Plasma membrane reductase that uses cytoplasmic ascorbate as an electron donor to reduce extracellular Fe(3+) into Fe(2+). Probably functions in dietary iron absorption at the brush border of duodenal enterocytes by producing Fe(2+), the divalent form of iron that can be transported into enterocytes. It is also able to reduce extracellular monodehydro-L-ascorbate and may be involved in extracellular ascorbate regeneration by erythrocytes in blood. May also act as a ferrireductase in airway epithelial cells (By similarity). May also function as a cupric transmembrane reductase (By similarity). Fe(3+)(out) + L-ascorbate(in) = Fe(2+)(out) + H(+) + monodehydro-L-ascorbate radical(in) Cu(2+)(out) + L-ascorbate(in) = Cu(+)(out) + H(+) + monodehydro-L-ascorbate radical(in) L-ascorbate(in) + monodehydro-L-ascorbate radical(out) = L-ascorbate(out) + monodehydro-L-ascorbate radical(in) Binds 2 heme b groups non-covalently. Homodimer. Localized at the brush border of duodenal cells. Highly expressed in all regions of the small intestine and colon studied in suckling animals. However, after weaning, when iron absorption declines significantly, strong expression is retained only in the duodenum. Also expressed in respiratory epithelium.