Antioxidant actions of β-carotene in liposomal and microsomal membranes: Role of carotenoid-membrane incorporation and α-tocopherol

β-Carotene and other carotenoids are widely regarded as biological antioxidants. However, recent clinical trials indicate that β-carotene supplements are not effective in disease prevention and raise questions about the biological significance of carotenoid antioxidant actions. To further explore this issue, we have reevaluated the antioxidant actions of β- carotene in liposomal and biological membrane systems. In dilinoleoylphosphatidylcholine liposomes in which 0.35 mol % β-carotene was incorporated into the bilayer during liposome preparation, the carotenoid inhibited lipid peroxidation initiated by 10 mM azobis[amidino-propane HCl] (AAPH). In carotenoid-free liposome suspensions to which the same amount of β-carotene was added, no antioxidant effect was observed. Supplementation of rat liver microsomes with β-carotene in vitro yielded microsomes containing 1.7 nmol β-carotene mg^-1 and 0.16 nmol α-tocopherol mg^-1 microsomal protein. In β-carotene supplemented microsomes incubated with 10 mM AAPH under an air atmosphere, lipid peroxidation did not occur until α-tocopherol was depleted by approximately 60%. β-Carotene exerted no apparent antioxidant effect and was not significantly depleted in the incubations. Similar results were obtained when the incubation was done at 3.8 torr O₂. In liver microsomes from Mongolian gerbils fed β-carotene-supplemented diets, β-carotene levels were 1637% of α-tocopherol levels. The kinetics of AAPH-induced lipid peroxidation were no different in β-carotene- supplemented microsomes than in microsomes from unsupplemented animals, although the kinetics of β-carotene and α-tocopherol depletion were similar. The results indicate that β-carotene is ineffective as an antioxidant when added to preformed lipid bilayer membranes and that α- tocopherol is a much more effective membrane antioxidant than β-carotene, regardless of the method of carotenoid-membrane incorporation. These results support a reevaluation of the proposed antioxidant role for β-carotene in biological membranes.