Catabolism of homologous and heterologous hemopexin in the rat and uptake of hemopexin by isolated perfused rat liver.

Iodinated hemopexin (Hx) from three different species, rabbit, human and rat, was injected into rats and its clearance from the plasma measured. Rabbit, human and rat apo-Hx were cleared from the plasma with a T 1/2 of 20--31 h, 31--32 h, and 48--60 h respectively. Heme injection (10 mg/kg) after equilibration of the protein immediately accelerates elimination of the hemopexins of all three species (T 1/2 of 4.5 to 10 h). This indicates that the two heterologous hemopexins maintain their function in heme transport. The T 1/2 of rabbit and human Hx return to pre-heme injection values 16 to 20 hours after the injection of heme. For rat Hx, however, the T 1/2, which was 54 +/- 3.5 h before heme injection, was reduced to 25 +/- 1.3 h 20 hours after heme injection. Administration of 1.2--1.3 mg of protoporphyrin IX or uroporphyrin III, after equilibration of iodinated rat Hx, did not change the T 1/2 of the protein, whereas the same amount of coproporphyrin III significantly reduced its T 1/2 from 54 +/- 3.5 to 37 +/- 0.4 h. In addition, the uptake of rat apo-Hx, heme-Hx and albumin by rat liver tissue was measured in an isolated liver perfusion system using radioiodinated proteins screened in vivo. The uptake of apo-Hx by the liver after 2 h (46.8 ml/100 g) was less than that of heme-Hx (67.3 ml/100 g). The amount of apo-Hx and heme-Hx associated with the liver, relative to that circulating in the perfusate, was greater than that of albumin (12.1 ml/100 g). These results are considered to represent selective uptake of Hx by the liver induced by its interaction with heme.