Antioxidant activity of 9-cis compared to all-trans beta-carotene in vitro

In the present in vitro study we compared the antioxidative efficiency of the 9-cis to that of the all-trans beta-carotene. The 9-cis isomer was isolated from the alga Dunaliella bardawil. The experimental system consisted of 80 mM methyl linoleate, 4 mM azo-bis-2,2'-dimethylvaleronitrile (AMVN) as a free radical generating agent, and 200 microM beta-carotene (synthetic all-trans, 9-cis or a mixture of the 9-cis and all-trans isomers, having a ratio of 2.3). During the incubation at 37 degrees C the mixtures were analyzed for methyl linoleate hydroperoxides, total beta-carotene concentration, and its isomeric composition. The content of 9-cis beta-carotene in the various systems was negatively correlated to the level of the hydroperoxides accumulated, and positively related to the residual beta-carotene amount. The HPLC analysis of the system containing both isomers revealed a continuous decrease in the 9-cis to all-trans isomer ratio. The results suggest that the 9-cis beta-carotene has a higher antioxidant potency than that of the all-trans isomer and, therefore, it protects the methyl linoleate, as well as the all-trans isomer, from oxidation. This isomeric difference might be explained by the higher reactivity of cis, compared to trans, bonds.