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) mainly due to the ease of cleavage of the S-N bond which consequently produces (*)NO. The reductive decomposition of S-nitrosoglutathione (GSNO) by l-ascorbic acid (vitamin C) yields (*)NO which was monitored both electrochemically (using NO-probe) and spectrophotometrically. The rate of reaction and (*)NO 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 = -dGSNO](t)/dt = ((k(a)H(+)](2) + k(b)H(+)]K(1) + k(c)K(1)K(2))/(H(+)](2) + K(1)H(+)] + K(1)K(2)))GSNO](t)H(2)A](t). k(a), 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 DeltaH(double dagger)(b) = 54.4 +/- 4.3 kJ mol(-1), DeltaS(double dagger)(b) = -106 +/- 16 J K(-1) mol(-1), DeltaH(double dagger)(c) = 80.5 +/- 7.5 kJ mol(-1), DeltaS(double dagger)(c) = 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 Cu(2+). Induced catalytic decomposition of GSNO only becomes significant above ca. 10 microM Cu(2+), but after this it shows linear dependency. To nullify any catalysis by Cu(2+) or any other transition metal ions, EDTA was added to all experimental reactions except those where catalysis by Cu(2+) was studied.