Nitrite Reductases, Enzymes That Generate Nitric Oxide in Denitrifying Bacteria

DISSIMILATORY NITRITE REDUCTASES, ENZYMES THAT GENERATE NITRIC OXIDE IN DENITRIFYING BACTERIA 

Denitrifying bacteria produce one or the other of two usually soluble but very dissimilar nitrite reductases, heme- and Cu-containing types (Brittain et al., 1992 Coyne et al., 1989), both of which are found in the periplasmic space of gram-negative bacteria (Coyne et al., 1990). 

It is remarkable that this enzyme can accept both O2 and nitrite as its oxidizing substrate, and in fact it was originally isolated as an oxidase rather than a nitrite reductase. Its role in nitrite reduction became clear when it was realized that its synthesis was induced only under anaerobic conditions and its oxidase activity was relatively small compared with the total O2-reducing activity of cells (Lam and Nicholas, 1969 Yamanaka, 1964 Yamanaka and Okunuki, 1963a). 

Absorption spectra of oxidized and dithionite-reduced nitrite reductase from Pseudomonas aeruginosa. Solid line, oxidized enzyme dashed line, dithionite-reduced enzyme, in 40 mM potassium phosphate buffer, pH 6.9. The enzyme concentration was 8 fiM and the spectra were recorded at room temperature (20°C) under N in a Thunberg cuvette with a light-path of 1 cm. From Barber etal. (1976). 

Cytochrome cd reduces nitrite by one electron to NO as the major product, providing a reducing system was used that does not reduce NO chemically (Kim and Hollocher, 1983, 1984 Mancinelli et al., 1986 ParretaL, 1976 Silvestrini et al., 1979 Timkovich et al., 1982 Yamanaka and Okunuki, 1963a). There is only one rapid kinetic study of cytochrome cd, with nitrite as the oxidant (Sil-vestrini et al., 1990). This work, carried out at pH 8.0, indicated that there was intramolecular electron transfer, c d, as in the O2 reaction, but that the 

---