KINETICS AND EQUILIBRIA OF THIOL DISULFIDE INTERCHANGE REACTIONS OF SELECTED BIOLOGICAL THIOLS AND RELATED MOLECULES WITH OXIDIZED GLUTATHIONE

Rate constants for reaction of coenzyme A and cysteine with oxidized glutathione (GSSG) and equilibrium constants for the reaction of coenzyme A, cysteine, homocysteine, cysteamine, and related thiols with GSSG by thiol/disulfide interchange were determined over a range of pD values by NMR spectroscopy. The rate constants for reaction of the thiolate anion forms of coenzyme A and cysteine with GSSG suggest that reduction of GSSG by coenzyme A and cysteine is a mechanistically uncomplicated S(N)2 reaction. Equilibrium constants for the thiol/disulfide interchange reactions show a strong dependence on the Bronsted basicity of the thiolate anion. In a similar way, DELTA-E-degrees', the difference between the half-cell potentials for the RSSR/RSH and GSSG/GSH redox couples, is linearly dependent on the difference between the pK(A) values of RSH and glutathione: DELTA-E-degrees' = 64-DELTA-pK(A) -7.7 where DELTA-E-degrees' is in units of mV. The reducing strength at a given pH is also determined by the fraction of the thiol present in the reactive thiolate form. At pD 7, the half-cell potentials for coenzyme A, cysteine, homocysteine, and cysteamine are close to that of glutathione, the major intracellular thiol redox system, which suggests that small changes in the intracellular redox potential can cause significant changes in the intracellular distribution of these biological thiols between their reduced and oxidized forms.