Ligand and lipid domain stabilization of a membraneous Ca 2+-ATPase during hyperthermia

Abstract

The susceptibility of the membranous Ca 2+-ATPase of sarcoplasmic reticulum to enzymatic inactivation at hyperthermic temperatures was investigated. Inactivation produced a break in the Arrhenius plot at 45–46°C and was accompanied by an increased mobility of spin label, covalently attached to the Ca 2+-ATPase. MgADP and MgATP exerted a markedly stabilizing effect on inactivation, both at pH 7.0 and in acidic media. By contrast, high-affinity Ca 2+ or Mg 2+ binding only moderately stabilized Ca 2+-ATPase (inactivation rates were decreased 2–3-times), and this effect was non-additive, i.e., only observed in the absence of the other divalent cation. But withdrawal of K + and Na + gave rise to a pronounced destabilization that could be reversed efficiently by high concentrations of Ca 2+ or Mg 2+. These results are compared with a previous study on detergent solubilized Ca 2+-ATPase (Møller, J.V., Lind, K.E. and Andersen, J.P. (1980) J. Biol. Chem. 255, 1912–1920) which showed the enzyme to be markedly stabilized by Ca 2+ as well as by nucleotide. It is concluded that, due to the presence of nucleotide, inactivation of Ca 2+-ATPase is not likely to occur during malignant hyperthermia and that the native environment of the lipid bilayer provides stabilization of the membrane-embedded and Ca 2+-translocating domain of the Ca 2+-ATPase.