Islet secretory granules contain cytochrome b₅₆₁

A cytochrome has been detected in secretory granules prepared from anglerfish islets of Langerhans. The heme moiety was determined to be of the b type, and the dithionite-reduced cytochrome exhibited an α-band maximum at 561 nm with an extinction coefficient of 13.8 mM^-1 · cm^-1. The protein was present at a concentration of 40 ± 4 pmol/mg of secretory granule protein. The cytochrome was found to be an integral membrane protein and to be reduced by ascorbic acid but not by NADH, NADPH, reduced glutathione (GSH), or succinate. Because of the similarity to previously characterized secretory granule cytochrome b₅₆₁'s from neuroendocrine tissues, this cytochrome is also referred to as cytochrome b₅₆₁. Although its function has not yet been elucidated, the apparent specificity for ascorbate suggests that it may be a component of the ascorbate-dependent peptidyl-glycine α-amidating monooxygenase system that functions in the amidation of islet hormones.