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

Abstract

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

GUN4-Protoporphyrin IX Is a Singlet Oxygen Generator with Consequences for Plastid Retrograde Signaling

“…In this model, an O 2 (a 1 ⌬ g ) signal is generated from GUN4-PPIX, and possibly oxidatively damaged ChlH-PPIX, when these molecules interact directly with a singlet oxygen-sensing system such as the membrane-associated EXECUTER 1 and 2 system. Due to the reactivity and short half-life of O 2 (a 1 ⌬ g ), the singlet oxygen producer must be in close proximity (10 -20 nm) to the singlet oxygen sensor (2,41,42). The proximity dependence is limited by the lifetime of singlet oxygen, which is ϳ1.6 s in vivo, with three times the lifetime corresponding to a root mean square radial diffusion distance of less than 20 nm (2, 41, 42).…”

Singlet oxygen and natural substrates: functional polyunsaturated models for the photooxidative degradation of carotenoids

“…was recently challenged for β-carotene [8]. Physical and chemical quenching constants were determined for numerous carotenoids and apocarotenoids and range from 10 8 to 10 10 M −1 s −1 [9].…”

Reactive oxygen species: Reactions and detection from photosynthetic tissues

“…In this respect, the finding that added β-carotene protects mammalian cells against 1 O 2 -induced cell death but does not shorten the lifetime of 1 O 2 [77] is highly interesting. The high viscosity of the cellular environment can partly explain the lack of an effect, as a three-fold increase in viscosity from C 6 H1 4 to CCl 4 is associated with a 30 % decrease of the quenching constant of 1 O 2 by β-carotene [48].…”