β-Carotene: an Unusual Type of Lipid Antioxidant

TLDR

The review discusses lipid peroxidation mechanisms and notes that β‑carotene, a proposed anticancer agent, has been suggested to act as a radical‑trapping antioxidant, yet experimental evidence has been lacking, especially under the low oxygen partial pressures typical of most tissues, a behavior that may also apply to other conjugated compounds. In vitro experiments reveal that β‑carotene is a novel biological antioxidant that scavenges radicals only below ~150 torr oxygen, while at higher oxygen pressures it loses antioxidant activity and becomes a prooxidant, particularly at high concentrations.

Abstract

The mechanism of lipid peroxidation and the manner in which antioxidants function is reviewed. β-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 β-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, β-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.