CAROTENOIDS AND PROTECTION OF PHOSPHOLIPIDS IN SOLUTION OR IN LIPOSOMES AGAINST OXIDATION BY PEROXYL RADICALS - RELATIONSHIP BETWEEN CAROTENOID STRUCTURE AND PROTECTIVE ABILITY

The ability of carotenoids to protect egg-yolk phosphatidylcholine (EY PC) lipids against oxidation by peroxyl radicals generated from ate-in itiators was studied. In homogeneous organic solution, all the caroten oids tested ameliorated lipid peroxidation by AMVN, but none was as ef fective as alpha-tocopherol. beta-Ring carotenoids showed a correlatio n between protective effect and rate of carotenoid destruction. beta,b eta-Carotene and zeaxanthin, which react with peroxyl radicals at simi lar rates, gave a similar degree of protection in organic solution. Th e reactivity and protective ability of the 4,4'-diketocarotenoids, ast axanthin and canthaxanthin was less. Carotenoids incorporated into ord ered membrane systems (EYPC liposomes) displayed different protective efficacies. Zeaxanthin and beta-cryptoxanthin were more effective than beta,beta-carotene against oxidation initiated in the aqueous and lip id phases. Astaxanthin and canthaxanthin afforded less protection to t he Liposomal lipids. Lycopene was destroyed most rapidly but was least effective as an antioxidant. Located in the hydrophobic inner core of the bilayer, the hydrocarbons lycopene and beta,beta-carotene would n ot be in a position to readily intercept free-radicals entering the me mbrane from the aqueous phase. Carotenoids with polar end groups span the bilayer with their end groups located near the hydrophobic-hydroph illic interface where free-radical attack from AAPH first occurs. Hydr ogen abstraction from C-4 may be one of the mechanisms of carotenoid a ntioxidant activity in this system. The chemical reactivity of a carot enoid is not the only factor that determines its ability to protect me mbranes against oxidation. The position and orientation of the caroten oid in the bilayer is also of importance. (C) 1997 Elsevier Science B. V.