Nitrite Reductase from the Magnetotactic Bacterium Magnetospirillum magnetotacticum

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Abstract
Cytochrome cd1 nitrite reductase was isolated from magnetite-containing cells of the magnetotactic bacterium Magnetospirillum (formerly Aquaspirillum) magnetotacticum, which was microaerobically cultivated under denitrifying conditions. The enzyme showed absorption maxima at 643 nm and 409 nm in the oxidized form, and at 663, 551, 522, and 418 nm in the reduced form. A distinctive split absorption band did not occur at about 550 nm. The pyridine ferrohemochrome spectra suggested the presence of heme c and heme d1 in the molecule. The enzyme was composed of two identical subunits each with a molecular mass of 54 kDa; each subunit contained one c-type and one d-type heme. The isoelectric point was 9.2. The redox potentials of heme c and heme d1 were estimated to be +191 mV and +180 mV, respectively. Although the enzyme showed cyanide-sensitive N,N,N',N'-tetramethyl-p-phenylenediamine-O2 oxidoreductase activity and N,N,N',N'-tetramethyl-p-phenylenediamine-nitrite oxidoreductase activity, the enzyme did not oxidize M. magnetotacticum ferrocytochrome c-550 and Pseudomonas aeruginosa ferrocytochrome c-551 in the presence of nitrite. Furthermore, sodium succinate did not cause the reduction of cytochrome cd1 in the crude cell-free extract prepared from the magnetite-containing bacterial cells. However, M. magnetotacticum cytochrome cd1 showed a novel Fe(II):nitrite oxidoreductase activity whereas P. aeruginosa cytochromes cd1 had no Fe(II):nitrite oxidoreductase activity. These results suggest that M. magnetotacticum cytochrome cd1 may function as a Fe(II)-oxidizing enzyme under microaerobic conditions using nitrite as electron acceptor.