Antioxidant reactions of vitamin E in rat liver

Abstract

Antioxidant reactions of vitamin E (α-tocopherol, α-TH) were studied by examining the fate of α-TH during oxidative challenge to isolated rat liver mitochondria and isolated perfused rat liver. The overall goal of this dissertation was to identify the products of antioxidant reactions of α-TH and to determine the relationship between the status of α-TH, lipid peroxidation, and mitochondrial function. In isolated mitochondria, products of α-TH oxidation induced by the radical initiator 2,2'-azobis(2-amidinopropane) dihydrochloride (ABAP) were α-tocopherolquinone (α-TQ), α-tocopherolquinone-2,3-oxide (α-TQE1), and α-tocopherolquinone-5,6-oxide (α-TQE2). ABAP induced lipid peroxidation after 50% of the initial α-TH was depleted and decreased the respiratory control ratio and state 3 and state 4 respiration. In isolated perfused rat liver, the principal products of α-TH oxidation induced by tert-butylhydroperoxide (t-BuOOH) were α-TQ and epoxyquinones α-TQE1/α-TQE2, which were formed from acid-labile 8a-substituted tocopherones and epoxyhydroperoxytocopherones, respectively. t-BuOOH increased gluconeogenesis and decreased ketogenesis, as indicated by increases in lactate and pyruvate and decreases in acetoacetate (AA) and β-hydroxybutyrate β-HBA) in the effluent. In addition, there was a shift to a more oxidized state in mitochondria, as evidenced by a decrease in the β-HBP/AA ratio, a measure of NADH/NAD⁺ ratio in the mitochondria. t-BuOOH increased lipid peroxidation, which was measured by thiobarbituric acid-reactive substances in the effluent and decreased the mitochondrial respiratory control ratio. Dietary supplementation with α-TH increased α-TH levels 7-10 fold, decreased hepatic lipid peroxidation and reversed the mitochondrial uncoupling effects of t-BuOOH, but did not effect changes in metabolism. These studies provide the first comprehensive description of the oxidative turnover of vitamin E in a biological system.