The heparin-neutralizing factor (PF4) has been isolated from the supernatant of thrombin-treated, washed human blood platelets. The major part of the factor is present as a PF4-proteoglycan complex which can be dissociated at higher ionic strength. PF4 thus obtained is a basic protein of molecular weight 29,700. The carrier proteoglycan will recombíne with a maximum of 4 molecules of PF4, whereby dimerization of the resulting complex is observed. This is in contrast to the naturally occurring, released complex, which has a sedimentation constant which is rather compatible with the presence of monomeric material. Heparin displaces PF4 from its proteoglycan carrier; thereby a 1:1 heparin PF4 complex is formed. Contrary to preparations obtained by zinc precipitation, neither the PF4-carrier complex, nor purified PF4 obtained by dissociation at high ionic strength form insoluble complexes with fibrinogen or with complexes formed from fibrin-monomers and fibrinogen degradation products. Pretreatment of zinc-precipitated PF4 with EDTA abolishes its paracoagulating properties.

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