Kinetics and equilibria of S-nitrosothiol-thiol exchange between glutathione, cysteine, penicillamines and serum albumin.

The kinetics and equilibria of S-nitrosothiol-thiol (SNO-SH) exchange reactions were determined using differential optical absorption. At pH 7.4 and 37 degrees C, k2 values ranged from 0.9 M-1.s-1 for the reaction between S-nitroso-glutathione (GSNO) and N-acetyl-penicillamine, and up to 279 M-1.s-1 for the exchange between S-nitroso-penicillamine (penSNO) and GSH. SNO-SH exchange involving GSH/GSNO and cysteine/cySNO was relatively rapid, k2 approx. 80 M-1.s-1 with an equilibrium constant slightly in favour of GSNO. GSNO was strongly favoured in equilibrium with penSNO, keq 0.0039. In the case of SNO-SH exchange between S-nitroso human serum albumin (albSNO) and GSH or cysteine k2 values were 3.2 and 9.1 M-1.s-1, respectively. The results show that the initial rate of SNO-SH exchange between physiological albSNO (7 microM) and venous plasma levels of GSH and cysteine is very slow, < 1%/min. On the other hand, if a nitrosothiol such as cySNO were to enter a cell, it would be rapidly converted to GSNO (43%/s).