Behavior of antithrombin III isoforms on immobilized heparins. Evidence that the isoforms bind to different numbers of low-affinity heparin sites.

Antithrombin III exists in plasma as major and minor isoforms differing in affinity for heparin. The nature of the binding of each purified isoform to immobilized heparins was investigated. Unfractionated, mixed-affinity heparin bound each isoform with both high affinity and concentration-dependent low affinity. The isoforms were resolved when filtered through low-affinity heparin (heparin repeatedly passed over immobilized antithrombin III) columns. Following chemical modification of a specific tryptophan residue required for heparin binding, each isoform failed to bind to either low-affinity or mixed-affinity heparin-agarose, but elution of the modified higher-affinity isoform was retarded on both gels. Because the modified lower-affinity isoform eluted with the similarly sized bovine serum albumin in these experiments, the difference in isoform affinity for heparin appears to be the result of a unique, secondary heparin-binding site in the higher-affinity isoform that can bind a heparin site with low affinity for antithrombin III. This interpretation was supported by the chromatographic behavior of the isoforms on mixed-affinity agarose during reverse gradient elution. Two other populations of each of the tryptophan-modified isoforms were identified. Since these isoforms bound tightly to mixed-affinity heparin-agarose but eluted at lower salt concentrations than the corresponding unmodified isoforms, both isoforms may contain additional secondary sites that interact weakly with heparin. A general model of heparin-antithrombin III interaction is proposed in which a high-affinity heparin site initially interacts with a primary site on antithrombin III. The subsequent conformational change leads to a cooperative, entropy-driven association between secondary sites on the protein and low-affinity sites on heparin, stabilizing antithrombin III in its activated form.