title: Characterization of Gd3+ and Tb3+ binding sites on Ca2+,Mg2+-adenosine triphosphatase of sarcoplasmic reticulum.
abstract: Interaction between Gd3+ and Tb3+ ions and Ca2+,Mg2+-ATPase of sarcoplasmic reticulum was studied. Three classes of lanthanide-ion binding sites with different affinities were distinguished. Binding of Gd3+ to the site with the highest affinity seemed to occur at less than 10(-6)M free Gd3+ and resulted in severe inhibition of ATPase activity. The reaction rates of both E-P formation and decomposition in the forward direction were inhibited in parallel with this binding, whereas ADP-dependent decay of E-P in the backward direction was not. At these Gd3+ concentrations, Ca2+-binding to the transport site was not inhibited. Binding of Gd3+ and Tb3+ to the Ca2+-transport site did occur, but more than 10(-5)M free Gd3+ or Tb3+ was required for effective competition with Ca2+ for that site. Gd3+ bound to the transport site in place of Ca2+ did not activate the E-P intermediate formation. Addition of 10(-1)M Tb3+ to a suspension of sarcoplasmic reticulum membranes resulted in marked enhancement of Tb3+ fluorescence, which is due to an energy transfer from aromatic amino acid residues of ATPase to Tb3+ ions bound to the low affinity site of the enzyme. Gd3+ and Mn2+ competed with Tb3+ for that site, but Ca2+, Zn2+, and Cd2+ did not.
mesh_terms: D001665:Binding Sites; D001667:Binding, Competitive; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D005682:Gadolinium / Q000378:metabolism* / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013725:Terbium / Q000378:metabolism*
chemical_list: D013725:Terbium; D005682:Gadolinium; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases