β-Carotene: an Unusual Type of Lipid Antioxidant

Abstract

The mechanism of lipid peroxidation and the manner in which antioxidants function is reviewed. beta-Carotene is a purported anticancer agent, which is believed by some to have antioxidant action of a radical-trapping type. However, definitive experimental support for such action has been lacking. New experiments in vitro show that beta-carotene belongs to a previously unknown class of biological antioxidants. Specifically, it exhibits good radical-trapping antioxidant behavior only at partial pressures of oxygen significantly less than 150 torr, the pressure of oxygen in normal air. Such low oxygen partial pressures are found in most tissues under physiological conditions. At higher oxygen pressures, beta-carotene loses its antioxidant activity and shows an autocatalytic, prooxidant effect, particularly at relatively high concentrations. Similar oxygen-pressure-dependent behavior may be shown by other compounds containing many conjugated double bonds.

Retinol, α-tocopherol and carotenoids in diabetes

“…Although the precise mechanism of many of the complications of diabetes is unknown, it is increasingly believed that oxidative tissue damage may play a role in their pathogenesis. Beta-carotene has been reported to be a potent free radical quencher, singlet oxygen scavenger and lipid antioxidant (Burton & Ingold, 1984;Krinsky, 1993); it is possible that the reduced serum concentrations of b-carotene in diabetic patients may be due to its depletion as a sacri®ced antioxidant (Bendich & Olson, 1989). …”

Vertebrate and invertebrate carotenoid-binding proteins

“…Many different carotenoids can be metabolized to products with retinoid activity [6,7]; however, because of the presence of oxygenated groups in terminal ionone rings, most xanthophylls do not fit the structural requirements for provitamin A activity so they serve other non-vitamin A functions [3,6]. Along with carotenes, oxygenated carotenoids are efficient antioxidants that can quench singlet oxygen, reactive oxygen species, and various other free radicals that are by-products of metabolic process in cells or from environmental pollutants [8,9,10]. They also can function as photoprotectants against light-induced damage in heavily exposed tissues such as skin and the eye [11,12].…”

Antioxidant activities of carotenes and xanthophylls

“…The interaction of 13-carotene with peroxyl radicals generated during lipid peroxidation has been suggested to lead to formation of a radical adduct [[3-car...RO2]" [16]. Mechanisms of free radical scavenging have been investigated [17] by pulse radiolytic generation of nitrogen dioxide radicals (NO~), thiyl radicals (RS') and sulphonyl radicals (RSO~), which are rapidly scavenged by I~-carotene.…”