A comparison of the strength of binding of antithrombin III, protamine and poly(L-lysine) to heparin samples of different anticoagulant activities.

The limiting concentrations, i.e., those concentrations of sodium chloride required to completely disrupt the complexes of heparin with antithrombin III, protamine and poly(L-lysine), were determined using fluorescence techniques, in order to compare the binding strengths of these complexes. From the limiting salt concentration values, poly(L-lysine) always exhibited stronger binding to heparin of a particular anticoagulant potency (degree of sulphation) than did protamine. The binding strengths of both complexes decreased as the degree of sulphation of the heparin participating in the complex was reduced. In contrast, the limiting salt concentration values for complexes formed between antithrombin III and heparin did not change with either the degree of sulphation or the biological potency of the heparin samples. A low-potency heparin simply contained a smaller number of molecules which possessed the intact antithrombin III binding site (thus being fully 'anticoagulant active') than a high-potency sample. Low-affinity heparin did not contain these binding sites and thus showed a low affinity for antithrombin III. High-potency heparin, being highly sulphated, possessed a higher affinity for protamine and poly(L-lysine) than for antithrombin III. However, after partial N-desulphation of heparin, the subsequent heparin-protamine complex was more weakly bound than a significant proportion of the corresponding heparin-antithrombin III complexes. These in vitro findings may have particular relevance in relation to the clinical condition termed 'heparin rebound'.