Cytochrome oxidase o has been isolated from the obligately aerobic, methylotrophic bacterium Methylophilus methylotrophus in the form of a cytochrome c L-o complex. The latter is comprised of cytochrome c L (M r 21 000) and cytochrome o (M r 29 000) in a 1–2:1 ratio, possibly in association with one or more minor polypeptides; the complex exhibits a high ascorbate-TMPD oxidase activity which is inhibited non-competitively by cyanide (K i ≈ 2 μM). In contrast, the oxidation of methanol by whole cells is inhibited uncompetitively by cyanide (K i≈4 μM), thus indicating the involvement in methanol oxidation of cytochrome oxidase aa 3 rather than o.
