Selenite is a substrate for calf thymus thioredoxin reductase and thioredoxin and elicits a large non-stoichiometric oxidation of NADPH in the presence of oxygen.
The thioredoxin system, comprising NADPH, thioredoxin reductase and thioredoxin reduces protein disulfides via redox-active dithiols. We have discovered that sodium selenite is a substrate for the thioredoxin system; 10 microM selenite plus 0.05 microM calf thymus thioredoxin reductase at pH 7.5 caused a non-stoichiometric oxidation of NADPH (100 microM after 30 min). In contrast, thioredoxin reductase from Escherichia coli showed no direct reaction with selenite, but addition of 3 microM E. coli thioredoxin also resulted in non-stoichiometric oxidation of NADPH, consistent with oxidation of the two active-site thiol groups in thioredoxin to a disulfide. Kinetically, the reaction was complex with a lag phase at low selenite concentrations. Under anaerobic conditions the reaction stopped after 1 mol selenite had oxidized 3 mol NADPH; the admission of air then resulted in continued consumption of NADPH consistent with autooxidation of selenium intermediate(s). Ferricytochrome c was effectively reduced by calf thymus thioredoxin reductase and selenite in the presence of oxygen. Selenite caused a strong dose-dependent inhibition of the formation of thiol groups from insulin disulfides with either the E. coli or calf-thymus thioredoxin system. Thus, under aerobic conditions selenite catalyzed, NADPH-dependent redox cycling with oxygen, a large oxygen-dependent consumption of NADPH and oxidation of reduced thioredoxin inhibiting its disulfide-reductase activity.
Kumar
S
S
Department of Biochemistry, Medical Nobel Institute, Stockholm, Sweden.
Björnstedt
M
M
Holmgren
A
A
D000693:Anaerobiosis; D000818:Animals; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism; D004220:Disulfides / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology; D007328:Insulin / Q000378:metabolism; D007700:Kinetics; D009249:NADP / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D012643:Selenium / Q000378:metabolism*; D018038:Sodium Selenite; D013880:Thioredoxin-Disulfide Reductase / Q000378:metabolism*; D013879:Thioredoxins / Q000378:metabolism*; D013950:Thymus Gland / Q000201:enzymology
D003574:Cytochrome c Group; D004220:Disulfides; D007328:Insulin; D013879:Thioredoxins; D009249:NADP; D013880:Thioredoxin-Disulfide Reductase; D012643:Selenium; D018038:Sodium Selenite; D010100:Oxygen