Antioxidant activity of palm oil carotenes in organic solution: effects of structure and chemical reactivity

The antioxidant effectiveness of palm oil alpha-carotene and comparison with beta-carotene in organic solution containing egg-yolk phosphatidycholine (EYPC) in the presence of lipid soluble 2,2(1)-azobis (2,4-dimethyl valeronitrile) (AMVN)-generated peroxyl radicals was investigated by measuring the formation of phosphatidyl choline hydroperoxide (PCOOH) and thiobarbituricacid reacting substances (TBARS). Lutein and zeaxanthin (xanthophylls), similar to alpha and beta carotenes, respectively, but differing in containing hydroxyl groups on the two rings (3,3(1)-diol), were also included in the investigation. The carotenes were more rapidly oxidised than the xanthophylls in the solution. The initial rates of oxidation of the carotenoid tested were 0.39 mu M min(-1) (alpha-carotene), 0.44 mu M min(-1) (beta-carotene), 0.30 mu M min(-1) (lutein) and 0.33 mu M min(-1) (zeaxanthin). Incubation of EYPC with AMVN at 37 degrees C induced the accumulation of PCOOH at the linear rate of 1.8 mu M min(-1). Although, all the carotenoids tested at 1 mol % relative to EYPC retarded (p < 0.05) the chain propagation reaction of PCOOH formation alpha-carotene had the highest activity but this was less than alpha-tocopherol. alpha-Tocopherol, alpha-carotene, beta-carotene, lutein and zeaxanthin reduced PCOOH accumulation by 78, 65, 40, 60 and 43%, respectively. AMVN incubated with EYPC for 2 h induced the formation of TEARS compared to the control (p < 0.001). alpha-Carotene significantly suppressed the TEARS formation by 68% whilst beta-carotene, lutein and zeaxanthin elicited 50, 64 and 53% reductions, respectively, alpha-Tocopherol retarded the TEARS formation by 80%. These results suggest that alpha-carotene, a carotenoid abundantly present in human diets, especially red palm oil, may better attenuate peroxyl radical-dependent lipid peroxidation than beta-carotene in organic solution. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.