In vivo metal substitution in Bacteroides fragilis superoxide dismutase

Bacteroides fragilis, an obligate anaerobe, synthesizes an azide-inhibitable iron-containing superoxide dismutase when grown in complex medium. Cells grown anaerobically in complex media containing desferrioxamine (Desferal, Ciba-Geigy) and graded concentrations of Mn synthesize the azide-resistant manganese-containing SOD. The fraction of MnSOD activity in dialyzed cell extracts increased progressively as the Mn concentration in the medium increased. The fraction of MnSOD activity also increased in extracts of cells grown in the medium with 1 mM Mn but with graded concentrations of desferrioxamine (0-10 micromolar). The SOD activity in the cells grown under the various conditions varied but not in a causal relationship with either Mn or desferrioxamine concentration. Electrophoresis revealed that the SOD activity in cells grown in the absence or presence of 1 mM Mn migrated with the same relative mobility and exhibited identical activity patterns when examined separately or as a mixture. These data are consistent with substitution of Mn for Fe in the B. fragilis apoprotein under anaerobic conditions and support the model of a single protein binding either Fe or Mn.