title: Reduction of dehydroascorbate to ascorbate by the selenoenzyme thioredoxin reductase.
abstract: Recycling of ascorbate from its oxidized forms is essential to maintain stores of the vitamin in human cells. Whereas reduction of dehydroascorbate to ascorbate is thought to be largely GSH-dependent, we reconsidered the possibility that the selenium-dependent thioredoxin system might contribute to ascorbate regeneration. We found that purified rat liver thioredoxin reductase functions as an NADPH-dependent dehydroascorbate reductase, with an apparent Km of 2. 5 mM for dehydroascorbate, and a kcat of 90 min-1. Addition of 2.8 microM purified rat liver thioredoxin lowered the apparent Km to 0.7 mM, without affecting the turnover (kcat of 71 min-1). Since thioredoxin reductase requires selenium, we tested the physiologic importance of this enzyme for dehydroascorbate reduction in livers from control and selenium-deficient rats. Selenium deficiency lowered liver thioredoxin reductase activity by 88%, glutathione peroxidase activity by 99%, and ascorbate content by 33%, but did not affect GSH content. NADPH-dependent dehydroascorbate reductase activity due to thioredoxin reductase, on the basis of inhibition by aurothioglucose, was decreased 88% in dialyzed liver cytosolic fractions from selenium-deficient rats. GSH-dependent dehydroascorbate reductase activity in liver cytosol was variable, but typically 2-3-fold that of NADPH-dependent activity. These results show that the thioredoxin system can reduce dehydroascorbate, and that this function is required for maintenance of liver ascorbate content.
authors.affiliation: Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-6303, USA. james.may@mcmail.vanderbilt.edu
authors.forename: J M
authors.initials: JM
authors.lastname: May
authors.forename: S
authors.initials: S
authors.lastname: Mendiratta
authors.forename: K E
authors.initials: KE
authors.lastname: Hill
authors.forename: R F
authors.initials: RF
authors.lastname: Burk
mesh_terms: D000818:Animals; D001205:Ascorbic Acid / Q000096:biosynthesis*; D003683:Dehydroascorbic Acid / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000201:enzymology / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D051381:Rats; D012643:Selenium / Q000172:deficiency; D013880:Thioredoxin-Disulfide Reductase / Q000378:metabolism*; D013879:Thioredoxins / Q000378:metabolism
chemical_list: D013879:Thioredoxins; D009249:NADP; D013880:Thioredoxin-Disulfide Reductase; D012643:Selenium; D001205:Ascorbic Acid; D003683:Dehydroascorbic Acid