A calorimetric and spectroscopic comparison of the effects of ergosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We performed comparative DSC and FTIR spectroscopic measurements of the effects of cholesterol (Chol) and ergosterol (Erg) on the thermotropic phase behavior and organization of DPPC bilayers. Ergosterol is the major sterol in the biological membranes of yeasts, fungi and many protozoa. It differs from Chol in having two additional double bonds, one in the steroid nucleus at C7-8 and another in the alkyl chain at C22-23. Erg also has an additional methyl group in the alkyl chain at C24. Our DSC studies indicate that the incorporation of Erg is more effective than Chol is in reducing the enthalpy of the pretransition. At lower concentrations Erg is also more effective than Chol in reducing the enthalpies of both the sharp and broad components of main phase transition. However, at sterol concentrations from 30 to 50 mol%, Erg is generally less effective at reducing the enthalpy of the broad components and does not completely abolish the cooperative hydrocarbon chain-melting phase transition at 50 mol%, as does Chol. Nevertheless, in this higher ergosterol concentration range, there is no evidence of the formation of ergosterol crystallites. Our FTIR spectroscopic studies demonstrate that Erg incorporation produces a similar ordering of liquid-crystalline DPPC bilayers as does Chol, but an increased degree of hydrogen bonding of the fatty acyl carbonyl groups in the glycerol backbone region of the DPPC bilayer. These and other results indicate that Erg is less miscible in DPPC bilayers at higher concentrations than is Chol. Finally, we provide a tentative molecular explanation for the comparative experimental and computation results obtained for Erg and Chol in phospholipid bilayers, emphasizing the dynamic conformational differences between these two sterols.     

Comparative calorimetric and spectroscopic studies of the effects of cholesterol and epicholesterol on the thermotropic phase behaviour of dipalmitoylphosphatidylcholine bilayer membranes

We carried out comparative differential scanning calorimetric and Fourier transform infrared spectroscopic studies of the effects of cholesterol (Chol) and epicholesterol (EChol) on the thermotropic phase behaviour and organization of dipalmitoylphosphatidylcholine (DPPC) bilayers. EChol is an epimer of Chol in which the axially oriented hydroxyl group of C3 of Chol is replaced by an equatorially oriented hydroxyl group, resulting in a different orientation of the hydroxyl group relative to sterol fused ring system. Our calorimetric studies indicate that the incorporation of EChol is more effective than Chol is in reducing the enthalpy of the pretransition of DPPC. EChol is also initially more effective than Chol in reducing the enthalpies of both the sharp and broad components of the main phase transition of DPPC. However, at higher EChol concentrations (~ 30-50 mol%), EChol becomes less effective than Chol in reducing the enthalpy and cooperativity of the main phase transition, such that at sterol concentrations of 50 mol%, EChol does not completely abolish the cooperative hydrocarbon chain-melting phase transition of DPPC, while Chol does. However, EChol does not appear to form a calorimetrically detectable crystallite phase at higher sterol concentrations, suggesting that EChol, unlike Chol, may form dimers or lower order aggregates at higher sterol concentrations. Our spectroscopic studies demonstrate that EChol incorporation produces more ordered gel and comparably ordered liquid-crystalline bilayers compared to Chol, which are characterized by increased hydrogen bonding in the glycerol backbone region of the DPPC bilayer. These and other results indicate that monomeric EChol is less miscible in DPPC bilayers than is Chol at higher sterol concentrations, but perturbs their organization to a greater extent at lower sterol concentrations, probably due primarily to the larger effective cross-sectional area of the EChol molecule. Nevertheless, EChol does appear to produce a lamellar liquid-ordered phase in DPPC bilayers.     

Differential scanning calorimetric study of the effect of sterol side chain length and structure on dipalmitoylphosphatidylcholine thermotropic phase behavior.

We have investigated the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers containing a series of cholesterol analogues varying in the length and structure of their alkyl side chains. We find that upon the incorporation of up to approximately 25 mol % of any of the side chain analogues, the DPPC main transition endotherm consists of superimposed sharp and broad components representing the hydrocarbon chain melting of sterol-poor and sterol-rich phospholipid domains, respectively. Moreover, the behavior of these components is dependent on sterol side chain length. Specifically, for all sterol/DPPC mixtures, the sharp component enthalpy decreases linearly to zero by 25 mol % sterol while the cooperativity is only moderately reduced from that observed in the pure phospholipid. In addition, the sharp component transition temperature decreases for all sterol/DPPC mixtures; however, the magnitude of the decrease is dependent on the sterol side chain length. With respect to the broad component, the enthalpy initially increases to a maximum around 25 mol % sterol, thereafter decreasing toward zero by 50 mol % sterol with the exception of the sterols with very short alkyl side chains. Both the transition temperature and cooperativity of the broad component clearly exhibit alkyl chain length-dependent effects, with both the transition temperature and cooperativity decreasing more dramatically for sterols with progressively shorter side chains. We ascribe the chain length-dependent effects on transition temperature and cooperativity to the hydrophobic mismatch between the sterol and the host DPPC bilayer (see McMullen, T. P. W., Lewis, R. N. A. H., and McElhaney, R. N. (1993) Biochemistry 32:516-522).(ABSTRACT TRUNCATED AT 250 WORDS)     

A calorimetric and spectroscopic comparison of the effects of lathosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We performed differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopic measurements to study the effects of lathosterol (Lath) on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine (DPPC) bilayer membranes and compared our results with those previously reported for cholesterol (Chol)/DPPC binary mixtures. Lath is the penultimate intermediate in the biosynthesis of Chol in the Kandutsch-Russell pathway and differs from Chol only in the double bond position in ring B, which is between C7 and C8 in Lath and between C5 and C6 in Chol. Our DSC studies indicate that the incorporation of Lath is more effective than Chol in reducing the temperature and enthalpy of the DPPC pretransition. At lower sterol concentrations (≤10 mol %), incorporation of both Lath and Chol decreases the temperature, enthalpy, and cooperativity of the sharp component of the main phase transition of DPPC to a similar extent, but at higher sterol concentrations, Lath is more effective at decreasing the phase transition temperature, enthalpy, and cooperativity than Chol. These results indicate that at higher concentrations, Lath is more disruptive of DPPC gel-state bilayer packing than Chol is. Moreover, incorporation of Lath decreases the temperature of the broad component of the main phase transition of DPPC, whereas Chol increases it; this difference in the direction and magnitude of the temperature shift is accentuated at higher sterol concentrations. Although at sterol concentrations of ≤20 mol % Lath and Chol are almost equally effective at reducing the enthalpy and cooperativity of the broad component of the main phase transition, at higher sterol levels Lath is less effective than Chol in these regards and does not completely abolish the cooperative hydrocarbon chain melting phase transition at 50 mol %, as does Chol. These latter results indicate that Lath both is more disruptive with respect to the low-temperature state of the sterol-enriched domains of DPPC bilayers and has a lower lateral miscibility in DPPC bilayers than Chol. Our FTIR spectroscopic studies suggest that Lath incorporation produces a less tightly packed bilayer than does Chol at both low (gel state) and high (liquid-crystalline state) temperatures, which is characterized by increased H-bonding between water and the carbonyl groups of the fatty acyl chains in the DPPC bilayer. Overall, our studies indicate that Lath and Chol incorporation can have rather different effects on the thermotropic phase behavior and organization of DPPC bilayers and thus that the position of the double bond in ring B of a sterol molecule can have an appreciable effect on the physical properties of sterol molecules.     

A comparative calorimetric and spectroscopic study of the effects of cholesterol and of the plant sterols β-sitosterol and stigmasterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We performed comparative DSC and FTIR spectroscopic measurements of the effects of β-sitosterol (Sito) and stigmasterol (Stig) on the thermotropic phase behavior and organization of DPPC bilayers. Sito and Stig are the major sterols in the biological membranes of higher plants, whereas cholesterol (Chol) is the major sterol in mammalian membranes. Sito differs in structure from Chol in having an ethyl group at C24 of the alkyl side-chain, and Stig in having both the C24 ethyl group and trans-double bond at C22. Our DSC studies indicate that the progressive incorporation of Sito and Stig decrease the temperature and cooperativity of the pretransition of DPPC to a slightly lesser and greater extent than Chol, respectively, but the pretransition persists to 10 mol % sterol concentration in all cases. All three sterols produce essentially identical effects on the thermodynamic parameters of the sharp component of the DPPC main phase transition. However, the ability to increase the temperature and decrease the cooperativity and enthalpy of the broad component decreases in the order Chol > Sito > Stig. Nevertheless, at higher Sito/Stig concentrations, there is no evidence of sterol crystallites. Our FTIR spectroscopic studies demonstrate that Sito and especially Stig incorporation produces a smaller ordering of the hydrocarbon chains of fluid DPPC bilayers than does Chol. In general, the presence of a C24 ethyl group in the alkyl side-chain reduces the characteristic effects of Chol on the thermotropic phase behavior and organization of DPPC bilayer membranes, and a trans-double bond at C22 magnifies this effect.     

Comparative calorimetric and spectroscopic studies of the effects of lanosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We carried out comparative DSC and Fourier transform infrared spectroscopic studies of the effects of cholesterol and lanosterol on the thermotropic phase behavior and organization of DPPC bilayers. Lanosterol is the biosynthetic precursor of cholesterol and differs in having three rather than two axial methyl groups projecting from the β-face of the planar steroid ring system and one axial methyl group projecting from the α-face, whereas cholesterol has none. Our DSC studies indicate that the incorporation of lanosterol is more effective than cholesterol is in reducing the enthalpy of the pretransition. Lanosterol is also initially more effective than cholesterol in reducing the enthalpies of both the sharp and broad components of the main phase transition. However, at sterol concentrations of 50 mol %, lanosterol does not abolish the cooperative hydrocarbon chain-melting phase transition as does cholesterol. Moreover, at higher lanosterol concentrations (~30–50 mol %), both sharp and broad low-temperature endotherms appear in the DSC heating scans, suggestive of the formation of lanosterol crystallites, and of the lateral phase separation of lanosterol-enriched phospholipid domains, respectively, at low temperatures, whereas such behavior is not observed with cholesterol at comparable concentrations. Our Fourier transform infrared spectroscopic studies demonstrate that lanosterol incorporation produces a less tightly packed bilayer than does cholesterol, which is characterized by increased hydration in the glycerol backbone region of the DPPC bilayer. These and other results indicate that lanosterol is less miscible in DPPC bilayers than is cholesterol, but perturbs their organization to a greater extent, probably due primarily to the rougher faces and larger cross-sectional area of the lanosterol molecule and perhaps secondarily to its decreased ability to form hydrogen bonds with adjacent DPPC molecules. Nevertheless, lanosterol does appear to produce a lamellar liquid-ordered phase in DPPC bilayers, although this phase is not as tightly packed as comparable cholesterol/DPPC mixtures.     

Sterol chemical configuration influences the thermotropic phase behaviour of dipalmitoylphosphatidylcholine bilayers containing 5α-cholestan-3β- and 3α-ol

It is commonly believed that all membrane sterols are rigid all-trans ring systems with a fully extended alkyl side-chain and that they similarly influence phospholipid bilayer physical properties. Here, we report the sterol concentration-dependent, thermotropic phase behaviour of binary dipalmitoylphosphatidylcholine (DPPC)/sterol mixtures containing two similar 5α-H sterols with different functional group orientations (3α-OH or 3β-OH), which adopt an ideal all-trans planar ring conformation but lack the deformed ring B conformation of cholesterol (Chol) and epicholesterol (Echol), using differential scanning calorimetry (DSC). Our deconvolution of the DSC main phase transition endotherms show differences in the proportions of sterol-poor (sharp) and sterol-rich (broad) domains in the DPPC bilayer with increasing sterol concentration, which delineate gel/liquid-crystalline (P_β′/L_α) and disordered gel (L_β)/liquid-ordered (l_o) phase regions. There are similarities in the DPPC main phase transition temperature, cooperativity and enthalpy for each 3β-ol and 3α-ol pair with increasing sterol concentration and differences in the parameters obtained for both the sterol-poor and sterol-rich regions. The sterol-poor domain persists over a greater concentration range in both 3α-ol/DPPC mixtures, suggesting that either those domains are more stable in the 3α-ols or that those sterols are less miscible in the sterol-rich domain. Corresponding parameters for the sterol-rich domain show that at sterol concentrations up to 20 mol%, the 5α-H,3β-ol is more effective at reducing the phase transition enthalpy of the broad component () than Chol, but is less effective at higher concentrations. Although mixtures containing Echol and 5α-cholestan-3α-ol have similar positive slopes below 7 mol% sterol, suggesting that they abolish the L_β/l_o phase transition equally effectively at low concentrations, Echol is more effective than the saturated 3α-ol at higher sterol concentrations. A comparison of obtained for the saturated and unsaturated pairs suggests that the latter sterols stabilize the l_o phase and broaden and abolish the DPPC main phase transition more effectively than the saturated sterols at physiologically relevant concentrations, supporting the idea that the double bond of Chol and Echol promotes greater sterol miscibility and the formation of l_o phase lipid bilayers relative to corresponding saturated sterols in biological membranes.     

Calorimetric and spectroscopic studies of the effects of cholesterol on the thermotropic phase behavior and organization of a homologous series of linear saturated phosphatidylglycerol bilayer membranes

We have examined the effects of cholesterol (Chol) on the thermotropic phase behavior and organization of aqueous dispersions of a homologous series of linear disaturated phosphatidylglycerols (PGs) by high-sensitivity differential scanning calorimetry and Fourier transform infrared and ³¹P NMR spectroscopy. We find that the incorporation of increasing quantities of Chol alters the temperature and progressively reduces the enthalpy and cooperativity of the gel-to-liquid-crystalline phase transition of the host PG bilayer. With dimyristoyl-PG:Chol mixtures, cooperative chain-melting phase transitions are completely or almost completely abolished at Chol concentrations near 50 mol%, whereas with the dipalmitoyl- and distearoyl-PG:Chol mixtures, cooperative hydrocarbon chain-melting phase transitions are still discernable at Chol concentrations near 50 mol%. We are also unable to detect the presence of significant populations of separate domains of the anhydrous or monohydrate forms of Chol in our binary mixtures, in contrast to previous reports. We ascribe the previously reported large scale formation of Chol crystallites to the fractional crystallization of the Chol and phospholipid phases during the removal of organic solvent from the binary mixture before the hydration of the sample. We further show that the direction and magnitude of the change in the phase transition temperature induced by Chol addition is dependent on the hydrocarbon chain length of the PG studied. This finding agrees with our previous results with phosphatidylcholine bilayers, where we found that Chol increases or decreases the phase transition temperature in a hydrophobic mismatch-dependent manner (Biochemistry 1993, 32:516-522), but is in contrast to our previous results for phosphatidylethanolamine (Biochim. Biophys. Acta 1999, 1416:119-234) and phosphatidylserine (Biophys. J. 2000, 79:2056-2065) bilayers, where no such hydrophobic mismatch-dependent effects were observed. We also show that the addition of Chol facilitates the formation of the lamellar crystalline phase in PG bilayers, as it does in phosphatidylethanolamine and phosphatidylserine bilayers, whereas the formation of such phases in phosphatidylcholine bilayers is inhibited by the presence of Chol. Moreover, the formation of the lamellar crystalline phase in PG bilayers at lower temperatures excludes Chol, resulting in an apparent Chol immiscibility in gel-state PG bilayers. We suggest that the magnitude of the effect of Chol on the thermotropic phase behavior of the host phospholipid bilayer, and its miscibility in phospholipids dispersions generally, depend on the strength of the attractive interactions between the polar headgroups and the hydrocarbon chains of the phospholipid molecule, and not on the charge of the polar headgroups per se.     

Investigations on the Δ23-, Δ24(28)- and Δ25-Sterols of zea mays

The sterols of Zea mays shoots were isolated and characterized by TLC, HPLC, GC/MS and ¹H NMR techniques. In all, 22 4-demethyl sterols were identified and they included trace amounts of the Δ²³-, Δ²⁴- and Δ²⁵-sterols, 24-methylcholesta-5,E-23-dien-3β-ol, 24-methylcholesta-5,Z-23-dien-3β-ol, 24-methylcholesta-5,25-dien-3β-ol, 24-ethylcholesta-5,25-dien-3β-ol and 24-ethylcholesta-5,24-dien-3β-ol. In the 4,4-dimethyl sterol fraction, cycloartenol and 24-methylenecycloartanol were the major sterol components but small amounts of the Δ²³-compound, cyclosadol, and the Δ²⁵-compound, cyclolaudenol, were recognized. These various Δ²³- and Δ²⁵-sterols may have some importance in alternative biosynthetic routes to the major sterols, particularly the 24β-methylcholest-5-en-3β-ol component of the C₂₈-sterols. Radioactivity from both [2-¹⁴C]MVA and [methyl-¹⁴C]methionine was incorporated by Z. mays shoots into the sterol mixture. Although 24-methylene and 24-ethylidene sterols were relatively highly labelled, the various Δ²³- and Δ²⁵-sterols contained much lower levels of radioactivity, which is possibly indicative of their participation in alternative sterol biosynthetic routes. (24R)-24-Ethylcholest-5-en-3β-ol (sitosterol) had a significantly higher specific activity than the 24-methylcholest-5-en-3β-ol indicating that the former is synthesized at a faster rate.     

Phase diagram of stigmasterol-dipalmitoylphosphatidylcholine mixtures dispersed in excess water

As a simple model of rafts in plant cells, the effect of stigmasterol, one of the predominant sterols in plant plasma membranes, on the phase behavior of dipalmitoylphosphatidylcholine (DPPC) multilayers has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and freeze-fracture electron microscopy (FFEM) techniques. A partial phase diagram of the binary system has been constructed. Particularly, the stigmasterol concentrations of the “left endpoint” and “right endpoint” of the three-phase line have been determined using the newly developed linear and nonlinear fitting method. They are 6.2 and 23.7 mol%, respectively. Furthermore, the resemblance and difference of phase diagrams of DPPC/stigmasterol, DPPC/cholesterol, and DPPC/ergosterol have been compared and the efficiency of these sterols in promoting the formation of the liquid-ordered domains (rafts) have also been discussed.     

Sterols from sponges of Anavilhanas

Sponges present a wide variety of metabolites, and are considered one of the hotspots in research on the chemistry of natural products. Sterols from sponges have received attention because they present patterns of branches that distinguish them from all other living organisms. Freshwater sponges, native to rivers and lakes, have been studied chemically throughout the world, but there have been no studies on sponges from the Amazon region. The present work describes the sterols present in freshwater sponges collected in Anavilhanas, the world's second largest river archipelago, in the Negro river (Amazonas-Brazil), focussing on species whose family has not been studied previously in regard to their chemistry of natural products. Using a set of derivatization reactions for identification by chromatographic and spectrometric techniques, it was observed that the steroid extracts of sponges of the species Metania reticulata, Drulia browni and Drulia uruguayensis (Metaniidae) present 24-ethyl-cholest-5,22-dien-3β-ol as the principal sterol. Cholesterol, the main sterol in Spongillidae and Lubomirskiidae, was already detected but as a minor component along with three other sterols.     

Codisterol and other Δ5-sterols in the seeds of Cucurbita maxima

A substantial amount (ca 18%) of the sterol found in the seeds of Cucurbita maxima had a Δ-bond and consisted of seven components. They were identified as 25(27)-dehydroporiferasterol, clerosterol, isofucosterol, stigmasterol, sitosterol, campesterol and codisterol. The C-24 configuration of each of the sterols was unequivocally established by a ¹H NMR spectral comparison with authentic standards. This is the first time codisterol has been found in a higher plant and also the first time the structures and configurations of the Δ⁵-sterols from a Cucurbitaceae species have been clearly characterized.     

Influence of plant sterols on the phase properties of phospholipid bilayers

The effects of stigmasterol, sitosterol, campesterol, and cholesterol on the phase properties of dipalmitoylphosphatidylcholine bilayers have been compared by differential scanning calorimetry and x-ray diffraction. The sterols were equally effective at progressively reducing the cooperativity and the enthalpy of the dipalmitoylphosphatidylcholine phase transition as their concentrations in the bilayer were increased. Moreover, both differential scanning calorimetry and x-ray diffraction indicated that the dipalmitoylphosphatidylcholine transition was eliminated by each of the sterols when they were present at a concentration of 33 mole%. This indicates that the interaction between phospholipid and both plant and animal sterols is stoichiometric, each sterol associating with two phospholipid molecules. At concentrations above 33 mole% the sterols were no longer completely solvated by the phospholipid, and sterol-sterol interaction resulted. Cholesterol, even at concentrations as high as 50 mole%, did not disrupt the lamellar structure of the bilayer. When these high concentrations of plant sterols were intercalated into the phospholipid, crystallinity, which presumably derives from sterol-sterol interaction, was detectable in the bilayer by x-ray diffraction. This observation is consistent with previous reports to the effect that the C₁₇ chains of the plant sterols render them less soluble in phospholipid than is cholesterol. It is clear that this solvation difference is of insufficient magnitude to affect the stoichiometry of dipalmitoylphosphatidylcholine-sterol interaction, but it could well account for the less effective modulation of lipid bilayer permeability exhibited by plant sterols in comparison with cholesterol.     

Calorimetric and spectroscopic studies of the phase behavior and organization of lipid bilayer model membranes composed of binary mixtures of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol

The thermotropic phase behavior of hydrated bilayers derived from binary mixtures of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry, Fourier-transform infrared spectroscopy and ³¹P-nuclear magnetic resonance spectroscopy. Binary mixtures of DMPC and DMPG that have not been annealed at low temperatures exhibit broad, weakly energetic pretransitions (∼11-15 °C) and highly cooperative, strongly energetic gel/liquid-crystalline phase transitions (∼23-25 °C). After low temperature incubation, these mixtures also exhibit a thermotropic transition form a lamellar-crystalline to a lamellar gel phase at temperatures below the onset of the gel/liquid-crystalline phase transition. The midpoint temperatures of the pretransitions and gel/liquid-crystalline phase transitions of these lipid mixtures are both maximal in mixtures containing ∼30 mol% DMPG but the widths and enthalpies of the same thermotropic events exhibit no discernable composition dependence. In contrast, thermotropic transitions involving the L_c phase exhibit a very strong composition dependence, and the midpoint temperatures and transition enthalpies are both maximal with mixtures containing equimolar amounts of the two lipids. Our spectroscopic studies indicate that the L_c phases formed are structurally similar as regards their modes of hydrocarbon chain packing, interfacial hydration and hydrogen-bonding interactions, as well as the range and amplitudes of the reorientational motions of their phosphate headgroups. Our results indicate that although DMPC and DMPG are highly miscible, their mixtures do not exhibit ideal mixing. We attribute the non-ideality in their mixing behavior to the formation of preferential PC/PG contacts in the L_c phase due to the combined effects of steric crowding of the DMPC headgroups and charge repulsion between the negatively charged DMPG molecules.     

The effects of amphotericin B on pure and ergosterol- or cholesterol-containing dipalmitoylphosphatidylcholine bilayers as viewed by 2H NMR

Amphotericin B (AmB) is a widely used polyene antibiotic to treat systemic fungal infections. This drug is known to be lethal to fungal cells but it has also side effect toxicity on mammalian cells. The mechanism of action of AmB is thought to be related to the difference of the main sterol present in the mammalian and the fungal cells, namely cholesterol and ergosterol, respectively. The effect of AmB has been investigated on pure dipalmitoylphosphatidylcholine (DPPC) and on cholesterol- and ergosterol-containing DPPC bilayers by 2H NMR spectroscopy. The 2H NMR results first confirm that AmB forms a complex with sterol-free DPPC bilayers, the interaction causing the structurization of the lipids and the increase of the gel-to-lamellar fluid DPPC phase transition temperature with increasing concentration of the antibiotic. The results also show that the effects of AmB on cholesterol- and ergosterol-containing DPPC bilayers are remarkably different. On one hand, the drug causes an increase of the orientational order of the lipid acyl chains in cholesterol-containing membranes, mostly in high cholesterol content membranes. On the other hand, the addition of AmB disorders the DPPC acyl chains when ergosterol is present. This is thought to be due to the direct complexation of the ergosterol by AmB, causing the sterol ordering effect to be weaker on the lipids.     

Study of the interaction of an α-helical transmembrane peptide with phosphatidylcholine bilayer membranes by means of densimetry and ultrasound velocimetry

We applied precise densimetry and ultrasound velocimetry methods to study the interaction of a synthetic α-helical transmembrane peptide, acetyl-K₂-L₂₄-K₂-amide (L₂₄), with model bilayer lipid membranes. The large unilamellar vesicles (LUVs) utilized were composed of a homologous series of n-saturated diacylphosphatidylcholines (PCs). PCs whose hydrocarbon chains contained from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us to analyze how the difference between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer influences the thermodynamical and mechanical properties of the membranes. We showed that the incorporation of L₂₄ decreases the temperature and cooperativity of the main phase transition of all LUVs studied. The presence of L₂₄ in the bilayer also caused an increase of the specific volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide concentration (mole ratio L₂₄:PC = 1:50). The overall volume compressibility of the peptide-containing lipid bilayers in the liquid-crystalline state was in general higher in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs and large multilamellar vesicles (MLVs) with and without L₂₄. As expected, a higher cooperativity of the changes of the thermodynamical and mechanical parameters took place for MLVs in comparison with LUVs. These results are in agreement with previously reported DSC and ²H NMR spectroscopy study of the interaction of the L₂₄ and structurally related peptides with phosphatidylcholine bilayers. An apparent discrepancy between ²H NMR spectroscopy and compressibility data in the liquid crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L₂₄ suggest that around each peptide a distorted region exists that involves at least 2 layers of lipid molecules.     

Magnetic alignment and orientational order of dipalmitoylphosphatidylcholine bilayers containing palmitoyllysophosphatidylcholine

Mixed bilayers of 1-palmitoyl-sn-glycero-3-phosphocholine (palmitoyllysophosphatidylcholine; PaLPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (dipalmitoyl phosphatidylcholine; DPPC) have been investigated by 2H-NMR and 31P-NMR spectroscopy. Binary phospholipid mixtures were studied in which the acyl chains of one or the other component were perdeuterated. At temperatures below the main order-disorder phase transition, the mixed PaLPC/DPPC bilayers appear to coexist with PaLPC micelles. The micelles disappear at temperatures above the phase transition, where mixed bilayers in the liquid-crystalline state are formed. The orientational order of the alkyl chains of the PaLPC component is essentially identical to that of the DPPC component in the mixed bilayers, both in the low temperature and liquid-crystalline phases. However, the presence of PaLPC perturbs the segmental ordering of DPPC as compared to the pure system. The order is increased in the low-temperature phase, where effective diffusion of the chains about their long axes occurs, but is decreased in the liquid-crystalline phase compared to pure DPPC bilayers. The mixed liquid-crystalline bilayers orient preferentially with their director axes perpendicular to the magnetic field. This alignment is easily observed in 31P- and 2H-NMR spectra, where the intensity of the perpendicular edges of the lineshapes is pronounced. One possible explanation of the magnetic alignment involves alteration of the curvature free energy of the DPPC bilayer due to incorporation of PaLPC in the mixed membranes.     

DNA and cholesterol biosynthesis in synchronized embryonic rat fibroblasts: I. Temporal relationships between HMG-CoA reductase activity,sterol biosynthesis and thymidine incorporation into DNA

Temporal relationships between hydroxymethylglutaryl-CoA reductase activity, biosynthesis of C₂₇ sterols, and [³H]thymidine incorporation into DNA were studied in a rat embryo fibroblast cell line synchronized by double thymidine block and cultured in cholesterol-containing medium. Cyclic variations of HMG-CoA reductase activity and C₂₇ sterols occurred, with two maxima in S and G₂M phases; the relative shortness of the G₁ phase (3 h) in these cells could be responsible for the shift of sterol synthesis in the S phase. No noticeable variation of the individual C₂₇ sterols was observed during the entire cell cycle. In each experiment, there was a good linear correlation between HMG-CoA reductase activity and C₂₇ sterol synthesis, but from one experiment to another, a given level of enzymatic activity led to varying levels of [2-¹⁴C]acetate incorporation into sterols. In our experimental conditions, total HMG-CoA reductase activity is measured, and the preceding observation could be explained by a varying degree of phosphorylation of the enzyme depending on the metabolic state of the cells at the start of the experiment. The cyclic variations of the enzyme activity seem to be due more to increased synthesis at given times of the cycle than to periodic dephosphorylation. We question the existence of a relationship between cell division and cyclic sterol synthesis occurring in cells cultured in cholesterol-containing medium.     

Transmembrane Peptides Influence the Affinity of Sterols for Phospholipid Bilayers

Cholesterol is distributed unevenly between different cellular membrane compartments, and the cholesterol content increases from the inner bilayers toward the plasma membrane. It has been suggested that this cholesterol gradient is important in the sorting of transmembrane proteins. Cholesterol has also been to shown play an important role in lateral organization of eukaryotic cell membranes. In this study the aim was to determine how transmembrane proteins influence the lateral distribution of cholesterol in phospholipid bilayers. Insight into this can be obtained by studying how cholesterol interacts with bilayer membranes of different composition in the presence of designed peptides that mimic the transmembrane helices of proteins. For this purpose we developed an assay in which the partitioning of the fluorescent cholesterol analog CTL between LUVs and mβCD can be measured. Comparison of how cholesterol and CTL partitioning between mβCD and phospholipid bilayers with different composition suggests that CTL sensed changes in bilayer composition similarly as cholesterol. Therefore, the results obtained with CTL can be used to understand cholesterol distribution in lipid bilayers. The effect of WALP23 on CTL partitioning between DMPC bilayers and mβCD was measured. From the results it was clear that WALP23 increased both the order in the bilayers (as seen from CTL and DPH anisotropy) and the affinity of the sterol for the bilayer in a concentration dependent way. Although WALP23 also increased the order in DLPC and POPC bilayers the effects on CTL partitioning was much smaller with these lipids. This indicates that proteins have the largest effect on sterol interactions with phospholipids that have longer and saturated acyl chains. KALP23 did not significantly affect the acyl chain order in the phospholipid bilayers, and inclusion of KALP23 into DMPC bilayers slightly decreased CTL partitioning into the bilayer. This shows that transmembrane proteins can both decrease and increase the affinity of sterols for the lipid bilayers surrounding proteins. This is likely to affect the sterol distribution within the bilayer and thereby the lateral organization in biomembranes.     

A calorimetric study of binary mixtures of dihydrosphingomyelin and sterols,sphingomyelin, or phosphatidylcholine

The thermotropic properties of binary mixtures of D-erythro-n-palmitoyl-dihydrosphingomyelin (16:0-DHSM), D-erythro-n-palmitoyl-sphingomyelin (16:0-SM), cholesterol, lathosterol, and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were studied by differential scanning calorimetry. Addition of sterol to 16:0-DHSM and 16:0-SM bilayers resulted in a progressive decrease in both the T(m) and the enthalpy of the main transition. The sterol-induced broad components in 16:0-DHSM endotherms had markedly lower enthalpies than those induced in 16:0-SM. Pretransitions recorded in 16:0-DHSM and 16:0-SM membranes responded differently to low concentrations of cholesterol. The presence of 5 mol % cholesterol increased the pretransition temperature in 16:0-SM bilayers, whereas it decreased the temperature in 16:0-DHSM membranes. Lathosterol behaved in general as cholesterol with regard to its effects on the thermotropic behavior of both sphingolipids, but it appeared to form more stable sterol-rich domains, as seen from the higher T(m) of the broad component, in comparison to cholesterol. Thermograms recorded on binary mixtures of 16:0-SM:16:0-DHSM and DPPC:16:0-DHSM showed that 16:0-SM mixed nearly ideally with 16:0-DHSM, whereas DPPC mixing was less ideal in a 16:0-DHSM membrane. In conclusion, we observed that 16:0-DHSM interactions with sterols differed from that seen with 16:0-SM, and that 16:0-DHSM mixed better with 16:0-SM than DPPC, which indicates that DHSM could function as a membrane organizer within laterally condensed domains.