The superoxide dismutase activity of desulfoferrodoxin from Desulfovibrio desulfuricans ATCC 27774

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Abstract
Desulfoferrodoxin (Dfx), a small iron protein containing two mononuclear iron centres (designated centre I and II), was shown to complement superoxide dismutase (SOD) deficient mutants of Escherichia coli[Pianzzola, M.J., Soubes M. & Touati, D. (1996) J. Bacteriol.178, 6736–6742]. Furthermore, neelaredoxin, a protein from Desulfovibrio gigas containing an iron site similar to centre II of Dfx, was recently shown to have a significant SOD activity [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. Biochem. 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 approximately 2.5‐fold upon incubation with cyanide. Cyanide binds specifically to Dfx centre II, yielding a low‐spin iron species with g‐values at 2.27 (g⊥) and 1.96 (g∥). Upon reaction of fully oxidized Dfx with the superoxide generating system xanthine/xanthine oxidase, Dfx centres I and II become partially reduced, suggesting that Dfx operates by a redox cycling mechanism, similar to those proposed for other SODs. Evidence for another SOD in D. desulfuricans is also presented – this enzyme 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 the first indication that a Cu‐containing protein may be present in a sulphate‐reducing bacterium.

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