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. Copyright © 1997 Elsevier Science Inc.