Comparative mechanisms and rates of free radical scavenging by carotenoid antioxidants

• Published in: FEBS letters 1997-11, Vol.418 (1), p.91-97
• Main Authors: Mortensen, Alan, Skibsted, Leif H., Sampson, Julia, Rice-Evans, Catherine, Everett, Steven A.
• Format: Article
• Language: English
• Subjects: Antioxidants; beta Carotene - analogs & derivatives; beta Carotene - chemistry; Canthaxanthin - chemistry; Carotenoid; Carotenoids - chemistry; Free Radical Scavengers; Free Radicals; Kinetics; Lutein - chemistry; Lycopene; Nitrogen Dioxide - chemistry; Nitrogen dioxide radical; Pulse radiolysis; Spectrophotometry - methods; Sulphonyl radical; Thiyl radical; Xanthophylls; Zeaxanthins
• ISSN: 0014-5793; 1873-3468
• DOI: 10.1016/S0014-5793(97)01355-0

The comparative mechanisms and relative rates of nitrogen dioxide (NO 2 ⋅), thiyl (RS ⋅) and sulphonyl (RSO 2 ⋅) radical scavenging by the carotenoid antioxidants lycopene, lutein, zeaxanthin, astaxanthin and canthaxanthin have been determined by pulse radiolysis. All the carotenoids under study react with the NO 2 ⋅ radical via electron transfer to generate the carotenoid radical cation (Car ⋅+). In marked contrast the glutathione and 2-mercaptoethanol thiyl radicals react via a radical addition process to generate carotenoid-thiyl radical adducts [RS-Car] ⋅. The RSO 2 ⋅ radical undergoes both radical addition, [RSO 2-Car] ⋅ and electron abstraction, Car ⋅+. Both carotenoid adduct radicals and radical cations decay bimolecularly. Absolute rate constants for radical scavenging were in the order of ∼10 7–10 9 M −1 s −1 and follow the sequence HO(CH 2) 2S ⋅>RSO 2 ⋅>GS ⋅>NO 2 ⋅. Although there were some discernible trends in carotenoid reactivity for individual radicals, rate constants varied by no greater than a factor of 2.5. The mechanism and rate of scavenging is strongly dependent on the nature of the oxidising radical species but much less dependent on the carotenoid structure.