Spectral Characteristics and Interconversions of the Reduced, Oxidized, and Oxygenated Forms of Purified Cytochrome o

Cytochrome o can be chemically reduced by excess dithionite and oxidized by ferricyanide and ammonium persulfate. The latter reagent is colorless and allows spectral observations in the Soret region. The spectrum of the form of the reduced cytochrome, which has absorption maxima at 576, 543, and 414 nm, is elicited in aerobic solution by enzymatic reduction with NADH, and when formed in this way the oxygenated form is very stable. When the solution is made anaerobic, the spectrum of the reduced (non-oxygenated) form appeared. The reduced form becomes spectrophotometrically detectable when the dissolved oxygen falls below 14 µm and is at half-maximal concentration at 6 µm oxygen. The reintroduction of oxygen results in reappearance of the spectrum of the oxygenated form. The addition of ammonium persulfate or sodium dithionite to the oxygenated form produced the oxidized or reduced cytochrome, respectively. Bubbling carbon monoxide through solutions of the oxygenated form shifted the absorption maxima to 566, 535, and 419 nm, which are the absorption maxima characteristic of the reduced cytochrome-carbon monoxide complex. Cyanide, which can bind to the oxidized form of cytochrome o, had no effect on the spectrum of the oxygenated form, but the formation of this form from oxidized cytochrome o and NADH was inhibited in the presence of cyanide. Hydrogen peroxide forms a complex with cytochrome o that has a spectrum similar to the spectrum of the oxygenated form.