Reaction of the desulfoferrodoxin from Desulfoarculus baarsii with superoxide anion - Evidence for a superoxide reductase activity

Desulfoferrodoxin is a small protein found in sulfate-reducing bacteria tha t contains two independent mononuclear iron centers, one ferric and one fer rous. Expression of desulfoferrodoxin from Desulfoarculus baarsii has been reported to functionally complement a superoxide dismutase deficient Escher ichia coli strain. To elucidate by which mechanism desulfoferrodoxin could substitute for superoxide dismutase in E. coli, we have purified the recomb inant protein and studied its reactivity toward O-2(radical anion). Desulfo ferrodoxin exhibited only a weak, superoxide dismutase activity (20 units m g(-1)) that could hardly account for its antioxidant properties. UV-visible and electron paramagnetic resonance spectroscopy studies revealed that the ferrous center of desulfoferrodoxin could specifically and efficiently red uce O-2(radical anion), with a rate constant of 6-7 x 10(8) M-1 s(-1). In a ddition, we showed that membrane and cytoplasmic E. coli protein extracts, using NADH and NADPH as electron donors, could reduce the O-2(radical anion ) oxidized form of desulfoferrodoxin. Taken together, these results strongl y suggest that desulfoferrodoxin behaves as a superoxide reductase enzyme a nd thus provide new insights into the biological mechanisms designed for pr otection from oxidative stresses.