Receptor-mediated heme uptake from hemopexin by human erythroleukemia K562 cells

Using human erythroleukemia K562 cells, existence of receptors for hemopexin has been investigated. Hemopexin was bound to the cells in saturable, time- and temperature-dependent manner. The cells exhibited approximately 8,400 binding sites/cell for hemopexin and apohemopexin. The dissociation constants (Kd) for hemopexin and apohemopexin were 4.79 nM and 10.8 nM, respectively. Specific binding of labeled hemopexin was inhibited with increasing concentrations of unlabeled hemopexin and apohemopexin, but unaffected by transferrin and serum albumin. Heme bound to hemopexin was incorporated into the cells at 37 degrees C, but not at 4 degrees C. These results indicate that heme in hemopexin was taken up by K562 cells via the receptors for hemopexin.