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Role of Haptoglobin on the Uptake of Native and β-Chain [Trimesoyl-(Lys82)β-(Lys82)β] Cross-Linked Human Hemoglobins in Isolated Perfused Rat Livers

Departments of Pharmaceutical Sciences (E.C.Y.C., L.L., K.S.P.) and Chemistry (N.S., R.K.), University of Toronto, Toronto, Ontario, Canada

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 β-chain [trimesoyl-(Lys82)β-(Lys82)β] cross-linked human hemoglobin were preincubated with haptoglobin-containing 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.