The copper(II)-catalyzed oxidation of glutathione

The kinetics and mechanisms of copper(II)-catalyzed GSH oxidation are examined in the light of its biological importance and in the use of blood and/or saliva sample for GSH monitoring. The rates of 'free thiol' consumption were measured spectrophotometrically via reaction with DTNB, showing that GSH is not auto-oxidized by oxygen in the absence of a catalyst. In the presence of Cu2+, reactions with two timescales were observed. The first step (short timescale) involves the fast formation of a copper-glutathione complex via the cysteine thiol. The second step (longer timescale) is the overall oxidation of GSH to GSSG catalyzed by copper(II). When the initial concentrations of GSH are at least three-fold in excess of Cu2+, the rate law is deduced to be. d[thiol]/dt = k[copper. glutathione complex][O-2](0.5)[H2O2]. 0.5. The order of reaction with respect to O-2 of 0.5th reveals a pre-equilibrium prior to the rate-determining step of GSSG formation. In contrast to [Cu2+] and [O-2], the rate of reactions decreases with increasing concentrations of GSH. This inverse relationship is proposed to be a result of competing formation of an inactive form of copper-glutathione complex (binding to glutamic and/or glycine moieties).