Antioxidant Reactions of all-transRetinol in Phospholipid Bilayers: Effect of Oxygen Partial Pressure, Radical Fluxes, and Retinol Concentration

Lipoperoxyl radical-scavenging activity of retinol in unilamellar soybean phosphatidylcholine liposomes was studied under a variety of conditions to appreciate to what extend retinol may be considered an effective antioxidant. Peroxidation, initiated by 2 mm2,2′-azobis(amidino-propane)hydrochloride (AAPH), was carried out at 160 torr O2or at 15 torr O2, in the absence or in the presence of 10 to 40 mmretinol. As evaluated by the length of the inhibition periods,tinh, and by the ratio between the inhibition and propagation rate,Rinh/Rp, the antioxidant activity of retinol was higher at 15 torr O2than at 160 torr O2. The consumption rate of retinol was markedly faster at 160 torr O2than at 15 torr O2and increased with the increase of retinol concentration under both oxygen tensions. When liposome peroxidation was carried out under N2, retinol consumption was independent of retinol concentration. Peroxyl radicals oxidize retinol to 5,6-retinol epoxide. The ratio between 5,6-epoxide formed and the retinol consumed was markedly higher at 15 torr O2than under air and decreased with the increased retinol concentrations. When butylated hydroxytoluene was included into the liposomal suspension, most of the consumed retinol was converted into 5,6-epoxide. Liposomes were incubated at 15 torr O2, in the presence of 0.5 to 10 mmAAPH. The antioxidant effectiveness of 40 mmretinol, as measured by theRinh/Rpratio, increased with the 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 lipoperoxyl radical scavenger at low, rather than at high partial pressure of oxygen. However, when self-oxidation of retinol is prevented, only a minor fraction of the antioxidant is allowed to effectively act as a radical scavenger, suggesting that the radical-trapping reactions are rate-limiting for the antioxidant process. Peroxyl radical concentration, by shifting the route of the retinol activity toward radical scavenging, brings about an increasingly more efficient radical trapping. It is concluded that all-transretinol behaves as a more effective antioxidant at low oxygen partial pressure, low retinol concentrations, and high radical flux.