The effect of molecular weight on heparin binding to platelets.

Low molecular weight heparin is reported to be less reactive with platelets than larger heparins. We probed the molecular basis for this pattern of reactivity by characterizing the saturable platelet binding of [3H]heparin in plasma using heparins of different molecular weights (Mr approximately 3000, approximately 5000, approximately 10,000, approximately 15,000). Binding affinity increased with increasing molecular weight, as expressed by decreasing apparent dissociation constants (Kdapp approximately 1.3 microM for Mr approximately 3000, to Kdapp approximately 0.31 microM for Mr approximately 15,000). After adjusting for the effect of antithrombin III in the plasma, true dissociation constants (Kd) could be calculated and these showed the same trend with molecular weight (Kd approximately 1.1 microM for Mr approximately 3000 to Kd approximately 0.096 microM for Mr approximately 15,000). Platelet binding capacity for the different heparin fractions also increased with molecular weight, although this correlation appeared to lessen with the largest species. Heparin antithrombin III affinity was shown not to affect heparin binding to platelets. We propose a model in which heparin binding to platelets is mediated by charge interaction. Larger molecules with more charge bind with greater affinity and to sites with a broader range of electronegativity than do smaller, less