Intracellular distribution of haem after uptake by different receptors. Haem-haemopexin and haem-asialo-haemopexin

Abstract

How the interaction of haemopexin with two different receptors affects its subsequent metabolism and 'intracellular' haem transport was examined by using mesohaem-haemopexin and mesohaem-asialo-haemopexin. The physical properties of the two haem proteins, including their absorption and c.d. spectra, are similar. Binding studies in vitro showed that haem-asialo-haemopexin interacts with both the haemopexin-specific and galactose-specific receptors on liver plasma membranes, but that haem-haemopexin interacts only with the haemopexin receptor. In vivo haem-asialo-haemopexin rapidly interacts with the liver via the galactose-specific receptor, since the protein is extensively catabolized and uptake is blocked by asialofetuin. Haem iron from haem-asialo-haemopexin is not accumulated in the liver to the same extent as from intact haem-haemopexin, and the native sialylated protein is not proteolysed. Moreover, after fractionation of homogenized liver by using colloidal-silica gradients, liver-associated haem-haemopexin and haem-asialo-haemopexin produced distinctly different patterns for both protein and ligand, consistent with their uptake by two distinct receptors. These results demonstrate that the interaction of haemopexin with different receptors influences its subsequent metabolic fate and that haem iron from haem-haemopexin is efficiently conserved only if it enters the liver cell via the specific haemopexin receptor.

Identification of the receptor scavenging hemopexin-heme complexes

“…39,40 Previous attempts to identify the Hx-heme receptor have not revealed any protein structure besides the hepatic asialoglycoprotein receptor, which is suggested to represent a receptor for uptake of "outdated" asialo-Hx. 15 However, several studies have published functional and structural aspects of cellular Hx-heme binding sites 11,12,14,22,41 different from the asialoglycoprotein receptor. Various affinities are reported but in the most recent of these studies the affinity estimated (K d ϭ 0.34-0.85 nM) on human cytotrophoblasts is close to the estimated value for binding of Hx-heme to purified placental LRP/CD91.…”

“…The asialoform of Hx has been reported to recognize the asialoglycoprotein receptor, but this hepatocyte-specific receptor is suggested to represent a secondary receptor for catabolism of outdated Hx, which has lost the terminal sialic acids of the N-linked carbohydrates. 15 To identify the primary receptor responsible for uptake of Hx-heme complexes in humans, we used a ligand-affinity purification approach similar to the one used for identification of the haptoglobin-hemoglobin receptor CD163. 10 This attempt led to purification of the 600-kDa LDL receptor-related protein (LRP), alias CD91, and the ␣ 2 -macroglobulin receptor, a multiligand receptor 16 expressed in a variety of cell types including those shown to possess Hx-heme receptor activity.…”

The haem–haemopexin utilization gene cluster (hxuCBA) as a virulence factor of Haemophilus influenzae

“…Rabbit haemopexin was prepared as described previously and the haem-haemopexin complexes were characterized by the typical features of their absorption spectra, which include the prominent shoulder at 290 nm that appears upon haem binding (Smith & Morgan, 1984;Smith, 1985).…”

Identification of a haem-utilization protein (Hup) in Haemophilus influenzae

“…Rabbit haemopexin was prepared as described previously and the haem-haemopexin complexes were characterized by the typical features of their absorption spectra, which include the prominent shoulder at 290 nm that appears upon haem binding (Smith, 1985;Smith & Morgan, 1984).…”