title: Zinc as a cofactor for heparin neutralization by histidine-rich glycoprotein.
abstract: We have studied the ability of histidine-rich glycoprotein (HRG) to neutralize the anticoagulant activity of heparin in plasma and in a purified component clotting assay. Addition of HRG to plasma or to the purified component assay did not neutralize the anticoagulant activity of heparin unless micromolar concentrations of zinc were present. Higher zinc concentrations were required for citrated than for heparinized plasmas due to competition of citrate with HRG for zinc binding. Zinc concentrations as low as 1.25 microM revealed HRG to be a powerful competitor of antithrombin for heparin in the purified component assays. HRG binding of heparin also was shown by affinity chromatography of HRG from immobilized heparin in the presence and absence of zinc. In the absence of zinc, HRG was eluted by 0.1 M NaCl, but, in the presence of zinc, elution of HRG required 1.0 M NaCl. Investigation of other divalent cations (copper and magnesium) indicated that augmentation of heparin binding by HRG in the presence of antithrombin was restricted to zinc. The HRG.Zn complex effectively competes with antithrombin for heparin, which restricts the availability of heparin to bind antithrombin and allows thrombin-mediated fibrinogenesis to proceed unimpeded. This could be initiated by zinc released from activated platelets.
authors.affiliation: Division of Experimental Pathology, Center for Reproduction and Transplantation Immunology, Methodist Hospital, Indianapolis, Indiana 46202, USA.
authors.forename: B A
authors.initials: BA
authors.lastname: Kluszynski
authors.forename: C
authors.initials: C
authors.lastname: Kim
authors.forename: W P
authors.initials: WP
authors.lastname: Faulk
mesh_terms: D000990:Antithrombin III / Q000378:metabolism; D001667:Binding, Competitive; D001682:Biological Availability; D001777:Blood Coagulation / Q000187:drug effects; D001792:Blood Platelets / Q000378:metabolism; D001798:Blood Proteins / Q000378:metabolism*; D019343:Citric Acid / Q000378:metabolism; D003300:Copper / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D005337:Fibrin / Q000378:metabolism; D005340:Fibrinogen / Q000378:metabolism; D006493:Heparin / Q000378:metabolism*; D006801:Humans; D008274:Magnesium / Q000378:metabolism; D011485:Protein Binding; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D013054:Spectrophotometry, Atomic; D013917:Thrombin / Q000737:chemistry; D013918:Thrombin Time; D015032:Zinc / Q000378:metabolism*; D000099053:Histidine-Rich Glycoprotein
chemical_list: D001798:Blood Proteins; D011506:Proteins; D000099053:Histidine-Rich Glycoprotein; D019343:Citric Acid; D003300:Copper; D000990:Antithrombin III; D005337:Fibrin; D005340:Fibrinogen; D006493:Heparin; D013917:Thrombin; D008274:Magnesium; D015032:Zinc