Plasma membrane reductase that uses cytoplasmic ascorbate as an electron donor to reduce extracellular Fe(3+) into Fe(2+) (PubMed:17068337). 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 (PubMed:11230685, PubMed:12547225). It is also able to reduce extracellular monodehydro-L-ascorbate and may be involved in extracellular ascorbate regeneration by erythrocytes in blood (PubMed:17068337). May also act as a ferrireductase in airway epithelial cells (By similarity). May also function as a cupric transmembrane reductase (PubMed:18498772). 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 the brush-border membrane of duodenal enterocytes (at protein level). Also expressed in liver and spleen. By iron deficiency. Up-regulated in duodenal mucosa of mice lacking transferrin or Hfe (at protein level). Mice are normal and do not display iron stores defects, even in the setting of iron deficiency. These results, reported by PubMed:15961514, suggest that Cybrd1 is not an essential component of intestinal iron apparatus. However, according to PubMed:16326980, no direct measurements of iron absorption were made by PubMed:15961514, suggesting that final conclusions can be drawn only when direct iron absorption studies are carried out or mice are maintained on a diet containing ferric iron only.