ABSTRACT:The role of haptoglobin in liver cell entry of acellular native hemoglobin, and cross-linked human hemoglobin, a potentially useful oxygen-carrier alternative in transfusion medicine, was examined in the recirculating, perfused rat liver preparation. Doses of tritiated native human or -chain [trimesoyl-(Lys82)-(Lys82)] crosslinked human hemoglobin were preincubated with haptoglobincontaining rat plasma or Krebs Henseleit bicarbonate buffer for 30 min and used for perfusion. Concentrations (dpm/ml) in reservoir, before and after separation of the hemoglobins and metabolites by gel filtration fast protein liquid chromatography column chromatography, were similar, showing mostly the presence of intact hemoglobin. Each hemoglobin species underwent a rapid distribution phase, followed by a protracted elimination phase. The radioactivity in bile at 3 h consisted of low molecular weight metabolites, and cumulative excretion was slightly higher when rat plasma was present: for native hemoglobin, 7.1 ؎ 1.6% versus 9.2 ؎ 2.1% dose; for cross-linked hemoglobin, 5.0 ؎ 1.7% versus 7.2 ؎ 0.8% dose. Data fit to a two-compartment model and physiologically based model revealed a significantly faster influx clearance (CL influx ) over the metabolic intrinsic clearance (CL int, met ). The ratios of CL influx /CL int, met were 125 and 535 for native hemoglobin in the absence and presence of rat haptoglobin, respectively, according to compartmental analyses; the ratios were 25 and 53, respectively, according to physiological modeling. The corresponding ratios for cross-linked hemoglobin in the absence and presence of rat haptoglobin were 55 and 81, respectively, and 24 and 70 for compartmental and physiological modeling. Although haptoglobin enhanced the hepatic internalization of the hemoglobins, the impact on the net clearance was lessened since degradation was the rate-limiting step.Hemoglobin is a tetrameric heme protein that is found within red blood cells. This protein is composed of a pair of ␣ dimers to form its tetrameric structure (Greer et al., 1981;Voet and Voet, 1995;Chang, 1999). In the tetrameric format, hemoglobin plays an important role in the binding to oxygen and the supply of oxygenated blood to tissues and organs [for review, see Winslow (1992)]. During intravascular hemolysis, hemoglobin is released from the red blood cells and is distributed into the plasma where free hemoglobin (HB) is rapidly bound to haptoglobin (Hp), a specific plasma binding protein of high affinity (K d Ϸ 1 ϫ 10 Ϫ15 M) toward the alpha chain on the hemoglobin (Putnam, 1976;McCormick and Atassi, 1990;Lim et al., 1998). The complex is then taken up by an unknown receptor on hepatocytes and by the CD163 hemoglobin scavenger receptor on macrophages that are present in the spleen, thymus, Kupffer cells, and bone marrow (Graversen et al., 2002;Polfliet et al., 2006).In cases of transfusion of acellular hemoglobin or severe hemolysis, high levels of hemoglobin are present and will easily saturate the haptoglobin present in plasma (...