Transport of heme by hemopexin to the liver: Evidence for receptor-mediated uptake

We used carefully defined heme-hemopexin complexes to investigate the role of hemopexin in the catabolism of heme in vivo. Uptake of rabbit ⁵⁹Fe]heme-¹²⁵I]hemopexin by rat liver was rapid. The liver-associated ¹²⁵I reached a maximum 5 minutes after injection, nearly 7-fold higher than apo-hemopexin, whereas liver-associated ⁵⁹Fe increased with time. This together with an inverse relationship of ¹²⁵I]hemopexin in the liver and serum during the course of heme transport suggests that hemopexin was released from the liver back to the circulation. Saturation of uptake with heme-hemopexin, reaching about 170 pmol ¹²⁵I]hemopexin (gm liver)^?1 5 minutes after injection of 11 nmol, indicates a receptor-mediated process.We conclude that hemopexin delivers heme to the liver via interaction with a finite number of receptors and returns to the circulation.