Proton nuclear magnetic resonance studies of sarcoplasmic reticulum membranes. Correlation of the temperature-dependent Ca2+ efflux with a reversible structural transition

Abstract

Proton nuclear magnetic resonance spectra of ATP-dependent Ca²⁺-accumulating sarcoplasmic reticulum vesicles isolated from rabbit leg white muscle show a reversible, temperature dependent structural transition. This transition involves a transformation of the choline methyl groups of lecithin from a rigid to a mobile state. The energy of the transformation is approx. 35 kcal/mole and involves nearly 60% of the lecithin molecules in the membrane. The fraction of mobile methyl groups as a function of temperature correlates with the temperature-dependent Ca²⁺ efflux measured in these preparations. As determined by nuclear magnetic resonance, other agents which alter protein structure and simultaneously increase Ca²⁺ efflux also increases the fraction of mobile choline methyl groups. Likewsise, irreversible heat denaturation of the membrane protein alters the temperature dependence of the transition. The proton magnetic resonance spectra are consistent with the organization of approx. 80% of the lipid fatty acid chains into a lamellar phase which permits only limited motion. On the other hand approx. 20% of the lipid molecules are in a fluid region of the membrane in which nearly isotropic motion occurs ( ).