Interaction of platelet factor 4 with human platelets.
Human washed resting platelets bound 125I-labeled platelet factor 4 in a reaction which was saturable and approached equilibrium within 15-30 min. Scatchard plot analysis of the binding isotherms suggested a single class of specific binding sites. Excess of unlabeled protein and low- and high-affinity heparin competed for platelet factor 4 binding sites on the platelet surface and caused a partial displacement of this molecule. Anti-platelet factor 4 Fab fragments caused inhibition of binding of 125I-platelet factor 4 to platelets. Most of the labeled platelet factor 4 which was bound to intact platelets was recovered in the Triton X-100-insoluble cytoskeletal fraction prepared from the same platelets after their stimulation by thrombin. The association with the cytoskeleton was inhibited by anti-platelet factor 4 Fab fragments and by low-affinity heparin. Anti-platelet factor 4 125I-labeled Fab fragments bound to resting platelets, and this binding was greatly increased following platelet stimulation with thrombin. This suggested that endogenously secreted platelet factor 4 also binds to the platelet surface. No significant binding to platelets of 125I-labeled beta-thromboglobulin and 125I-labeled anti-beta-thromboglobulin Fab fragments was observed. Fab fragments of monospecific anti-human platelet factor 4 antibody raised in rabbits inhibited platelet aggregation and secretion induced by low concentrations of thrombin. Fab fragments of anti-beta-thromboglobulin antibody had no inhibitory effect. We suggest that the binding of alpha-granule-derived platelet factor 4 to the specific sites on the surface of platelets may modulate platelet aggregation and secretion induced by low levels of platelet agonists.
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000378:metabolism; D000941:Antigens / Q000032:analysis; D001792:Blood Platelets / Q000502:physiology*; D003599:Cytoskeleton / Q000032:analysis; D006801:Humans; D007457:Iodine Radioisotopes; D008970:Molecular Weight; D010974:Platelet Aggregation; D010978:Platelet Factor 4 / Q000276:immunology / Q000502:physiology*; D013917:Thrombin / Q000494:pharmacology
D000941:Antigens; D007457:Iodine Radioisotopes; D010978:Platelet Factor 4; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D013917:Thrombin