Hemopexin joins transferrin as representative members of a distinct class of receptor-mediated endocytic transport systems.
Receptor-mediated transport of heme by hemopexin in vivo and in vitro results in catabolism of heme but not the protein, suggesting that intact apohemopexin recycles from cells. However, until now, the intracellular transport of hemopexin by receptor-mediated endocytosis remained to be established. Biochemical studies on cultured human HepG2 and mouse Hepa hepatoma cells demonstrate that hemopexin is transported to an intracellular location and, after endocytosis, is subsequently returned intact to the medium. During incubation at 37 degrees C, hemopexin accumulated intracellularly for ca. 15 min before reaching a plateau while surface binding was saturated by 5 min. No internalization of ligand took place during incubation at 4 degrees C. These and other data suggest that hemopexin receptors recycle, and furthermore, incubation with monensin significantly inhibits the amount of cell associated of heme-[125I]hemopexin during short-term incubation at 37 degrees C, consistent with a block in receptor recycling. Ammonium chloride and methylamine were less inhibitory. Electron microscopic autoradiography of heme-[125I]hemopexin showed the presence of hemopexin in vesicles of the classical pathway of endocytosis in human HepG2 hepatoma cells, confirming the internalization of hemopexin. Colloidal gold-conjugated hemopexin and electron microscopy showed that hemopexin bound to receptors at 4 degrees C is distributed initially over the entire cell surface, including microvilli and coated pits. After incubation at 37 degrees C, hemopexin-gold is located intracellularly in coated vesicles and then in small endosomes and multivesicular bodies. Colocalization of hemopexin and transferrin intracellularly was shown in two ways. Radioiodinated hemopexin was observed in the same subcellular compartment as horseradish peroxidase conjugates of transferrin using the diaminobenzidine-induced density shift assay. In addition, colloidal gold derivatives of heme-hemopexin and diferric transferrin were found together in coated pits, coated vesicles, endosomes and multivesicular bodies. Therefore, hemopexin and transferrin act by a similar receptor-mediated mechanism in which the transport protein recycles after endocytosis from the cell to undergo further rounds of intracellular transport.