Recombinant expression and functional characterization of human hephaestin: A multicopper oxidase with ferroxidase activity

Human hephaestin (Hp) is a transmembrane protein that has been implicated in duodenal iron export. Mutations in the murine hephaestin gene (sla) produce microcytic, hypochromic anemia that is refractory to oral iron therapy. Hp shares similar to 50% sequence identity with the plasma multicopper ferroxidase ceruloplasmin including conservation of residues involved in disulfide bond formation and metal coordination. On the basis of this similarity to ceruloplasmin, human hephaestin may also bind copper and possess ferroxidase activity. To test this hypothesis, human hephaestin cDNA has been cloned by reverse transcription of human duodenal mRNA. Following in vitro mutagenesis to make the encoded polypeptide suitable for expression and purification, the hephaestin cDNA was cloned into the expression vector pNUT and introduced into baby hamster kidney cells. After selection with methotrexate, the baby hamster kidney cells secreted the recombinant human hephaestin into the medium at a level of similar to 2 mg/L. Purification was achieved by a single immunoaffinity chromatography step. As judged by SDS-PAGE, N-terminal sequence analysis, and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry, the purified hephaestin was homogeneous with a mass of 129600 Da, suggesting a carbohydrate content of 7.7%. Inductively coupled plasma mass spectrometry revealed that recombinant hephaestin contained an average of 3.13 atoms of copper per protein molecule. A visible absorption maximum was observed at 607 nm, consistent with the presence of a Type 1 copper site. By using ferrous ammonium sulfate as a substrate, recombinant hephaestin was shown to have ferroxidase activity with a K-m of 2.1 mu M for Fe(II). Lastly, urea PAGE showed that hephaestin was able to catalyze formation of diferric transferrin from Fe(II) and apotransferrin.