Heterogeneous halothane binding in the SR Ca 2+-ATPase

Abstract

The activity of various Ca 2+-ATPases is affected by volatile anesthetics, such as halothane, commonly used in clinical practice. The effect on the enzyme in skeletal muscle sarcoplasmic reticulum (SR) is biphasic, including stimulation at clinical anesthetic concentrations and subsequent inhibition at higher concentrations. We have previously proposed that the action of a volatile anesthetic on Ca 2+-ATPases results from its binding in the interior of the enzyme molecule [Lopez, M.M. and Kosk-Kosicka, D. (1995) J. Biol. Chem. 270, 28239–28245]. Presently, we investigated whether the anesthetic interacts directly with the skeletal muscle SR Ca 2+-ATPase (SERCA1) as evidenced by binding. Photoaffinity labeling with [ 14C]halothane demonstrated that the anesthetic binds saturably to SR membranes, and that ≈80% of the binding is specific, with a K I of 0.6 mM. The K I value agrees well with the concentration at which halothane half-maximally activates SERCA1. SDS gel electrophoresis of labeled membranes indicates that 38–56% of [ 14C]halothane incorporates into SERCA1, and 38–53% in lipids. Distribution of label among the three fragments produced by controlled tryptic digestion of SERCA1 suggests heterogeneous halothane binding presumably in discrete sites in the enzyme. The results provide the first direct evidence that halothane binds to SERCA1. Potentially this binding could be related to anesthetic effect on enzyme's function.