omega-Hydroxylation of alpha-tocopheryl quinone reveals a dual function for cytochrome P450-4F2 in vitamin E metabolism

alpha-Tocopherol (alpha-TOH) is the primary lipophilic radical trapping antioxidant in human tissues. Oxidative catabolism of alpha-tocopherol (alpha TOH) is initiated by omega-hydroxylation of the terminal carbon (C-13) of the isoprenoid sidechain followed by oxidative transformations that sequentially truncate the chain to yield the 2,5,7,8-tetramethyl(3' carboxyethyl)-6-hydroxychroman (alpha-CEHC). After conjugation to glucuronic acid, 3'-carboxyethyl-6-hydroxychroman glucuronide is excreted in urine. We report here that the same enzyme that accomplishes this task, the cytochrome P450 monooxygenase CYP-4F2, can also omega-hydroxylate the terminal carbon of alpha-tocopheryl quinone. A standard sample of omega-OH-alpha-tocopheryl quinone (omega-OH-alpha-TQ) was synthesized as a mixture of stereoisomers by allylic oxidation of alpha-tocotrienol using SeO2 followed by double-bond reduction and oxidation to the quinone. After incubating human liver microsomes or insect cell microsomes expressing only recombinant human CYP-4F2, cytochrome b5, and NADPH P450 reductase with d(6)-alpha-tocopheryl quinone (d(6)-alpha TQ), we showed that the omega-hydroxylated (13-OH) d(6)-alpha-TQ was produced. We further identified the production of the terminal carboxylic acid d(6)-13-COOH-alpha TQ. The ramifications of this discovery to the understanding of tocopherol utilization and metabolism, including the quantitative importance of the alpha TQ-omega-hydroxylase pathway in humans, are discussed.