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 (PubMed:3597367, PubMed:1623014, PubMed:18501187). 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 (Probable). 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. Expressed in the adrenal medulla and all brain regions, but not in visceral organs. Extended N-terminus.