Selenite incubated with NADPH and mammalian thioredoxin reductase yields selenide, which inhibits lipoxygenase and changes the electron spin resonance spectrum of the active site iron

Selenite and selenodiglutathione (GS-Se-SG) efficiently inhibited 5-lipoxygenase activity in sonicates of human monoclonal B-lymphocytes. The apparent IC50 of GS-Se-SG was 0.5 mu M. The inhibitory effect of these compounds was observed within 10 min of incubation. In order to elucidate if the mechanism of inhibition by these compounds was result of direct interference with lipoxygenase or indirectly mediated by cellular factors, pure 15-lipoxygenase from soybeans was used as a model system for enzyme assays and electron spin resonance (ESR) measurements. Incubation of 15-lipoxygenase with a mixture of human placenta thioredoxin reductase (TR) or calf-thymus TR, selenite, and NADPH blocked the activity of the enzyme. Neither TR and NADPH nor selenite inhibited soybean lipoxygenase when incubated separately. These results suggest that selenite must be reduced to selenide in order to inhibit 5- and 15-lipoxygenase activities. Preincubation anaerobically of 15-lipoxygenase with chemically generated selenide (6 mu M) resulted in a strong inhibition of activity, in assays with arachidonic acid in the presence of oxygen. In contrast, selenide exposed to air prior to preincubation did not inhibit the enzyme. Since selenide is known to be efficiently oxidized by oxygen and to form elemental selenium the results evidence that selenide was the inhibitor of lipoxygenase activity in the anaerobic preincubations. After incubation with TR, NADPH, and selenite or with chemically generated selenide, the ESR spectrum of 15-lipoxygenase changed: the dominant axial component with a peak at g = 6.1 decreased, and a rhombic form with a feature at g = 4.28 grew. The results suggest that selenide produced by the reduction of selenite reduces the active site iron to the ESR invisible state and changes the ligation geometry of the oxidized form.