RESUMEN
X-ray diffraction, fluorescence polarization of trans-parinaric acid, and fluorescence photobleaching recovery of dioctadecyltrimethyneindolecarbocyanine have been used to characterize the phase composition and liquid phase fluidity of bovine luteal cell membranes and membrane lipids for functional corpora lutea collected at midcycle and for regressing corpora lutea collected after treatment with prostaglandin F2 alpha. These results support previous observations of gel phases in microsomal preparations of regressed luteal cells at physiological temperatures and further suggest that the plasma membrane may be the main source of this gel phase. Analysis of the overall lipid composition of the microsomal preparations from these cells indicates a role for sphingomyelin, in the presence of cholesterol, for the generation of a gel phase at physiological temperatures.
Asunto(s)
Membrana Celular/fisiología , Cuerpo Lúteo/ultraestructura , Luteólisis , Fluidez de la Membrana , Animales , Bovinos , Colesterol/farmacología , Dinoprost , Estro , Femenino , Polarización de Fluorescencia , Lípidos de la Membrana/fisiología , Fosfolípidos/fisiología , Embarazo , Prostaglandinas F/farmacología , Temperatura , Difracción de Rayos XRESUMEN
The conformations of liquid n-alkanes have been studied using neutron scattering techniques to better understand the conformational forces present in membrane lipid interiors. We have studied hydrocarbon chains having lengths comparable to those found for esterified membrane lipid fatty acids, and find that the steric constraints of packing in the liquid state do not change the conformational distributions of hydrocarbon chains from those imposed by the intrachain forces present in the gas phase. It follows that the central region of membranes containing lipids in the disordered state should contain hydrocarbon chain conformations determined primarily by intrachain forces.
Asunto(s)
Alcanos , Matemática , Conformación Molecular , Neutrones , Dispersión de Radiación , Relación Estructura-ActividadRESUMEN
Fluorescence polarization of cis- and trans-parinaric acid and high angle x-ray diffraction are used to examine the physical effects of in vitro sterol depletion of LM cell plasma membranes. Measurements on lipids isolated from the normal plasma membrane show that most of the lipid is fluid between 5 and 45 degrees C. When no sterol is present, approximately 15% of the phospholipids undergo gel phase formation, detectable by x-ray measurements, in this temperature range. Partial sterol repletion of the isolated phospholipids serves to lower the onset temperature of the phase transition, as well as to decrease the fraction of lipid undergoing a transition. Intact plasma membranes are sterol depleted by in vitro incubation with egg phosphatidylcholine liposomes. This induces a phase transition with an onset temperature increasingly above the physiological temperature as more sterol is removed from the membranes. The transition can be suppressed by returning sterol to the membranes. These experiments extend earlier work (Rintoul, D. A., Chou, S.-M., and Silbert, D. F. (1979) J. Biol. Chem. 254, 10070-10077) on isolated plasma membrane lipids by showing that sterol suppresses a phase transition in intact membranes as well as in isolated lipids. The correlation between gel phase formation upon sterol loss at the physiological temperature and loss of membrane glucose transport function is discussed.
Asunto(s)
Lípidos de la Membrana/análisis , Fosfolípidos/análisis , Animales , Membrana Celular/análisis , Células L/análisis , Ratones , Conformación Molecular , Espectrometría de Fluorescencia , Difracción de Rayos XRESUMEN
The ability of seeds to withstand dehydration indicates that their membranes may maintain structural integrity even when dry. Analysis of polar lipids (the principal lipidic constituents of the membranes) from soybean seeds (Glycine-max (L.) Merr.) by X-ray diffraction indicated that even in the dehydrated state the lipids retained a lamellar (bilayer) configuration. As the degree of hydration was raised, evidence of some structural alteration (apparent as an abrupt increase in bilayer spacing) was obtained from diffraction patterns of both the extracted lipid and particles of seed tissue. In seed tissue this increase in bilayer spacing occurred at a hydration level just above that at which free water could be detected by nuclear-magnetic-resonance analysis. The water content at which the increase in bilayer spacing occurred was higher in the seed tissue than in the extracted polar lipids, probably because other cell components restricted the availability of free water in the seed.