Antioxidant and prooxidant role of β-carotene in murine normal and tumor thymocytes: Effects of oxygen partial pressure

The effects of the partial pressure of oxygen (pO₂) on antioxidant efficiency of β-carotene in inhibiting radical-initiated lipid peroxidation were studied in murine normal and tumor thymocytes. At 150 mm Hg pO₂ (the pressure of oxygen in normal air), β-carotene acted as an antioxidant, inhibiting radical-induced lipid peroxidation in both normal and tumor thymocytes. At 760 mm Hg pO₂, β-carotene lost its antioxidant activity in normal thymocytes and exhibited a dose-dependent prooxidant effect in tumor thymocytes. In these cells, the prooxidant effect of β-carotene was also accompanied by an increase of endogenous α-tocopherol loss. β-Carotene radical-trapping and autooxidation reactions were faster at 760 mm Hg pO₂ than at 150 mm Hg pO₂ in both normal and tumor thymocytes and the carotenoid was more rapidly consumed in tumor cells. These data point out a key role of the oxygen tension on the antioxidant effectiveness of β-carotene. They also show a selective prooxidant effect of β-carotene under 100% oxygen in tumor cells.