Heparin is an acceleratory cofactor for antithrombin, a circulating inhibitor of blood coagulation enzymes. The presence of heparin on blood vessel walls is believed to contribute to the nonthrombogenic properties of those surfaces. In apparent opposition to this function, heparin was found to greatly accelerate the in vitro inactivation of antithrombin by neutrophil elastase. Inactivation rates in solution were potentiated several hundredfold by specific heparin fractions with anticoagulant activity. Although the data suggest that a heparin-antithrombin complex is essential for the inactivation by elastase to occur, the enzyme itself interacts tightly with heparin. These results suggest a mechanism which, if operating in vivo, could lead to a localized neutralization of the anticoagulant function of heparin at the endothelial surface.

The gene encoding the major envelope glycoprotein of the HIV-SF2 isolate was engineered for the secretion of recombinant gp120 (rgp120SF2) from permanent Chinese hamster ovary (CHO) cell lines. Cellular production methods were scaled up and a method for purification of the secreted glycoprotein was devised. Mild purification conditions were selected in order to preserve the native structure of the protein. rgp120SF2 exhibits a molecular weight of 120 kDa in reduced or nonreduced SDS gels; thus the polypeptide chain is intact. Deglycosylated rgp120SF2 has the predicted molecular weight of the polypeptide backbone, 54 kDa. Gel-filtration HPLC in a nondenaturing buffer at neutral pH yields a molecular weight estimate of approximately 120 kDa. Purified rgp120 closely resembles authentic viral gp120 by several physical, chemical, and immunochemical tests. rgp120SF2 reacts strongly with human HIV-positive sera, monoclonal antibodies reactive with HIV-SF2 and HIV-MN viral envelope, and a human virus-neutralizing monoclonal antibody that maps to a conserved discontinuous epitope on HIV-1 gp120. Purified rgp120SF2 forms a 1:1 molecular complex with soluble recombinant human CD4 (rCD4) receptor, as demonstrated by gel-filtration HPLC; binding is high affinity (Kd approximately 2 x 10(-9) M).

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In apparent contradiction to its anticoagulant activity, we have observed a previously undetected, and potentially opposing function for heparin: a distinct heparin-dependency for the in vitro inactivation of highly-purified human antithrombin by neutrophil elastase. Similar to its ability to accelerate antithrombin-mediated inhibition of coagulation enzymes, anticoagulantly-active heparin was also found to stimulate the rate of inactivation of antithrombin by the neutrophil enzyme.In the absence of heparin, or in the presence of the heparin antagonists platelet factor 4 or polybrene, little or no inactivation of antithrombin occurred. Catalytic amounts of heparin and elastase caused the complete inactivation of antithrombin (approximate molar ratio of 1:1:400 respectively) in 5-10 minutes. The loss of heparin binding affinity by the elastase-cleaved form of antithrombin permitted its separation from active antithrombin by heparin-agarose chromatography.The purified elastase-inactivated antithrombin was injected into rabbits for determination of its comparative clearance behavior. In contrast to intact, functional antithrombin (t 1/2 >30 hours) and the thrombin-antithrombin (T-AT) complex (t 1/2 previously shown to be minutes), elastase-inactivated antithrombin circulated for approximately 13 hours. This prolonged clearance relative to the T-AT complex may suggest an alternative explanation for the circulating, non-functional antithrombin observed in certain coagulopathic states. In summary, these results point to a potential and unexpected role for heparin in directing the inactivation of antithrombin and suggest a possible in vivo mechanism for neutralizing the usually non-thrombogenic nature of the vascular lining.

The rate of inhibition of coagulation enzymes by antithrombin III is greatly increased by an interaction between the inhibitor and a limited, anticoagulantly-active subfraction of heparin molecules. We have observed that this same heparin sub-fraction also greatly stimulates the rate of inactivation of antithrombin by neutrophil elastase. Inactivation rates were increased several hundred-fold by catalytic amounts of the anticoagulantly active heparin species and were unaffected by the inactive heparin fraction or other glycosaminoglycans. Addition of catalytic amounts of elastase (1:400) to a solution of antithrombin in the presence of saturating levels of anticoagulantly-active heparin caused an approximately 30% decrease in the ultraviolet fluorescence emission of the inhibitor. The rate of fluorescence loss corresponded exactly with the loss of inhibitory activity in a parallel incubation under the same conditions. The use of the fluorescence technique for kinetic measurement of inactivation rates indicated a Km of less than 1 uM for the heparin-antithrombin complex substrate and a turnover of several hundred per minute per enzyme molecule. Although the specificity of the heparin effect appears to be at the level of its interaction with antithrombin, an elastase-heparin interaction could also be detected in kinetic analyses. Chromatographic studies employing immobilized heparin confirmed that elastase itself binds tightly to the complex carbohydrate. These results suggest a subtle mechanism, of potential relevance to thrombosis associated with inflammatory conditions, in which both heparin and elastase act catalytically to direct the inactivation of antithrombin. Since anticoagulantly-active heparin species are present on endothelial cells, the above mechanism could markedly affect the balance between procoagulant and anticoagulant processes on the usually non-thrombogenic endothelial surface.