THE FATE OF THE OXIDIZING TYROSYL RADICAL IN THE PRESENCE OF GLUTATHIONE AND ASCORBATE - IMPLICATIONS FOR THE RADICAL SINK HYPOTHESIS

Cellular systems contain as much as millimolar concentrations of both ascorbate and GSH, although the GSH concentration is often 10-fold tha t of ascorbate. It has been proposed that GSH and superoxide dismutase (SOD) act in a concerted effort to eliminate biologically generated r adicals. The tyrosyl radical (Tyr(.)) generated by horseradish peroxid ase in the presence of hydrogen peroxide can react with GSH to form th e glutathione thiyl radical (GS(.)), GS(.) can react with the glutathi one anion (GS(-)) to form the disulfide radical anion (GSSG(radical) ( anion)). This highly reactive disulfide radical anion will reduce mole cular oxygen, forming superoxide and glutathione disulfide (GSSG). In a concerted effort, SOD will catalyze the dismutation of superoxide, r esulting in the elimination of the radical. The physiological relevanc e of this GSH/SOD concerted effort is questionable, In a tyrosyl radic al-generating system containing ascorbate (100 mu M) and GSH (8 mM), t he ascorbate nearly eliminated oxygen consumption and diminished GS(.) formation. In the presence of ascorbate, the tyrosyl radical will oxi dize ascorbate to form the ascorbate radical. When measuring the ascor bate radical directly using fast-flow electron spin resonance, only mi nor changes in the ascorbate radical electron spin resonance signal in tensity occurred in the presence of GSH. These results indicate that i n the presence of physiological concentrations of ascorbate and GSH, G SH is not involved in the detoxification pathway of oxidizing free rad icals formed by peroxidases.