Lanthanide binding to cardiac and skeletal muscle microsomes: Effects of adenosine triphosphate, cations, and ionophores

Abstract

Lanthanide (gadolinium, Gd) binding to cardiac and skeletal muscle microsomes was studied, and high- and low-affinity sites were identified. The high-affinity constant was 10 6 M −1, and there were 131 and 107 nmol/mg bound to this site in dog heart and rabbit skeletal muscle, respectively. Zn 2+, Cd 2+, Al 3+, and Ca 2+ (5 m m) inhibited binding, especially of the high-affinity site. Ionophores X537A (10 μ m) and A23187 (1–2 μ m) increased lanthanide binding and did not cause release. Addition of ATP in low concentration (20–50 μ m) increased the binding of Gd without hydrolysis of the ATP. The extra binding induced by ATP was blocked by heating the microsomes and was reversed by [ethylenebis(oxyethylenenitrilo)]tetraacetic acid. High concentrations (10 −4–10 −3, m) of ATP blocked extra Gd binding by competitive chelation. The Ca 2+-activated ATPase was inhibited by Gd and stimulated by X537A. The Gd did not block the ionophore-stimulated increase in Ca 2+-ATPase activity. It is postulated that lanthanides bind predominantly to the ionophoric component of the Ca-transport site rather than the hydrolytic site and that ATP may facilitate such binding without being split.