Endogenous ascorbate regenerates vitamin E in the retina directly and in combination with exogenous dihydrolipoic acid

Vitamin E ($aL-tocopherol) is the major lipid-soluble antioxidant of retinal membranes whose deficiency causes retinal degeneration. Its antioxidant function is realized via scavenging peroxyl radicals as a result of which phenoxyl radicals of $aL-tocopherol are formed. Our hypothesis is that $aL-tocopherol phenoxyl radicals can be reduced by endogenous reductants in the retina, providing for $aL-tocopherol recycling. The results of this study demonstrate for the first time that: (i) endogenous ascorbate (vitamin C) in retinal homogenates and in rod outer segments is able to protect endogenous $aL-tocopherol against oxidation induced by UV-irradiation by reducing the phenoxyl radical of $aL-tocopherol, (ii) in the absence of ascorbate, neither endogenous nor exogenously added glutathione (GSH) is efficient in protecting $aL-tocopherol against oxidation; (iii) GSH does not substantially enhance the protective effect of ascorbate against $aL-tocopherol oxidation; (iv) exogenous dihydrolipoic acid (DHLA), although inefficient in direct reduction of the $aL-tocopherol phenoxyl radical, is able to enhance the protective effect of ascorbate by regenerating it from dehydroascorbate. Thus, regeneration of $aL-tocopherol from its phenoxyl radical can enhance its antioxidant effectiveness in the retina. The recycling of $aL-tocopherol opens new avenues for pharmacological approaches to enhance antioxidants of the retina.