Carotenoids as scavengers of free radicals in a Fenton reaction: Antioxidants or pro-oxidants?

The spin trapping EPR technique was used to study the influence of caroteno ids (beta -carotene, 8'-apo-beta -caroten-8'-al, canthaxanthin, and ethyl 8 '-apo-beta -caroten-8'-oate) on the yield of free radicals in the Fenton re action (Fe2+ + H2O2 --> Fe3+ + (OH)-O-. + -OH) in the organic solvents, DMS O, and methanol. DMPO and PBN were used as spin trapping agents. It was dem onstrated that carotenoids could increase or decrease the total yield of fr ee radicals depending on the oxidation potential of the carotenoids and the nature of the radicals. A reaction mechanism is suggested which includes t he reduction of Fe3+ to Fe2+ by carotenoids. The effectiveness of this caro tenoid-driven Fenton reaction increases with a decrease of the scavenging r ates for free radicals and with decreasing oxidation potentials of caroteno ids. (C) 2001 Elsevier Science Inc.