title: Fibrinogen binding to human platelet plasma membranes. Identification of two steps requiring divalent cations.
abstract: Fibrinogen binding to platelet plasma membranes, which is a prerequisite for platelet aggregation, was determined by incubating 125I-labeled fibrinogen with isolated membranes and measuring the amount of radioactivity sedimenting with the membranes through 15% sucrose. Fibrinogen binding was optimal at 10(-3) M Ca2+. Scatchard analyses of the fibrinogen binding showed that the membrane capacity for fibrinogen was 1.6 X 10(-12) mol/mg of membrane protein, with a dissociation constant (Kd) = 1.2 X 10(-8) M. When Ca2+ levels were manipulated by the addition of varying amounts of EGTA at a fixed Mg2+ concentration of 3 X 10(-3) M, specific binding of fibrinogen to platelet membranes occurred only at Ca2+ concentrations greater than or equal to 10(-6) M. Membranes isolated from platelets of an individual with Glanzmann's thrombasthenia bound only 12% as much fibrinogen as control platelets. The data in the present study suggest that there are two divalent cation binding sites that must be occupied for fibrinogen to bind: one site is specific for calcium and is saturated at 10(-6) M Ca2+; the other site is less specific and is saturated at a 10(-3) M concentration of either Ca2+ or Mg2+. Fibrinogen binding to intact platelets and, consequently, platelet aggregation only required 10(-3) M extracellular divalent cation and was not specific for Ca2+. These data indicate that the cytoplasm is a potential source for the requirement of 10(-6) M Ca2+, and that changes in the intracellular concentration of Ca2+ may cause the expression of fibrinogen receptors during ADP-induced platelet activation.
mesh_terms: D001791:Blood Platelet Disorders / Q000097:blood; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000097:blood; D002413:Cations, Divalent / Q000097:blood*; D005340:Fibrinogen / Q000378:metabolism*; D006801:Humans; D008274:Magnesium / Q000097:blood; D008565:Membrane Proteins / Q000378:metabolism; D010980:Platelet Membrane Glycoproteins; D011956:Receptors, Cell Surface / Q000378:metabolism; D012794:Sialic Acids / Q000097:blood
chemical_list: D002413:Cations, Divalent; D008565:Membrane Proteins; D010980:Platelet Membrane Glycoproteins; D011956:Receptors, Cell Surface; D012794:Sialic Acids; D005340:Fibrinogen; D008274:Magnesium; D002118:Calcium