Transmembrane reductase that uses ascorbate as an electron donor in the cytoplasm and transfers electrons across membranes to reduce iron cations Fe(3+) into Fe(2+) in the lumen of the late endosome and lysosome (PubMed:16911521). Reduced iron can then be extruded from the late endosome and lysosome to the cytoplasm by divalent metal-specific transporters (Probable). It is therefore most probably involved in endosomal and lysosomal cellular iron homeostasis (Probable). Fe(3+)(out) + L-ascorbate(in) = Fe(2+)(out) + H(+) + monodehydro-L-ascorbate radical(in) Binds 2 heme b groups non-covalently. Homodimer. Present in lung, spleen, thymus and testis. Present at low level in brain, heart, liver and kidney. Expressed in the alveolar macrophages of the lung, in the white pulp of the spleen, widespread in the thymus, and in the Sertoli cells of the testis (at protein level). At 17.5 dpc, it is primarily expressed in lung, spleen, thymus, testis, placenta, small intestine and stomach. N-glycosylated.