A Spin Label Study of Sarcoplasmic Vesicles

Abstract

Spin labeled fatty acids have been incorporated as structural probes into sarcoplasmic vesicles isolated from rabbit skeletal muscle. The evaluation of the electron paramagnetic resonance spectra has yielded the following results: 1 The spin labels I (m,n) display a fast, anisotropic rotation in the sarcoplasmic membrane. The first 7 carbon-carbon bonds adjacent to the carboxyl group of the spin labeled fatty acid experience a highly ordered environment, whereas for n > 7 a pronounced increase in the flexibility of the hydrocarbon chain is observed although the system is still more ordered than pure lipid dispersions. The fluidity of the membrane is due to its lipid constituents, since a removal of the lipids leads to spectra characteristic for immobilized spin labels. 2 The comparison of several model systems leads to the assumption that a direct lipidprotein interaction must play an essential role in the organization of the membranous lipids. The effect of this interaction is to increase the stiffness of the hydrocarbon chains of the lipid moieties. 3 The activity of the calcium dependent ATPase is directly related to the fluidity of the membrane. If the enzymatic activity of a lipid deficient membrane is restored by the addition of oleic acid, the oleic acid assumes a physical state which is very similar to that of the natural membranous lipids.