Free radicals, myocytes and reperfusion injury.

established, the mechanism by which /?-carotene exerts a protective function against cancer remains unknown. However, there are several lines of evidence which suggest that the generation of reactive oxygen species may play an important role in the development of cancer [14]. The present work emphasizes that attention should be extended from /?-carotene to lycopene and other carotenoids. Lycopene has a plasma concentration slightly higher than /?-carotene and both these carotenoids were found in lowdensity lipoproteins [ IS]. The relative physical quenching abilities of the tocopherol homologues decreased in the following order: a, /?, y, and &tocopherol. With the tocopherols, the ability of ‘02 quenching depends on a free hydroxyl group in position 6 of the chromane ring. Chemical reactivity of the tocopherol homologues were low, accounting for 0.1 to 1.5”/0 of the physical quenching. Among the biological thiols, cysteine was the most effective quencher of lo,, followed by lipoate (disulphide form of the dithiol lipoate), coenzyme A, glutathione, cysteamine and dihydrolipoate. Pharmacologically active thiols like Nacetylcysteine, mesna, WR1065 and captopril significantly differed in their quenching abilities. The pD dependence of the chemical quenching indicated that ‘0, reacts with the thiolate anion. As compared to their overall quenching abilities, cysteine, WR1065, cysteamine, mesna and dihydrolipoate had significant chemical quenching abilities. other thiols tested had a chemical quenching rate less than S%, most of them less than I%, of their overall quenching ability. Compared to carotenoids, other classes of compounds, e.g. bilirubin, tocopherols and thiols were less active in singlet oxygen quenching. But these may also be biologically important in ’02 quenching because of their higher concentration and/or different subcellular location in biological targets, besides solubility characteristics.