Intramolecular Transformation Reaction of the Glutathione Thiyl Radical into a Non-sulphur-centred Radical: A Pulse-radiolysis and EPR Study

Abstract

The thiyl radical derived from glutathione (GSH) is shown to decay rapidly in aqueous solution by intramolecular rearrangement reactions into the non-sulphur-centred radical 1. The reaction is induced by OH⁻ with a rate constant of 5 × 10⁹ dm³ mol⁻¹ and is also observable at near-neutral conditions (at physiological pH values around 7·5 the rate of formation of 1 amounts to ∼1 × 10³ s⁻¹). The activation enthalpy and entropy at pH 8·4 and 20°C were found to be 26·7 kJ mol⁻¹ and −77 J mol⁻¹ K⁻¹, respectively. Radical 1 was unequivocally identified by EPR as the α-amino radical at the glutamyl residue of GSH. It is relatively long-lived with typical bimolecular decay rate constants of the order of (2–20) × 10⁶ dm³ mol⁻¹ s⁻¹. At higher GSH concentrations the formation of 1 is retarded but not inhibited. All radicals, sulphur- as well as non-sulphur-centred ones are connected via equilibria, partly under the action of ‘repair’ processes of GSH. These repair processes, however, are slow (k < 1·4 × 10⁵ dm³ mol⁻¹ s⁻¹). The equilibria are established quite rapidly and were found to be far on the side of the non-sulphur-centred radical under all conditions employed. Radical 1 possesses reducing properties as evidenced by its fast reaction with 4-nitro-acetophenone (PNAP) to yield PNAP^.− (k = 7 × 10⁸ dm³ mol⁻¹ s⁻¹).