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.