Transmembrane reductase that uses ascorbate as an electron donor in the cytoplasm and transfers electrons across membranes to reduce monodehydro-L-ascorbate radical in the lumen of secretory vesicles (Probable). It is therefore involved the regeneration and homeostasis within secretory vesicles of ascorbate which in turn provides reducing equivalents needed to support the activity of intravesicular enzymes (By similarity). L-ascorbate(in) + monodehydro-L-ascorbate radical(out) = L-ascorbate(out) + monodehydro-L-ascorbate radical(in) Binds 2 heme b groups non-covalently. Secretory vesicle containing catecholamines and amidated peptides. Abundantly distributed in a number of neuroendocrine tissues. Mice lacking Cyb561 have significantly decreased amounts of norepinephrine and normetanephrine in the adrenal gland and brain while the biosynthesis of dopamine, the norepinephrine precursor, is normal pointing to a defect in the catecholamine biosynthesis downstream of dopamine probably at the level of the dopamine beta synthase.