Cytochrome b-561 (CYB561)

Cytochrome b561 is a major transmembrane protein that is specific to catecholamine and neuropeptide secretory vesicles of the adrenal medulla, pituitary gland, and other neuroendocrine tissues. This 30-kD cytochrome is present in both the small synaptic vesicles and the large dense core vesicles (chromaffin granules) of the tissues. Its role is to supply reducing equivalents to 2 monooxygenases, dopamine beta-hydroxylase  in chromaffin granules and peptidylglycine alpha-amidating monooxygenase in neurosecretory vesicles.

The cytochrome fulfills this role by catalyzing the transfer of electrons from a cytoplasmic donor, ascorbate, across a phospholipid bilayer to the luminal acceptor, semidehydroascorbate, in the interior of the vesicles.