Intramolecular transformation reaction of the glutathione thiyl radical into a non-sulphur-centred radical: a pulse-radiolysis and EPR study.
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 x 10(9) 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 approximately 1 x 10(3) s-1). The activation enthalpy and entropy at pH 8.4 and 20 degrees 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 alpha-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) x 10(6) 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 much less than 1.4 x 10(5) 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 x 10(8) dm3 mol-1 s-1).
Grierson
L
L
Max-Planck-Institut für Strahlenchemie, Mülheim an der Ruhr, Germany.
Hildenbrand
K
K
Bothe
E
E
D003037:Cobalt Radioisotopes; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D005978:Glutathione / Q000737:chemistry / Q000528:radiation effects*; D006878:Hydroxides; D017665:Hydroxyl Radical; D011675:Pulse Radiolysis; D012996:Solutions; D013455:Sulfur; D014867:Water
D003037:Cobalt Radioisotopes; D005609:Free Radicals; D006878:Hydroxides; D012996:Solutions; D014867:Water; D017665:Hydroxyl Radical; D013455:Sulfur; D005978:Glutathione