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A material with antiheparin activity released by thrombin from human washed platelets was precipitated by 0.04 M ZnSO4 and applied on heparin-agarose column. Fractions eluted at 0.5 M and 1.5 M NaCl contained antiheparin activities and were denoted as low affinity (LA-PF4) and high affinity (HA-PF4) platelet factor 4, respectively. In Factor Xa-heparin inhibition assay 1 mg of each LA-PF4, HA-PF4 and protamine sulfate neutralized 0.86, 7.6 and 11.7 units of heparin, respectively.LA-PF4 and HA-PF4 were homogenous in SDS electrophoresis and had apparent MW of 13,700 and 10,300, respectively. Treatment with reducing agents had no effect on the migration of HA-PF4 but it decreased apparent MW of LA-PF4 by 50%. It has been shown by immunodiffusion and by radioimmunoassay that LA-PF4 and β-thromboglobulin (βTG) share common antigenic determinants. LA-PF4 and HA-PF4 were antigenically different. LA-PF4 and human PF4 prepared previously in our laboratory by DEAE cellulose column chromatography appeared to bè identical proteins. βTG and LA-PF4 had similar patterns of migration on SDS electrophoresis in reduced and non-reduced systems and similar amino acid composition. However, on cellulose acetate electrophoresis, LA-PF4 appeared as one homogenous band both at pH 8.4 and 9.9 which migrated in the γ-globulin region in contrast to βTG migrating with the β-globulin mobility. Migration of LA-PF4 was not affected by Zn ions. Secretion of LA-PF4 from stimulated platelets occurred in parallel with ADP and serotonin and preceded secretion of HA-PF4. In conclusion, thrombin releases from human platelets two distinct proteins with different antiheparin activities.

Low Affinity Platelet Factor 4 and High Affinity Platelet Factor 4 - Two Antiheparin Proteins Secreted by Human Platelets