L. Tesoriere et al., ANTIOXIDANT REACTIONS OF ALL-TRANS RETINOL IN PHOSPHOLIPID-BILAYERS -EFFECT OF OXYGEN PARTIAL-PRESSURE, RADICAL FLUXES, AND RETINOL CONCENTRATION, Archives of biochemistry and biophysics, 343(1), 1997, pp. 13-18
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