Peroxidase-like activity of cytochrome b5 is triggered upon hemichrome formation in alkaline pH

In alkaline media (pH 12) a catalytic peroxidase activity of cytochrome b5 was found associated to a different conformational state. Upon incubation at this pH, cytochrome b5 electronic absorption spectrum was altered, with disappearance of characteristic bands of cytochrome b5 at pH 7.0. The appearance of new electronic absorption bands and EPR measurements support the formation of a cytochrome b5 class B hemichrome with an acquired ability to bind polar ligands. This hemichrome is characterized by a negative formal redox potential and the same folding properties than cytochrome b5 at pH 7. The acquired peroxidase-like activity of cytochrome b5 found at pH 12, driven by a hemichrome formation, suggests a role of this protein in peroxidation products propagation.