Kinetics and mechanism of the decomposition of S-nitrosoglutathione by L-ascorbic acid and copper ions in aqueous solution to produce nitric oxide

S-Nitrosothiols serve as a good source of nitric oxide ((NO)-N-.) mainly due to the ease of cleavage of the S-N bond which consequently produces (NO)-N-.. The reductive decomposition of S-nitrosoglutathione (GSNO) by L-ascorbic acid (vitamin C) yields (NO)-N-. which was monitored both electrochemically (using NO-probe) and spectrophotometrically. The rate of reaction and (NO)-N-. release was found to be pH dependent in a manner which drastically increases with pH demonstrating that the L-ascorbic acid dianion (A(2-)) is by far the most reactive species of L-ascorbic acid (H(2)A). The derived rate expression (measuring the disappearance of the absorption at ca, 336 nm due to GSNO) was established as rate = -d[GSNO](t)/dt = ((k(a)[H+](2) + k(b)[H+]K-1 + k(c)K(1)K(2))/([H+](2) + K-1[H+] + K1K2))[GSNO](t)[H(2)A](t).k(u), k(b), and k(c) are second-order rate constants via the H(2)A, HA(-), and A(2-) pathways, respectively, while K-1 and K-2 represent the first and second equilibrium dissociation constants of L-ascorbic acid. There is little or no reaction at low pH (below 5,5), where H(2)A is a predominant species, and as a result the rate constant (k(a)) via this route was found to be negligible. At 25 degrees C, k(b) = 5.23 +/- 1.47 x 10(-3) dm(3) mol(-1) s(-1) and k(c) = 1.22 +/- 0.04 x 10(3) dm(3) mol(-1) s(-1), activation parameters Delta H-b(double dagger) = 54.4 +/- 4.3 kJ mol(-1), Delta S-b(double dagger) = -106 +/- 16 J K-1 mol(-1), Delta H-c(double dagger) = 80.5 +/- 7.5 kJ mol(-1), Delta S-c(double dagger) = 84 +/- 7 kJ mol(-1). The experimental rate and activation parameters suggest that this redox process follows an outer-sphere electron transfer mechanism. GSNO is relatively stable in the dark, aqueous medium and even in the presence of trace quantities of Cu2+, Induced catalytic decomposition of GSNO only becomes significant above ca, 10 mu M Cu2+, but after this it shows linear dependency. To nullify any catalysis by Cu2+ or any other transition metal ions, EDTA was added to all experimental reactions except those where catalysis by Cu2+ was studied. (C) 2000 Academic Press.