Plasma membrane reductase that uses cytoplasmic ascorbate as an electron donor to reduce extracellular Fe(3+) into Fe(2+) (PubMed:30272000). 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:30272000). 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 (Probable). 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. Activated by chelators like citrate, malate, and oxalate specially at alkaline pH. Homodimer. Localized at the brush border of duodenal cells. Present in erythrocyte membranes (at protein level). Also expressed in respiratory epithelium. By iron deficiency (at protein level). Genetic variations in CYBRD1 may act as modifier of iron overload expression and account for the variance observed in serum ferritin levels in patients with hereditary hemochromatosis.