title: Calcium and temperature regulation of the stability of the human platelet integrin GPIIb/IIIa in solution: an analytical ultracentrifugation study.
abstract: The human platelet integrin GPIIb/IIIa (228 kDa), a Ca-dependent heterodimer formed by the alpha IIb subunit (GPIIb, 136 kDa) and the beta 3 subunit (GPIIIa, 92 kDa), serves as the fibrinogen receptor at the surface of activated platelets. The degree of dissociation of the GPIIb/IIIa heterodimer (s degrees 20*, 8.9 S) into its constituent glycoproteins (GPIIb, 5.8 S; and GPIIIa, 3.9 S) has been assessed by analytical ultracentrifugation in Triton X100 buffers, and its Ca(2+)- and temperature-dependence correlated with Ca(2+)-binding to GPIIb/IIIa and its temperature dependence. At 21 degrees C half-maximal dissociation of GPIIb/IIIa occurs at 5.5 +/- 2.5 x 10(-8) M Ca2+, very close to the dissociation constant of the high affinity Ca-binding site of GPIIb/IIIa (Kd1 8 +/- 3 x 10(-8) M) (Rivas and González-Rodriguez, 1991) and much lower than the Kd of the 3.4 medium affinity Ca-binding sites (Kd2 4 +/- 1.5 x 10(-5) M), which seems to demonstrate that the stability of the heterodimer in solution at room temperature is regulated by the degree of saturation of the high-affinity Ca-binding site. At 4 degrees C, the stability of the heterodimer is apparently Ca(2+)-independent, while at room and physiological temperatures (15-37 degrees C) the degree of dissociation of the heterodimer is regulated by the degree of dissociation of the high- and medium-affinity Ca-binding sites, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Instituto de Química Física, C.S.I.C., Madrid, Spain.
authors.forename: G A
authors.initials: GA
authors.lastname: Rivas
authors.forename: P
authors.initials: P
authors.lastname: Usobiaga
authors.forename: J
authors.initials: J
authors.lastname: González-Rodriguez
mesh_terms: D002118:Calcium / Q000494:pharmacology*; D002850:Chromatography, Gel / Q000379:methods; D004355:Drug Stability; D006801:Humans; D046911:Macromolecular Substances; D008823:Micelles; D010980:Platelet Membrane Glycoproteins / Q000737:chemistry* / Q000187:drug effects / Q000302:isolation & purification; D014461:Ultracentrifugation / Q000379:methods
chemical_list: D046911:Macromolecular Substances; D008823:Micelles; D010980:Platelet Membrane Glycoproteins; D002118:Calcium