The superoxide dismutase activity of desulfoferrodoxin from Desulfovibrio desulfuricans ATCC 27774

Desulfoferrodoxin (Dfx), a small iron protein containing two mononuclear ir on centres (designated centre I and II), was shown to complement superoxide dismutase (SOD) deficient mutants of Escherichia coli [Pianzzola, M.J., So ubes M. & Touati, D. (1996) J. Bacteriol. 178, 6736-6742]. Furthermore, nee laredoxin, a protein from Desulfovibrio gigas containing an iron site simil ar to centre II of Dfx, was recently shown to have a significant SOD activi ty [Silva, G., Oliveira, S., Gomes, C.M., Pacheco, I., Liu, M.Y., Xavier, A .V., Teixeira, M., Le Gall, J. & Rodrigues-Pousada, C. (1999) Eur. J. Bioch em. 259, 235-243]. Thus, the SOD activity of Dfx isolated from the sulphate -reducing bacterium Desulfovibrio desulfuricans ATCC 27774 was studied. The protein exhibits a SOD activity of 70 U.mg(-1), which increases approximat ely 2.5-fold upon incubation with cyanide. Cyanide binds specifically to Df x centre II, yielding a low-spin iron species with g-values at 2.27 (g(perp endicular to)) and 1.96 (g(parallel to)). Upon reaction of fully oxidized D fx with the superoxide generating system xanthine/xanthine oxidase, Dfx cen tres I and II become partially reduced, suggesting that Dfx operates by a r edox cycling mechanism, similar to those proposed for other SODs. Evidence for another SOD in D. desulfuricans is also presented - this enzym e is inhibited by cyanide, and N-terminal sequence data strongly indicates that it is an analogue to Cu,Zn-SODs isolated from other sources. This is t he first indication that a Cu-containing protein may be present in a sulpha te-reducing bacterium.