Solubilization and assay of an hepatic receptor for the haptoglobin-hemoglobin complex

Hemoglobin released into the bloodstream is tightly bound by haptoglobin. The resulting complex (HpHb) is promptly cleared from the circulation and accumulates in the liver. A binding protein with a high affinity for HpHb has been solubilized from an acetone powder of rat liver and freed from an endogenous inhibitor by passage over a column of immobilized hemoglobin. An assay procedure has been developed whereby the bound HpHb is selectively precipitated by polyethylene glycol 6000. Employing this assay, the binding reaction was shown to be linear and saturable with respect to the ligand. In contrast to several previously described receptors for glycoproteins, the carbohydrate moiety of haptoglobin did not appear to participate in the binding of HpHb by the soluble receptor.