Do carotenoids serve as transmembrane radical channels?

Carotenoids are a diverse group of terpenoid pigments that originated in prokaryotes over 3 billion years ago. Their primary function in plants is to serve as photomodulators of the oxidizing side of Photosystem II. In animals, which must acquire carotenoids from their diets, carotenoids serve a host of functions and are viewed primarily as efficient scavengers of singlet oxygen and radicals within the domain of membranes where they reside. Recently it has been demonstrated that carotenoids react cooperatively and synergistically with vitamin C and E, serving to regenerate a pro-oxidant radical carotenoid after the antioxidant reduction of a radical species. The exact location and behavior of carotenoids within biological membrane systems remain largely unknown. A hypothesis is proposed suggesting that carotenoids may serve as transmembrane radical channels. In this capacity carotenoids may reduce radicals in one biological compartment, while simultaneously being reduced in another. The benefit of rapid radical quenching across membrane compartments by transmembrane-spanning carotenoids such as zeaxanthin and lutein may be especially advantageous to intra- and extracellular redox control.