ZINC AS A COFACTOR FOR HEPARIN NEUTRALIZATION BY HISTIDINE-RICH GLYCOPROTEIN

We have studied the ability of histidine-rich glycoprotein (HRG) to ne utralize the anticoagulant activity of heparin in plasma and in a puri fied component clotting assay. Addition of HRG to plasma or to the pur ified 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 p lasmas due to competition of citrate with HRG for zinc binding. Zinc c oncentrations as low as 1.25 mu M revealed HRG to be a powerful compet itor of antithrombin for heparin in the purified component assays. HRG binding of heparin also was shown by affinity chromatography of HRG f rom immobilized heparin in the presence and absence of zinc. In the ab sence of zinc, HRG was eluted by 0.1 M NaCl, but, in the presence of z inc, elution of HRG required 1.0 M NaCl. Investigation of other divale nt cations (copper and magnesium) indicated that augmentation of hepar in binding by HRG in the presence of antithrombin was restricted to zi nc. The HRG Zn complex effectively competes with antithrombin for hepa rin, 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.