Cholesterol is excluded from the phospholipid annulus surrounding an active calcium transport protein

Degenerate perturbations of protein structure as the mechanism of anaesthetic action

The interaction of the n-alkanols with lipid bilayers and excitable membranes shows that there is no simple correlation between conduction block and any of the perturbations of bilayer structure currently proposed as unitary mechanisms of local anaesthetic action. We propose instead that the n-alkanols act by direct interaction with target proteins to cause perturbations which depend directly on the precise structure of the alcohol.

The phospholipid headgroup specificity of an ATP-dependent calcium pump

We have replaced the lipid associated with a purified calcium transport protein with a series of defined synthetic dioleoyl phospholipids in order to determine the effect of phospholipid headgroup structure on the ATPase activity of the protein. At 37 degrees C the zwitterionic phospholipids (dioleoyl phosphatidylcholine and dioleoyl phosphatidylethanolamine) support the highest activity, while a phospholipid with two negative charges (dioleoyl phosphatidic acid) supports an activity which is at least twenty times lower. Dioleoyl phospholipids with a single net negative charge support at intermediate ATPase activity which is not affected by the precise chemical structure of the phospholipid headgroup. The protocol used to determine the phospholipid headgroup specificity of calcium transport protein is novel because it establishes the composition of the lipid in contact with the protein without the need to isolate defined lipid-protein complexes. This allows the lipid specificity to be determined using only very small quantities of test lipids. We also determined the ability of the same phospholipids to support calcium accumulation in reconstituted membranes. Two requirements had to be met. The phospholipid had to support the ATPase activity of the pump protein and it had to form sealed vesicles as determined by electron microscopy. Since a number of phospholipids met those requirements it is clear that in vitro the lipid specificity of the calcium-accumulating system is rather broad.

Interaction of cholesterol, phospholipid and apoprotein in high density lipoprotein recombinants

To examine the effect of incorporation of cholesterol into high density lipoprotein (HDL) recombinants, multilamellar liposomes of 3H cholesterol/14C dimyristoyl phosphatidylcholine were incubated with the total apoprotein (apoHDL) and principal apoproteins (apoA-1 and apoA-2) of human plasma high density lipoprotein. Soluble recombinants were separated from unreacted liposomes by centrifugation and examined by differential scanning calorimetry and negative stain electron microscopy. At 27 degrees C, liposomes containing up to approx. 0.1 mol cholesterol/mol dimyristoyl phosphatidylcholine (DMPC) were readily solubilized by apoHDL, apoA-1 or apoA-2. However, the incorporation of DMPC and apoprotein into lipoprotein complexes was markedly reduced when liposomes containing a higher proportion of cholesterol were used. For recombinants prepared from apoHDL, apoA-1, or apoA-2, the equilibrium cholesterol content of complexes was approx. 45% that of the unreacted liposomes. Electron microscopy showed that for all cholesterol concentrations, HDL recombinants were predominantly lipid bilayer discs, approx. 160 X 55 A. Differential scanning calorimetry of cholesterol containing recombinants of DMPC/cholesterol/apoHDL or DMPC/cholesterol/apoA-1 showed, with increasing cholesterol content, a linear decrease in the enthalpy of the DMPC gel to liquid crystalline transition, extrapolating to zero enthalpy at 0.15 cholesterol/DMPC. The enthalpy values were markedly reduced compared to control liposomes, where the phospholipid transition extrapolated to zero enthalpy at approx. 0.45 cholesterol/DMPC. The calorimetric and solubility studies suggest that in high density lipoprotein recombinants cholesterol is excluded from 55% of DMPC molecules bound in a non-melting state by apoprotein.

Alterations of membrane phosphorylation in erythrocyte membranes from patients with Duchenne muscular dystrophy

The phosphorylation of spectrin, band 3 protein, and the phospholipids of erythrocyte membranes (ghosts) was examined in 10 patients with Duchenne muscular dystrophy (DMD) and in healthy age- and sex-matched controls. The rates of phosphorylation of spectrin and band 3 protein were significantly higher in ghosts prepared from patient blood than from control blood at both 30 degrees C and 37 degrees C. However, the mean increases in the rate of phosphorylation of both spectrin and band 3 protein in response to a temperature change from 30 degrees C to 37 degrees C were identical in ghosts from patient and controls. Phosphorylation of phospholipid and its temperature response did not differ between patients and controls. These results complement previous observations of differences in erythrocytes from patients with DMD. The similarity of the changes in phosphorylation of both spectrin and band 3 protein indicates a common cause, possibly their lipid environment.

Changes in the form of Arrhenius plots of the activity of glucagon-stimulated adenylate cyclase and other hamster liver plasma-membrane enzymes occurring on hibernation

1. Arrhenius plots of the glucagon-stimulated adenylate cyclase, 5'-nucleotidase, (Na+ + K+)-stimulated adenosine triphosphatase and Mg2+-dependent adenosine triphosphatase activities of control hamster liver plasma membranes exhibited two break points at around 25 and 13 degrees C, whereas Arrhenius plots of their activities in hibernating hamster liver plasma membranes exhibited two break points at around 25 and 4 degrees C. 2. A single break occurring between 25 and 26 degrees C was observed in Arrhenius plots of the activities of fluoride-stimulated adenylate cyclase, basal adenylate cyclase and cyclic AMP phosphodiesterase of liver plasma membranes from both control and hibernating animals. 3. Arrhenius plots of phosphodiesterase I activity showed a single break at 13 degrees C for membranes from control animals, and a single break at around 4 degrees C for liver plasma membranes from hibernating animals. 4. The temperature at which break points occurred in Arrhenius plots of glucagon- and fluoride-stimulated adenylate cyclase activity were decreased by about 7--8 degrees C by addition of 40 mm-benzyl alcohol to the assays. 5. Discontinuities in the Arrhenius plots of 4-anilinonaphthalene-1-sulphonic acid fluorescence occurred at around 24 and 13 degrees C for liver plasma membranes from control animals, and at around 25 and 4 degrees C for membranes from hibernating animals. 6. We suggest that in hamster liver plasma membranes from control animals a lipid phase separation occurs at around 25 degrees C in the inner half of the bilayer and at around 13 degrees C in the outer half of the bilayer. On hibernation a change in bilayer asymmetry occurs, which is expressed by a decrease in the temperature at which the lipid phase separation occurs in the outer half of the bilayer to around 4 degrees C. The assumption made is that enzymes expressing both lipid phase separations penetrate both halves of the bilayer, whereas those experiencing a single break penetrate one half of the bilayer only.

Freeze-fracture identification of sterol-digitonin complexes in cell and liposome membranes

To advance our understanding of the organization of cholesterol within cell membranes, we used digitonin in freeze-fracture investigations of model lipid vesicles and tissues. Cholesterol suspensions or multilamellar liposomes composed of phosphatidylcholine with and without cholesterol were exposed to digitonin. Freeze-fracture replicas of those multilamellar liposomes containing cholesterol displayed either 50--60-nm wide intramembrane corrugations or extramembrane tubular complexes. Comparable intramembrane hemitubular scallops and extra-cellular free tubular complexes were observed in thin sections. Exposure of sperm, erythrocytes (whole and ghosts), and intact tissues (skin, liver, adrenal gland, epididymis) to digitonin produced the same types of intra- and extramembrane complexes or furrows as were formed in liposomes. The plasma membrane of guinea pig serum tail had two unfurrowed regions: the annulus and the zipper. Incubating erythrocyte membranes with digitonin resulted in rapid displacement of cholesterol, accompanied by intramembrane particle clustering and membrane faceting, a feature which we did not see in the intact epithelia studied. In freeze-fractured epithelia, we found that plasma membranes, lysosomes, and some vesicular organelles commonly furrowed, but that mitochondrial membranes and nuclear envelopes were generally spared, correlating well with their known cholesterol content. Finally, plasma membrane corrugations approached but did not impinge on either gap or tight junctions, or on coated vesicles. We conclude that freeze-fracture of membranes exposed to digitonin: (a) reveals distinctive cholesterol-digitonin structural complexes; (b) distinguishes cholesterol-rich and -poor organelle membranes; and (c) demonstrates membrane domains rich or poor in cholesterol.

The activity of glucagon-stimulated adenylate cyclase from rat liver plasma membranes is modulated by the fluidity of its lipid environment

1. The local anaesthetic benzyl alcohol progressively activated glucagon-stimulated adenylate cyclase activity up to a maximum at 50 mM-benzyl alcohol. Further increases in benzyl alcohol concentration inhibited the activity. The fluoride-stimulated adenylate cyclase activity was similarly affected except for an inhibition of activity occurring at low benzyl alcohol concentrations (approx. 10 mM. 2. The fluoride-stimulated adenylate cyclase activity of a solubilized enzyme preparation was unaffected by any of the benzyl alcohol concentrations tested. 3. Increases in 3-phenylpropan-1-ol and 5-phenylpentan-1-ol concentrations progressively activated both the fluoride- and glucagon-stimulated adenylate cyclase activities up to a maximum, above which further increases in alcohol concentration inhibited the activities. 4. The 'break' points in Arrhenius plots of glucagon-stimulated adenylate cyclase activity in native plasma membranes, and in plasma membranes fused with synthetic dimyristoyl phosphatidylcholine so as to constitute 60% of the total lipid pool, were decreased by approx. 6 degrees C by addition of 40 mM-benzyl alcohol. This was accompanied by a fall in the associated activation energies. 6. Arrhenius plots of fluoride-stimulated adenylate cyclase activity in the presence and absence of 40 mM-benzyl alcohol were linear, although addition of benzyl alcohol caused a dramatic decrease in the associated activation energy of the reaction. 7. 5'-Nucleotidase activity was stimulated by benzyl alcohol, and the 'break' point in the Arrhenius plot of its activity was decreased by about 6 degrees C by addition of 40 mM-benzyl alcohol to the assay. 8. It is suggested that benzyl alcohol effects a fluidization of the bilayer, which is clearly demonstrated by its ability to lower the temperature of a lipid phase separation occurring at 28 degrees C in the outer half of the bilayer to around 22 degrees C. The increase in bilayer fluidity relieves a physical constraint on the membrane-bound adenylate cyclase, activating the enzyme. 9. The various inhibition phenomena are discussed in detail, together with the suggestion that the interaction between the uncoupled catalytic unit of adenylate cyclase and the lipids of the bilayer is altered on its physical coupling to the glucagon receptor.

Comparison of the (Ca2+ + Mg2+)-ATPase proteins from normal and dystrophic chicken sarcoplasmic reticulum

The involvement of membrane protein in dystrophic chicken fragmented sarcoplasmic reticulum alterations has been examined. A purified preparation of the (Ca2+ + Mg2+)-ATPase protein from dystrophic fragmented sarcoplasmic reticulum was found to have a reduced calcium-sensitive ATPase activity and phosphoenzyme level, in agreement with alterations found in dystrophic chicken fragmented sarcoplasmic reticulum. An amino acid analysis of the ATPase preparations showed no difference in the normal and dystrophic (Ca2+ + Mg2+)-ATPase. The (Ca2+ + Mg2+)-ATPase was investigated further by isoelectric focusing and proteolytic digestion of the fragmented sarcoplasmic reticulum. Neither of these methods indicated any alteration in the composition of the dystrophic (Ca2+ + Mg2+)-ATPase. We have concluded that the alterations observed in dystrophic fragmented sarcoplasmic reticulum are not due to increased amounts of non-(Ca2+ + Mg2+)-ATPase protein, and that the normal and dystrophic (Ca2+ + Mg2+)-ATPase protein are not detectably different.

Effect of liposomes containing cholesterol on adenylate cyclase activity of cultured mammalian fibroblasts

Liposomes prepared with cholesterol and dipalmitoyl phosphatidylcholine were incubated with a clone of normal rat kidney fibroblast of cells in culture. The cells took up [14C]cholesterol in proportion to the concentration of liposomes in the incubation medium, and the uptake increased with time over the four hours of study. Two cell membrane enzymes, adenylate cyclase and (Na+ + K+)-ATPase, exhibited decreased activity after treatment with cholesterol-containing liposomes. The decrease in adenylate cyclase activity was directly proportional to the uptake of [14C]cholesterol. When a variety of subclones of NRK 5W were examined some were found to respond to cholesterol treatment and some did not. These data are consistent with the view that membrane cholesterol content plays a role in controlling the activity of some plasma membrane enzymes.

Influence of increased membrane cholesterol on membrane fluidity and cell function in human red blood cells

Cholesterol and phospholipid are the two major lipids of the red cell membrane. Cholesterol is insoluble in water but is solubilized by phospholipids both in membranes and in plasma lipoproteins. Morever, cholesterol exchanges between membranes and lipoproteins. An equilibrium partition is established based on the amount of cholesterol relative to phospholipid (C/PL) in these two compartments. Increases in the C/PL of red cell membranes have been studied under three conditions: First, spontaneous increases in vivo have been observed in the spur red cells of patients with severe liver disease; second, similar red cell changes in vivo have been induced by the administration of cholesterol-enriched diets to rodents and dogs; third, increases in membrane cholesterol have been induced in vitro by enriching the C/PL of the lipoprotein environment with cholesterol-phospholipid dispersions (liposomes) having a C/PL of greater than 1.0. In each case, there is a close relationship between the C/PL of the plasma environment and the C/PL of the red cell membrane. In vivo, the C/PL mole ratio of red cell membranes ranges from a normal value of 0.09--1.0 to values which approach but do not reach 2.0. In vitro, this ratio approaches 3.0. Cholesterol enrichment of red cell membranes directly influences membrane lipid fluidity, as assessed by the rotational diffusion of hydrophobic fluorescent probes such as diphenyl hexatriene (DPH). A close correlation exists between increases in red cell membrane C/PL and decreases in membrane fluidity over the range of membrane C/PL from 1.0 to 2.0; however, little further change in fluidity occurs when membrane C/PL is increased to 2.0--3.0. Cholesterol enrichment of red cell membranes is associated with the transformation of cell contour to one which is redundant and folded, and this is associated with a decrease in red cell filterability in vitro. Circulation in vivo in the presence of the spleen further modifies cell shape to a spiny, irregular (spur) form, and the survival of cholesterol-rich red cells is decreased in the presence of the spleen. Although active Na-K transport is not influenced by cholesterol enrichment of human red cells, several carrier-mediated transport pathways are inhibited. We have demonstrated this effect for the cotransport of Na + K and similar results have been obtained by others in studies of organic acid transport and the transport of small neutral molecules such as erythritol and glycerol. Thus, red cell membrane C/PL is sensitive to the C/PL of the plasma environment. Increasing membrane C/PL causes a decrease in membrane fluidity, and these changes are associated with a reduction in membrane permeability, a distortion of cell contour and filterability and a shortening of the survival of red cells in vivo.

Effects of streptolysin O on transport of amino acids, nucleosides, and glucose analogs in mammalian cells

The transport of several amino acids, nucleosides, and glucose analogs was studies in HeLa cells treated with sublethal concentrations of streptolysin O. A differential effect of toxin on the various solutes tested was observed. The uptake of the neutral amino acids alanine, alpha-aminoisobutyric acid, leucine, and phenylalanine was reduced by 60 about to 70%. Less inhibition of transport was observed with acidic and basic amino acids, and the uptake of nucleosides and glucose analogs was reduced by 20% or less. The decreased uptake of alpha-aminoisobutyric acid could be explained by the inability of toxin-treated cells to retain this amino acid. The altered transport of phenylalanine and lysine, however, appeared to be due to decreased initial rates of uptake rather than to the loss of these amino acids from intracellular pools in toxin-treated cells. After treatment with sublethal concentrations of streptolysin O, the cells recovered their ability to transport and accumulate alpha-aminoisobutyric acid in about 4 h. The data suggest that the alterations in membrane transport observed in toxin-treated cells are due to an effect of streptolysin O on specific transport systems, rather than to the production of holes or pores in the membrane.

Modulation of macrophage tumoricidal capability by components of normal serum: a central role for lipid

The tumoricidal capabilities of macrophages can be reversibly inhibited by a lipoprotein of high molecular weight, and the inhibition appears to be reproduced by enrichment of macrophage plasma membranes with cholesterol. A second serum component of lower molecular weight enhances macrophage cytotoxicity. The presence of these components in normal serums suggests a physiological role for such factors in the regulation of macrophage function.

Crystalline patterns of myelin lipids visualized by freeze fracture

Freeze fracture of rat optic nerve reveals smooth, particle-free regions on the lammellar fracture faces of myelin when prepared by standard procedures. When the fixed, glycerin-impregnated tissue is incubated at 6 degrees C for two or more days, crystalline patterns indicative of a phase transition can be seen in the particle-free regions. The crystalline patterns can be destroyed by subsequent incubation at 37 degrees C and are not seen when the initial incubation is at room temperature or 37 degrees C. Butylated hydroxytoluene has no effect on the formation of the crystalline patterns. The time course of the formation of the crystalline patterns suggest that the rate-limiting step in the process is not the phase transition itself. We propose that the lipids associated with the particles in vivo are involved in the formation of the crystalline patterns.

Photochemical functionality of rhodopsin-phospholipid recombinant membranes

Purified rhodopsin was incorporated into phospholipid bilayers to give recombinant membranes. The photochemical functionality in these systems was examined by low-temperature spectroscopy and by kinetic spectrophotometry. Changes in the absorption spectra of glycerol-water mixtures of rhodopsin-egg phosphatidylcholine and rhodopsin-asolectin recombinants were monitored after the sample was cooled to -196 degrees C, presented with light of wavelength greater than 440 nm, and then warmed gradually to room temperature. Absorption characteristics indicative of the spectral intermediates prelumirhodopsin, lumirhodopsin, metarhodopsin I, and metarhodopsin II were observed. The kinetics of the metarhodopsin I -o metarhodopsin II transition in these recombinants was studied by flash photolytic observation of the decay of meta I and the formation of meta II. Recombinants prepared from unsaturated phospholipids, e.g., asolectin, egg phosphatidylcholine, egg phosphatidylethanolamine, and dioleoylphosphatidylcholine, showed first-order kinetics for the transition with rates comparable to that of rod outer segment membranes. Recombinants prepared from saturated phosphatidylcholines have a retarded rate of conversion from meta I to meta II and are considered to be nonfunctional. The photochemical functionality of rhodopsin-phospholipid recombinants is dependent upon the presence of phospholipid unsaturation and the fluidity of the phospholipid hydrocarbon chains, and is independent of the polar head group of the phospholipid.

The erythrocyte membrane site for the effect of temperature on osmotic fragility

The osmotic fragility of human erythrocytes is well known to decrease as the temperature is elevated. The cellular site for the temperature effect was studied by assessing possible roles of hemoglobin and of membrane lipids and by taking advantage of the unique response of camel erythrocytes to temperature. It is concluded that the erythrocyte membrane is the site for the temperature effect on osmotic fragility. The human erythrocyte is likely to rupture in protein--lipid boundary regions in the membrane, from which cholesterol is apparently excluded.

Labelling of egg phosphatidylcholine vesicles and myelin membrane with a photoreactive lipophilic reagent

The preparation and isolation of [3H]phenyl azide, a photosensitive non-polar probe, is reported. The reagent partitions into the lipid bilayer of egg phosphatidylcholine vesicles and bovine myelin membranes. On photoactivation to generate the nitrene grouping, as much as 90% of the covalently attached label is associated with the fatty acyl residues of the constituent phospholipid molecules. The remainder is found in the polar head groups. The cholesterol component of myelin membranes is also heavily labelled. These results suggest that such reagents may be used to probe the hydrophobic regions of natural membranes.