  Desulfoferrodoxin is a small protein found in sulfate-reducing bacteria that
contains two independent mononuclear iron centers, one ferric and one ferrous.
Expression of desulfoferrodoxin from Desulfoarculus baarsii has been reported
to functionally complement a superoxide dismutase deficient Escherichia coli
strain. To elucidate by which mechanism desulfoferrodoxin could substitute for
superoxide dismutase in E. coli, we have purified the recombinant protein and
studied its reactivity toward O-(2). Desulfoferrodoxin exhibited only a weak
superoxide dismutase activity (20 units mg(-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 reduce O-(2), with a rate constant of 6-7 x
10(8) M(-1) s(-1). In addition, we showed that membrane and cytoplasmic E. coli
protein extracts, using NADH and NADPH as electron donors, could reduce the
O-(2) oxidized form of desulfoferrodoxin. Taken together, these results
strongly suggest that desulfoferrodoxin behaves as a superoxide reductase
enzyme and thus provide new insights into the biological mechanisms designed
for protection from oxidative stresses.
