Cytochrome b561, ascorbic acid, and transmembrane electron transfer.

Cytochrome b561 is a transmembrane protein unique to neuroendocrine secretory vesicles. It acts as an electron channel and mediates equilibration of ascorbate-semidehydroascorbate inside the secretory vesicle with the ascorbate redox pair in the cytoplasm. The role for this function is to regenerate ascorbate inside the secretory vesicle for use by monooxygenases. Elucidation of the structure and mechanism of redox activity of cytochrome b561 may demonstrate paradigms for other ascorbate-utilizing enzymes as well as provide insights into long-range biological electron transfer.