The transport to the liver of heme by hemopexin and of hemoglobin by haptoglobin are tissue-specific, receptor-mediated processes. As a result, biologically valuable iron is conserved and accumulation of toxic heme is prevented.
The aim of the research proposed here is to elucidate the role of a recently-discovered heme-binding protein component (HBC) in heme accumulation and degradation by liver cells. During these processes heme bound initially to HBC in the hepatocyte plasma membrane is transported to its site of catabolism by heme oxygenase in the endoplasmic reticulum. To attain this goal, this heme-binding membrane protein will be isolated and characterized, and its function in hepatic heme transport examined.
The specific aims are: 1. to isolate the HBC from rabbit liver plasma membrane and characterize its molecular size, number and type of subunits (if present) and chemical composition; 2. to use purified HBC to raise monospecific polyclonal antibodies to HBC; 3. to characterize the heme-HBC complex with respect to the affinity and stoichiometry of heme binding as well as the amino acid residues involved in binding; 4. to investigate the orientation and distribution of HBC in the plasma membrane lipid bilayer; 5. to begin to assess the role of HBC in intracellular heme transport by liver cells employing pulse-chase, subcellular fractionation and immunoprecipitation techniques. In addition to extending the examination of the mechanism of action of HBC as indicated by on-going results, the relationship of HBC with other membrane heme-binding proteins such as those in the plasma membrane of duodenal mucosal cells and of Friend erythroleukemia cells and those found in association with ferritin will be explored.
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