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

Abstract

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 μM selenite plus 0.05 μM calf thymus thioredoxin reductase at pH 7.5 caused a non‐stoichiometric oxidation of NADPH (100 μM after 30 min). In contrast, thioredoxin reductase from Escherichia coli showed no direct reaction with selenite, but addition of 3 μM 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 thioredoxine 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.