 2 OH and NO 2 m are evidences of NO m formation. The mechanism of the reaction was found to be very complicated and involved three clear stages. NO and NO m are postulated to form in the first and third stages, respectively. All three stages showed cysteine dependence and were also affected by the pH of the solution. The first two stages resembled stepwise reversible two-electron reduction of NO + to NO m and are similar in rates. However the third stage is the slowest irreversible and rate-determining substitution of NO m by cysteine in the reduced nitroprusside ion. Increased absorption occurs between 250 and 500 u nm for the first two stages with absorption maxima at about 340 u nm for the first two stages which signals NO reduction in nitroprusside. A decrease in absorption in the same region, which signal loss of NO m was observed for the third stage of the reaction sequence. Cyanide release occurs in the first stage and the addition of NaCN significantly retards the reaction rate proving the reversible loss of CN m from the nitroprusside ion. Transition metal ions are known to catalyze the decay of nitrosocompounds and addition of Cu 2+ to reaction solutions showed significant catalysis and was especially pronounced in the final stage. Keywords: Nitric OxideNitroxyl AnionSodium NitroprussideReduction KineticsCysteine