Platelet factor four and protamine sulfate neutralization of heparin fractionated according to anionic charge density.

The role of heparin structure in neutralization by the neutralizing substances (NS) platelet factor 4 (PF4) and protamine sulfate (PS) was investigated using a thrombin clotting assay and a series of more homogeneous heparin fractions varying systematically in charge density (Z). For a given heparin, plotting inverse clotting times measured without NS, and in the presence of PF4 or PS, vs heparin concentration yielded approximately parallel straight lines displaced horizontally according to the amount of NS. Potencies of heparin fractions in the absence of NS, or in the presence of PF4 of PS, depended almost identically upon Z2. Small but significant quantitative differences in potency among equivalent fractions from different heparins showed both PF4 and PS had a slight preference for the least active subfraction of decolorized heparins, but for the most active subfraction of undecolorized heparins. Neutralization of heparin by PF4 and PS probably proceeds by similar mechanisms, but the details of structure outside the antithrombin III-binding oligosaccharide of heparin may enter in differently.