Action of β-Carotene as an Antioxidant against Lipid Peroxidation

Abstract

The action and activity of β-carotene as a radical-scavenging antioxidant against lipid peroxidation have been studied. β-Carotene suppressed the free-radical-mediated oxidations of methyl linoleate in benzene solution and soybean phosphatidylcholine liposomal membranes in aqueous dispersions in a dose-dependent manner, but the antioxidant activity of β-carotene was much smaller than that of α-tocopherol. β-Carotene was 32 times less reactive toward peroxyl radical than α-tocopherol and approximately as reactive as 2,6-di-tert-butyl-4-methylphenol in benzene solution. Toward carbon-centered radical, β-carotene was 11 times less reactive than α-tocopherol. When β-carotene and α-tocopherol were present together in homogeneous solution, α-tocopherol was consumed predominantly and β-carotene was spared. When they were incorporated simultaneously into the same dimyristoyl phosphatidylcholine liposomal membranes and the radicals were formed in the aqueous phase, α-tocopherol was consumed faster than β-carotene, but the sparing efficacy was much smaller than in homogeneous solution. On the contrary, β-carotene was consumed faster than α-tocopherol when the radicals were generated within the lipophilic compartment of the membranes, implying that β-carotene is relatively more favorable than α-tocopherol for scavenging lipophilic radicals within the membranes. In contrast to an efficient synergistic inhibition by a combination of α-tocopherol and ascorbic acid, cooperative interaction between β-carotene and ascorbic acid was not observed. β-Carotene underwent autooxidation to give polymeric products and the rate of consumption of β-carotene increased with increasing concentrations of itself and oxygen and decreased with increasing con centration of lipids. The formation of polymeric products was confirmed by gel permeation chromatography. It was concluded that β-carotene is less potent as an antioxidant than α-tocopherol because β-carotene is less reactive toward peroxyl radical than α-tocopherol and the stable β-carotene radical reacts with oxygen to give β-carotene peroxyl radical which is not stable but able to attack lipid to continue chain oxidation. The higher antioxidant activity of β-carotene at lower oxygen pressure is attributed primarily to an unfavorable formation of β-carotene peroxyl radical at higher oxygen pressure rather than a higher reactivity of β-carotene toward carbon-centered radical than peroxyl radical.