Antioxidant ß-carotene does not quench singlet oxygen in mammalian cells.

Carotenoids, and ß-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 ß-carotene efficiently quenches singlet oxygen in solution-phase systems is invariably invoked when explaining the biological antioxidative properties of ß-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 ß-carotene. This observation requires a re-evaluation of ß-carotene's role as an antioxidant in mammalian systems and now underscores the importance of mechanisms by which ß-carotene inhibits radical reactions.