Dynamic structure of bovine adrenal cortex microsomal membranes studied by time-resolved fluorescence anisotropy of all-trans-1,6-diphenyl-1,3,5-hexatriene

Alteration in membrane lipid order and composition in metabolically hyperactive fatty rat adipocytes

We have previously shown that adipose cells from young genetically obese Zucker rats are characterized by very high metabolic activity together with an increase in a wide range of membrane-mediated functions. The aim of the present study was to examine whether the physical properties of the membranes and the composition of the membrane lipids were altered in these cells. Plasma membranes and two intracellular membrane fractions were prepared by differential ultracentrifugation from inguinal adipose cells of 30-day-old obese (fa/fa) and lean (Fa/fa) littermates. The lipid order as measured by steady-state fluorescence polarization of diphenylhexatriene used as probe was markedly decreased in the plasma membranes of obese rat adipose cells. Consistent with this, the cholesterol-to-phospholipid ratio was significantly decreased, and the degree of unsaturation of the phospholipid fatty acids was significantly increased. In intracellular membranes, none of these parameters were altered by the different genotype. In fat cells from obese rats, both plasma and intracellular membranes exhibited a 2-fold decrease in the ratios of n-6/n-3 fatty acids mainly due to an enrichment in docosahexaenoic acid (22:6n-3). The data show that the fatty genotype is a determinant of membrane lipid order and composition in adipose cells. The alterations reported here for young obese Zucker rat adipocytes might be related to the metabolic hyperactivity of these cells.

Studies of conformation and interaction of the cyclohexenone and acetyl group of progesterone with liposomes

The conformations of the A-ring and the 17-acetyl groups of progesterone were examined within liposomes, which were prepared from L-alpha-phosphatidylcholine in the presence or absence of cholesterol in the buffer, using qualitative nuclear magnetic resonance and circular dichroism of the progesterone spectra in the wavelength regions of 260-360 nm. The preferred conformational assignments, in the rotational conformations of the 17-acetyl group and invertible conformations of the cyclohexenone of progesterone were discussed on the basis of the elliptical strength of the Cotton effect and an energy estimation of the preferred conformers. Energetically unstable conformers of the acetyl group and alpha,beta-unsaturated cyclohexenone of progesterone remarkably increased with an increase in the concentration of the liposomes. The liposomes containing 10% cholesterol were similar to the effect of the liposomes lacking cholesterol on the 17-acetyl group and the alpha,beta-unsaturated cyclohexenone but those containing 50% cholesterol showed an increase in the number of energetically stable conformers of the alpha,beta-unsaturated cyclohexenone. The nuclear magnetic resonance signal from liposomes together with the progesterone indicated the existence of the progesterone adjacent to a double bond or ester moiety in the lipid molecule. Therefore, it was apparent that the liposomes and the cholesterol within the liposomes regulated the conformational populations of both the cyclohexone and acetyl groups of the progesterone molecule.

Relation between membrane phospholipid composition, fluidity and function in mitochondria of rat brown adipose tissue. Effect of thermal adaptation and essential fatty acid deficiency

Male weanling rats were maintained either at 28 degrees C (thermoneutrality) or at 5 degrees C (cold adaptation). During 9 weeks they were fed either a 2% hydrogenated coconut oil diet deficient in essential fatty acids or a diet containing 2% sunflower oil. The respective incidences of cold adaptation and of EFA deficiency on lipid composition of mitochondrial membranes from brown adipose tissue (BAT) were investigated. Using 1,6 diphenylhexatriene (DPH) as a probe, the parameters of membrane fluidity were estimated by steady-state fluorescence polarization measurements (rs) and by time-resolved fluorescence anisotropy decay (order parameter S). Cold acclimation induced a decrease of phosphatidylcholine to phosphatidylethanolamine (PC/PE ratio), an increase of the total fatty acid unsaturation index (T.U.). EFA deficiency had the same effect as cold on the PC/PE ratio, but decreased T.U. Cold adaptation induced a larger decrease of S than of rs, whereas EFA deficiency only increased rs and did not modify S. In liposomes prepared from mitochondrial lipids, rs values were smaller than in whole mitochondria. Both in cold-adapted and in EFA-deficient rats the variations of rs were correlated with lipid unsaturation. Comparison between BAT thermogenic activity, assessed by GDP binding and proportions of PE and PC showed a high correlation suggesting a change in the membrane occurring with the increase of mitochondrial activity that could be related to phospholipid composition rather than to membrane fluidity.

The physical state of the erythrocyte membrane in myotonic dystrophy

The molecular pathology of myotonic dystrophy is believed to be expressed at the plasma membrane level. Previous assessments of membrane fluidity, a marker of the biochemical state of the membrane, have yielded conflicting results. In this study, erythrocyte membrane fluidity was reevaluated using highly sensitive fluorescence probe techniques. Steady-state anisotropy was measured with diphenylhexatriene (DPH), trimethylaminophenyl-hexatriene (TMA-DPH) and phenylhexatrienylphenylpropionic acid, probing different regions of the membrane. In the patients, significantly increased steady-state anisotropy was obtained with DPH, probing the hydrophobic core of the membrane, while slightly reduced anisotropy was found with TMA-DPH. The dynamic properties of the membrane lipids were further examined by means of time-resolved measurements with DPH. The excited state decay kinetics could best be described by a bi-exponential decay model. A large redistribution of the probe populations and a reduction of the average order parameter were found in the patients indicating a less ordered or more fluid lipid matrix. These perturbations might be induced by a protein abnormality and altered protein-lipid interaction within the erythrocyte membrane.

Alcohol abuse increases the lipid structural order in human erythrocyte membranes. A steady-state and time-resolved anisotropy study

The effect of ethanol abuse on the lipid ordering of the human erythrocyte membranes was studied by steady-state and time-resolved fluorescence anisotropy measurements of DPH and its polar analogue TMA-DPH, which probe different membrane regions. Steady-state anisotropy values with DPH as a probe were slightly but significantly increased (+3%) in erythrocyte membranes from alcoholic patients. A resistance to the ethanol fluidizing effect was evidenced in these membranes with DPH and TMA-DPH. No difference in the probe lifetimes was detected between the control and the alcoholic subjects. In the alcoholic patients as compared to the healthy controls, the residual anisotropy for DPH was significantly increased (+7%) corresponding to an increase in the orientational order parameter of 4%; a decrease of the apparent correlation time value was also observed. Nevertheless, no differences between the two erythrocyte populations were observed with TMA-DPH.

Effects of temperature and benzyl alcohol on the structure and adenylate cyclase activity of plasma membranes from bovine adrenal cortex

Adenylate cyclase activation by corticotropin (ACTH), fluoride and forskolin was studied as a function of membrane structure in plasma membranes from bovine adrenal cortex. The composition of these membranes was characterized by a very low cholesterol and sphingomyelin content and a high protein content. The fluorescent probes 1,6-diphenylhexa-1,3,5-triene (DPH) and a cationic analogue 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) were, respectively, used to probe the hydrophobic and polar head regions of the bilayer. When both probes were embedded either in the plasma membranes or in liposomes obtained from their lipid extracts, they exhibited lifetime heterogeneity, and in terms of the order parameter S, hindered motion. Under all the experimental conditions tested, S was higher for TMA-DPH than for DPH but both S values decreased linearly with temperature within the range of 10 to 40 degrees C, in the plasma membranes and the liposomes. This indicated the absence of lipid phase transition and phase separation. Addition to the membranes of up to 100 mM benzyl alcohol at 20 degrees C also resulted in a linear decrease in S values. Membrane perturbations by temperature changes or benzyl alcohol treatment made it possible to distinguish between the characteristics of adenylate cyclase activation with each of the three effectors used. Linear Arrhenius plots showed that when adenylate cyclase activity was stimulated by forskolin or NaF, the activation energy was similar (70 kJ.mol-1). Fluidification of the membrane with benzyl alcohol concentrations of up to 100 mM at 12 or 24 degrees C produced a linear decrease in the forskolin-stimulated activity, that led to its inhibition by 50%. By contrast, NaF stabilized adenylate cyclase activity against the perturbations induced by benzyl alcohol at both temperatures. In the presence of ACTH, biphasic Arrhenius plots were characterized by a well-defined break at 18 degrees C, which shifted at 12.5 degrees C in the presence of 40 mM benzyl alcohol. These plots suggested that ACTH-sensitive adenylate cyclase exists in two different states. This hypothesis was supported by the striking difference in the effects of benzyl alcohol perturbation when experiments were performed below and above the break temperature. The present results are consistent with the possibility that clusters of ACTH receptors form in the membrane as a function of temperature and/or lipid phase fluidity.(ABSTRACT TRUNCATED AT 400 WORDS)

Time-resolved fluorescence anisotropy for systems with lifetime and dynamic heterogeneity

The time dependence of the fluorescence anisotropy expected when a fluorophore exists in distinct environments having different fluorescence decay and motional behavior is illustrated by simulation calculations. A wide range of behavior is observed. The analysis of such decays in terms of the underlying physical parameters is also illustrated and discussed. In particular, the analysis of 'associated' heterogeneous behavior using a homogeneous environment model with complex motional behavior is evaluated. It is argued that anisotropy decays that exhibit a high initial anisotropy and that rise at long times must be due to a heterogeneous environment. Anisotropic rotor diffusion cannot give rise to behavior of this type. A similar conclusion is reached for anisotropies that exhibit downward curvature. On the other hand, anisotropy decays that are monotonically decreasing and have a positive second derivative at all times cannot be analyzed in a unique fashion and therefore an ambiguity exists in the interpretation of such data in terms of motional behavior.

Effect of cholesterol and protein content on membrane fluidity and 3 beta-hydroxysteroid dehydrogenase activity in mitochondrial inner membranes of bovine adrenal cortex

The steroid biosynthetic enzymes in the adrenal cortex are localised in endoplasmic reticulum and mitochondrial membranes. For some of the enzymes in endoplasmic reticulum the activity appears to be modulated by lipid fluidity, (21-hydroxysteroid hydroxylase and 3 beta-hydroxysteroid dehydrogenase). A mechanism for the regulation of corticosteroid biosynthesis mediated by the membrane fluidity has been suggested. Therefore a study of the mitochondrial inner membrane of the bovine adrenal cortex has been undertaken in comparison with a previous study of the endoplasmic reticulum. The kinetic parameters of the 3 beta-hydroxysteroid dehydrogenase were studied as a function of pH and temperature. No thermal transition can be observed in the Arrhenius plot for this enzyme in contrast with the results obtained for the microsomal enzyme. Membrane fluidity using, as fluorescent probes, diphenylhexatriene and a set of n-(9-anthroyloxy)fatty acids has been also studied as a function of temperature with or without addition of cholesterol. No thermal transition in the lipid phase can be observed. The addition of cholesterol to total mitochondrial membrane as to a lipid extract of the membrane decreases fluidity to the same extent as it does with microsomes. The presence of a large amount of protein in mitochondria has an effect which is additive to that of the cholesterol.

Steroid-lipid interactions in sonicated dipalmitoyl phosphatidyl choline vesicles: a steady-state and time-resolved fluorescence anisotropy study with all trans-1,6-diphenyl-1,3,5-hexatriene as probe

The effect of steroid chemical structure on steroid-lipid interaction is studied in sonicated dipalmitoyl phosphatidylcholine vesicles by steady-state and time-resolved fluorescence anisotropy techniques using diphenylhexatriene as probe. A large number of cholesterol derivatives and steroids, some of them being precursors involved in corticosteroids biosynthesis, has been studied. Addition of a polar group on the side chain in carbon-20 or 22 position evokes considerable difference for the interaction with the bilayer. The carbon-22 position appears to be of critical importance. This observation may be relevant for the functioning of mitochondria from steroid producing tissues, where a stereospecific hydroxylation of cholesterol at the carbon-22R position is required for pregnenolone synthesis.