Reactions of beta-carotene with cigarette smoke oxidants. Identification of carotenoid oxidation products and evaluation of the prooxidant/antioxidant effect.

Recent intervention trials reported that smokers given dietary beta-carotene supplementation exhibited an increased risk of lung cancer and overall mortality. beta-Carotene has been hypothesized to promote lung carcinogenesis by acting as a prooxidant in the smoke-exposed lung. We have examined the interactions of cigarette smoke with beta-carotene in model systems. Both whole smoke and gas-phase smoke oxidized beta-carotene in toluene to several products, including carbonyl-containing polyene chain cleavage products and beta-carotene epoxides. A major product of the reaction was identified as 4-nitro-beta-carotene, which was formed by nitrogen oxides in smoke. Both cis and all-trans isomers of 4-nitro-beta-carotene were detected. The hypothesis that smoke-driven beta-carotene autoxidation exerts prooxidant effects was tested in a liposome system. Lipid peroxidation in dilinoleoylphosphatidylcholine liposomes exposed to gas-phase smoke was modestly inhibited by the incorporation of 0.1 mol % beta-carotene. Both the lipid soluble antioxidant alpha-tocopherol and the water soluble antioxidant ascorbate were oxidized more slowly by gas-phase smoke exposure in liposomes containing beta-carotene. These data indicate that beta-carotene exerts weak antioxidant effects against smoke-induced oxidative damage in vitro. It is unlikely that a prooxidant effect of beta-carotene occurs under biologically relevant conditions or is responsible for an increased incidence of lung cancer observed in smokers who consume beta-carotene supplements.