Both PF4 and PS have been postulated to neutralize heparin by electrostatic complex formation. The present study examined the role of charge density (Z) of the heparin in neutralization reactions with PF4 and PS. Fractions varying systematically in charge density were isolated from 3 hog mucosal (HM) heparins (2 decolorized and 1 undecolorized) using 2 phase sequential extraction system. Fractions were similarly isolated from a decolorized beef lung (BL) heparin. Thrombin inhibition (ThI)

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... were performed in the absence and presence of a constant amount of PF4 or PS for each fraction and for a series of dilutions of a standard heparin. The potencies of fractionated heparins varied systematically in a non-linear manner with Z2. However, in all heparins studied, potency curves generated in the presence of PF4 and PS paralleled each other closely. With the 2 decolorized heparins, these curves also closely paralleled the potency curves generated in the ThI assay in the absence of PF4 or PS (ThI-buffer curves), but for the undecolorized heparin, the ThI-buffer curve was distinctly of steeper slope with greater potency. In contrast, with BL heparin , the ThI-buffer curve was of shallower slope and of lower potency.The parallel behavior of PF4 and PS curves supports a similarity in the mechanism of neutralization. The potency of heparin fractions is related to their charge density. With decolorized HM heparins, PF4 and PS do not selectively neutralize a subfraction with different potency, but with the undecolorized heparin, PF4 and PS is somewhat selective towards neutralizing more potent heparin species. In contrast, with BL heparin, a less active heparin species is apparently preferentially inactivated by PF4 and PS, and moreover, the activity is less subject to neutralization.