Antioxidant β-Carotene Does Not Quench Singlet Oxygen in Mammalian Cells

Carotenoids, and beta-carotene in particular, are important natural antioxidants. Singlet oxygen, the lowest excited state of molecular oxygen, is an intermediate often involved in natural oxidation reactions. The fact that beta-carotene efficiently quenches singlet oxygen in solution-phase systems is invariably invoked when explaining the biological antioxidative properties of beta-carotene. We recently developed unique microscope-based time-resolved spectroscopic methods that allow us to directly examine singlet oxygen in mammalian cells. We now demonstrate that intracellular singlet oxygen, produced in a photosensitized process, is in fact not efficiently deactivated by beta-carotene. This observation requires a re-evaluation of beta-carotene's role as an antioxidant in mammalian systems and now underscores the importance of mechanisms by which beta-carotene inhibits radical reactions.