Transmembrane reductase that may use ascorbate as an electron donor in the cytoplasm and transfer electrons across endoplasmic reticulum membranes to reduce monodehydro-L-ascorbate radical and iron cations Fe(3+) in the lumen of that compartment. L-ascorbate(in) + monodehydro-L-ascorbate radical(out) = L-ascorbate(out) + monodehydro-L-ascorbate radical(in) Fe(3+)(out) + L-ascorbate(in) = Fe(2+)(out) + H(+) + monodehydro-L-ascorbate radical(in) Binds 2 heme b groups non-covalently. E(0) is +141 +/-9 mV for the high potential heme and +43 +/-9 mV for the low potential heme. Highly expressed in the brain, lung, liver, and kidney (PubMed:17938141). Moderately expressed in the heart, placenta, skeletal muscle, and pancreas (PubMed:17938141).