THE ROLE OF IRON CHELATORS AND OXYGEN IN THE REDUCED NICOTINAMIDE ADENINE-DINUCLEOTIDE PHOSPHATE-CYTOCHROME P450 OXIDOREDUCTASE-DEPENDENT CHROMIUM(VI) REDUCTION

Chromium(VI) reduction was studied in a system composed of reduced nicotinamide adenine dinucleotide phosphate-cytochrome P450 oxidoreductase (NADPH-P450 reductase) and different iron chelators and iron sources, In an aerobic phosphate buffer containing iron(II), chromium(VI) was not reduced by the Fe2+ probably because of spontaneous autoxidation of Fe2+, but freshly made Fe2+, added directly to a Cr-VI-containing buffer, reduced Cr-VI, Under anaerobic conditions, iron(II) reduced chromium(VI) stoichiometrically. A system containing ethylenediaminetetraacetic acid (EDTA)-Fe3+, NADPH-P450 reductase and NADPH effectively reduced chromium(VI) anaerobically, Under aerobic conditions this reaction was inhibited by about 45%, Adenosine diphosphate (ADP)-Fe3+, which is a poor acceptor of electrons from NADPH-P450 reductase, reduced chromium(VI) only marginally, Mannitol slightly increased the aerobic Cr-VI reduction, Addition of superoxide dismutase and catalase, which both regenerate some O-2, led to inhibition of Cr-VI reduction. Ferritin, NADPH-P450 reductase and the iron chelators, EDTA and citrate, reduced Cr-VI, indicating mobilization of Fe2+ from ferritin, Low levels of EDTA (55 pmol l(-1)) and citrate (100 mu mol l(-1)) in contrast to high levels (5 mmol l(-1)) did not increase Cr-VI reduction in microsomes, Using 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid buffer instead of phosphate buffer, the Cr-VI-reducing activity was increased.