Cytochromes and in membrane vesicles from aerobically grown were readily reduced by succinate; one cytochrome, which we propose should be called cytochrome , reacted with CO in the Fe(ll) state to give a photodissociable CO adduct. The photodissociation spectrum (photolysed pre-photolysis) at sub-zero temperatures had a relatively high γ/α absorbance ratio, indicating a high-spin haem, which, in the reduced state, probably contributes little to the sharp α absorbance of the oxidase complex in membranes. Reaction with oxygen of the unliganded high-spin haem between −132 °C and −95 °C following photolytic activation gave a product that is identified as the oxygenated form, being spectrally similar to, but not identical with, the CO adduct. In membranes, the forward velocity constant at −95 °C was 61 M S, and the dissociation constant was 1·6 × 10 M O, as it is in intact cells. These data clearly distinguish the oxygen-trapping strategy of the cytochrome ' in this oxidase from that of cytochrome and also suggest that the presence of the soluble flavohaemoglobin (Hmp) in intact cells is without effect on such measurements of the primary oxygen reaction. In view of recent findings that this oxidase complex contains predominantly one mole of haem O and one of haem B, a revised nomenclature for the oxidase complex is proposed, namely, cytochrome '.


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