Abstract
Ferricytochrome b5 was found to convert oxyhaemoglobin into methaemoglobin under conditions previously found to be optimal for complex-formation between ferricytochrome b5 and methaemoglobin [Mauk & Mauk (1982) Biochemistry 21, 4730-4734]. As this reaction is completely inhibited by CO, it is proposed that oxyhaemoglobin is oxidized after O2 dissociation, as has been suggested for the oxidation of oxyhaemoglobin by inorganic complexes. From the present analysis, ferricytochrome b5 seems unlikely to contribute significantly to methaemoglobin formation in vivo. Nevertheless, this observation provides a relatively convenient means of investigating the mechanism by which these two proteins interact.