The histidine-rich glycoprotein (HRG) of plasma has become the subject of increasing attention due to its ability to interact with a variety of compounds present in the circulation. HRG binds metals (Zn, Ni, Cu), heme and heparin, and associates with plasminogen and thrombospondin. Interestingly, HRG competes with antithrombin IV and heparin cofactor II for heparin, and the activation of plasminogen by tissue plasminogen activator is enhanced in the presence of HRG and thrombospondin. These binding activities point to a biological function for HRG in hemostatis. The basic hypothesis of this research proposal is that HRG contains several ligand-binding functional domains that act to accomplish the biological role of HRG In hemostatis. Although this role cannot yet be defined, eventual understanding of the mechanisms of action of HRG in hemostatis will be facilitated if its structure is known and if the activity of each domain is characterized. This hypothesis is supported by the recent isolation of a histidine-proline-glycine-rich peptide (Mr 30000) from HRG (Mr 94000) which binds heme, heparin and metals; of a peptide of more usual composition (Mr 45000) which has two preferential heme-binding sites and interacts with plasminogen; and of a peptide (Mr 16000) which binds heparin also.
The specific aims are: 1) to complete the determination of the amino acid sequence of HRG; 2) to continue to delineate the biochemical mechanisms of the interaction of HRG with its various ligands, especially heparin, plasminogen and thrombospondin; 3) to continue characterization of the physico- chemical and biological properties of peptides isolated after protease cleavage of HRG; 4) to delineate the interrelationships among the ligand binding and structural domains of intact HRG employing both physical and chemical methods and the monoclonal antibodies already in hand. The ultimate goal of this project is to elucidate the mechanisms of action of HRG in hemostasis to allow development of improved diagnostic and therapeutic measures. The basic information to be gathered in the first phase of this research program is expected to be of vital importance in attaining this goal.
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