title: The interaction of Tb3+ with the human platelet surface.
abstract: The interaction of the lanthanide Tb3+ with washed, human platelets was examined. When bound to the platelet surface, the fluorescence of this Ca2+ analog was increased approximately 200-fold, most likely by a Förster mechanism involving platelet surface protein aromatic residues. The binding of Tb3+ to the unactivated platelet was specific and saturable with an apparent approximate Kd of 195 microM. Both Ca2+ and La3+ effectively displaced Tb3+ from platelet surface sites, but neither cation did so completely. Plasmin treatment of the platelet surface reduced Tb3+ fluorescence by 68% at saturation without significantly affecting the approximate apparent Kd. Activating washed, aspirinated platelets with ADP induced a 78% increase in Tb3+ fluorescence at saturation. Tb3+ competed effectively and completely for platelet surface-bound 45Ca2+ with an approximate IC50 of 10 microM. These data indicate the potential utility of this fluorescent lanthanide in characterizing Ca2+-binding sites on the human platelet.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D001665:Binding Sites; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D005341:Fibrinolysin / Q000378:metabolism; D006801:Humans; D007811:Lanthanum / Q000378:metabolism; D010974:Platelet Aggregation; D013050:Spectrometry, Fluorescence; D013725:Terbium / Q000378:metabolism*
chemical_list: D013725:Terbium; D000244:Adenosine Diphosphate; D007811:Lanthanum; D005341:Fibrinolysin; D002118:Calcium