ANTIOXIDANT REACTIONS OF ALL-TRANS RETINOL IN PHOSPHOLIPID-BILAYERS -EFFECT OF OXYGEN PARTIAL-PRESSURE, RADICAL FLUXES, AND RETINOL CONCENTRATION

Lipoperoxyl radical-scavenging activity of retinol in unilamellar soyb ean phosphatidylcholine liposomes was studied under a variety of condi tions to appreciate to what extend retinol may be considered an effect ive antioxidant. Peroxidation, initiated by 2 mM 2,2'-azobis(amidino-p ropane) hydrochloride (AAPH), was carried out at 160 torr O-2 or at 15 torr O-2, in the absence or in the presence of 10 to 40 mM retinol. A s evaluated by the length of the inhibition periods, t(inh), and by th e ratio between the inhibition and propagation rate, R-inh/R-p, the an tioxidant activity of retinol was higher at 15 torr O-2 than at 160 to rr O-2. The consumption rate of retinol was markedly faster at 160 tor r O-2 than at 15 torr O-2 and increased with the increase of retinol c oncentration under both oxygen tensions. When liposome peroxidation wa s carried out under N-2, retinol consumption was independent of retino l concentration. Peroxyl radicals oxidize retinol to 5,6-retinol epoxi de. The ratio between 5,6-epoxide formed and the retinol consumed was markedly higher at 15 torr O-2 than under air and decreased with the i ncreased retinol concentrations. When butylated hydroxytoluene was inc luded into the liposomal suspension, most of the consumed retinol was converted into 5,6-epoxide. Liposomes were incubated at 15 torr O-2, i n the presence of 0.5 to 10 mM AAPH. The antioxidant effectiveness of 40 mM retinol, as measured by the Ri(inh)/R-p ratio, increased with th e increase of the radical fluxes. The results suggest, besides radical trapping, that a major consumption of retinol during lipid oxidation occurs through self-oxidation reactions, which are concentration- and oxygen-dependent. A decreased self-oxidation makes retinol a better li poperoxyl radical scavenger at low, rather than at high partial pressu re of oxygen. However, when self-oxidation of retinol is prevented, on ly a minor fraction of the antioxidant is allowed to effectively act a s a radical scavenger, suggesting that the radical-trapping reactions are rate-limiting for the antioxidant process. Peroxyl radical concent ration, by shifting the route of the retinol activity toward radical s cavenging, brings about an increasingly more efficient radical trappin g, It is concluded that all-trans retinol behaves as a more effective antioxidant at low oxygen partial pressure, low retinol concentrations , and high radical flux. (C) 1997 Academic Press