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