The polar nature of 7-ketocholesterol determines its location within membrane domains and the kinetics of membrane microsolubilization by apolipoprotein A-I

7-Ketocholesterol is an oxidized derivative of cholesterol with numerous physiological effects. In model membranes, 7-ketocholesterol and cholesterol were compared by physical measures of bilayer order and polarity, formation of detergent resistant domains (DRM), phase separation, and membrane microsolubilization by apolipoprotein A-I. In binary mixtures of a saturated phosphatidylcholine (PC), dipalmitoyl-PC (DPPC), and cholesterol or 7-ketocholesterol, the sterols modulate bilayer order and polarity and induce DRM formation to a similar extent. Cholesterol induces formation of ordered lipid domains (rafts) in tertiary mixtures with dioleoyl-PC (DOPC) and DPPC, or DOPC and sphingomyelin (SM). In tertiary mixtures, cholesterol increased lipid order and reduces bilayer polarity more than 7-ketocholesterol. This effect was more pronounced when the mixtures were in a miscible liquid-disordered (L(d)) phase. Substitution of 7-ketocholesterol for cholesterol dramatically reduced the extent of DRM formation in DOPC/DPPC and DOPC/SM bilayers and ordered lipid phase separation in mixtures of a spin-labeled PC with DPPC and with SM. Compared to cholesterol, 7-ketocholesterol decreased the rate for the microsolubilization of dimyristoyl-PC multilamellar vesicles by apolipoprotein A-I. The membrane effects of 7-ketocholesterol were dependent on the phospholipid matrix. In L(d) phase phospholipids, a model for 7-ketocholesterol indicates that the proximity of the 7-keto and 3beta-OH groups puts both polar moieties at the lipid-water interface to tilt the sterol nucleus to the plane of the bilayer. 7-Ketocholesterol was less effective in forming ordered lipid domains, in decreasing the level of bilayer hydration, and in forming phase boundary bilayer defects. Compared to cholesterol, 7-ketocholesterol can differentially modulate membrane properties involved in protein-membrane association and function.     

Membrane microdomains: Role of ceramides in the maintenance of their structure and functions

Free-standing giant unilamellar vesicles were used to visualize the complex lateral heterogeneity, induced by ceramide in the membrane bilayer at micron scale using C₁₂-NBD-PC probe partitioning under the fluorescence microscope. Ceramide gel domains exist as leaf-like structures in glycerophospholipid/ceramide mixtures. Cholesterol readily increases ceramide miscibility with glycerophospholipids but cholesterol-ceramide interactions are not involved in the organization of the liquid-ordered phase as exemplified by sphingomyelin/cholesterol mixtures. Sphingomyelin stabilizes the gel phase and thus decreases ceramide miscibility in the presence of cholesterol. Gel/liquid-ordered/liquid-disordered phase coexistence was visualized in quaternary phosphatidylcholine/sphingomyelin/ceramide/cholesterol mixtures as occurrence of dark leaf-like and circular domains within a bright liquid phase. Sphingomyelin initiates specific ceramide-sphingomyelin interactions to form a highly ordered gel phase appearing at temperatures higher than pure ceramide gel phase in phosphatidylcholine/ceramide mixtures. Less sphingomyelin is engaged in formation of liquid-ordered phase leading to a shift in its formation to lower temperatures. Sphingomyelinase activity on substrate vesicles destroys micron L_o domains but induces the formation of a gel-like phase. The activation of phospholipase A₂ by ceramide on heterogeneous membranes was visualized. Changes in the phase state of the membrane bilayer initiates such morphological processes as membrane fragmentation, budding in and budding out was demonstrated.     

Comparison of the liquid-ordered bilayer phases containing cholesterol or 7-dehydrocholesterol in modeling Smith-Lemli-Opitz syndrome

The phase behavior of egg sphingomyelin (ESM) mixtures with cholesterol or 7-dehydrocholesterol (7-DHC) has been investigated by independent methods: fluorescence microscopy, X-ray diffraction, and electron spin resonance spectroscopy. In giant vesicles, cholesterol-enriched domains appeared as large and clearly delineated domains assigned to a liquid-ordered (L_o) phase. The domains containing 7-DHC were smaller and had more diffuse boundaries. Separation of a gel phase assigned by X-ray examination to pure sphingomyelin domains coexisting with sterol-enriched domains was observed at temperatures less than 38°C in binary mixtures containing 10-mol% sterol. At higher sterol concentrations, the coexistence of liquid-ordered and liquid-disordered phases was evidenced in the temperature range 20°–50°C. Calculated electron density profiles indicated the location of 7-DHC was more loosely defined than cholesterol, which is localized precisely at a particular depth along the bilayer normal. ESR spectra of spin-labeled fatty acid partitioned in the liquid-ordered component showed a similar, high degree of order for both sterols in the center of the bilayer, but it was higher in the coexisting disordered phase for 7-DHC. The differences detected in the models of the lipid membrane matrix are said to initiate the deleterious consequences of the Smith-Lemli-Opitz syndrome.     

Simulations of simple Bovine and Homo sapiens outer cortex ocular lens membrane models with a majority concentration of cholesterol

The lipid composition of bovine and human ocular lens membranes has been probed, and a variety of lipids have been found including phosphatidylcholine (PC), phosphatidylethanolamine (PE), sphingomyelin (SM), and cholesterol (CHOL) with cholesterol being present in particularly high concentrations. In this study, we use the all-atom CHARMM36 force field to simulate binary, ternary, and quaternary mixtures as models of the ocular lens. High concentration of cholesterol, in combination with different and varying diversity of phospholipids (PL) and sphingolipids (SL), affect the structure of the ocular lens lipid bilayer. The following analyses were done for each simulation: surface area per lipid, component surface area per lipid, deuterium order parameters (S_CD), electron density profiles (EDP), membrane thickness, hydrogen bonding, radial distribution functions, clustering, and sterol tilt angle distribution. The S_CD show significant bilayer alignment and packing in cholesterol-rich bilayers. The EDP show the transition from liquid crystalline to liquid ordered with the addition of cholesterol. Hydrogen bonds in our systems show the tendency for intramolecular interactions between cholesterol and fully saturated lipid tails for less complex bilayers. But with an increased number of components in the bilayer, the acyl chain of the lipids becomes a less important characteristic, and the headgroup of the lipid becomes more significant. Overall, cholesterol is the driving force of membrane structure of the ocular lens membrane where interactions between cholesterol, PL, and SL determine structure and function of the biomembrane. The goal of this work is to develop a baseline for further study of more physiologically realistic ocular lens lipid membranes.This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.     

An X-ray diffraction study of model membrane raft structures

Protein sorting and assembly in membrane biogenesis and function involves the creation of ordered domains of lipids known as membrane rafts. The rafts are comprised of all the major classes of lipids, including glycerophospholipids, sphingolipids and sterol. Cholesterol is known to interact with sphingomyelin to form a liquid-ordered bilayer phase. Domains formed by sphingomyelin and cholesterol, however, represent relatively small proportions of the lipids found in membrane rafts and the properties of other raft lipids are not well characterized. We examined the structure of lipid bilayers comprised of aqueous dispersions of ternary mixtures of phosphatidylcholines and sphingomyelins from tissue extracts and cholesterol using synchrotron X-ray powder diffraction methods. Analysis of the Bragg reflections using peak-fitting methods enables the distinction of three coexisting bilayer structures: (a) a quasicrystalline structure comprised of equimolar proportions of phosphatidylcholine and sphingomyelin, (b) a liquid-ordered bilayer of phospholipid and cholesterol, and (c) fluid phospholipid bilayers. The structures have been assigned on the basis of lamellar repeat spacings, relative scattering intensities and bilayer thickness of binary and ternary lipid mixtures of varying composition subjected to thermal scans between 20 and 50 °C. The results suggest that the order created by the quasicrystalline phase may provide an appropriate scaffold for the organization and assembly of raft proteins on both sides of the membrane. Co-existing liquid-ordered structures comprised of phospholipid and cholesterol provides an additional membrane environment for assembly of different raft proteins.     

Comparative study of in vivo stigmasterol biosynthesis in Nicotiana tabacum and Hordeum vulgare

Six-day-old tobacco (Nicotiana tabacum) and barley (Hordeum vulgare) seedlings rapidly incorporated and metabolized exogenously supplied [4-¹⁴C]sitosterol but neither plant was able to convert it into stigmasterol. However, a sterol metabolite was isolated from both species and the acetate derivative was slightly more polar, on AgNO₃—silica gel TLC, than stigmasteryl acetate. A similar metabolite was also obtained with [4-¹⁴C]cholesterol, indicating a general metabolic reaction of plants to exogenous sterols. Both species incorporated [2-¹⁴C]mevalonic acid into sitosterol and stigmasterol. We suggest that in vascular plants, whether monocotyledons or dicotyledons, the pathway of stigmasterol biosynthesis is not via sitosterol but through a common precursor which is derived from mevalonic acid.     

N-palmitoyl-sulfatide participates in lateral domain formation in complex lipid bilayers

Sulfatides (galactosylceramidesulfates) are negatively charged glycosphingolipids that are important constituents of brain myelin membranes. These membranes are also highly enriched in galactosylceramide and cholesterol. It has been implicated that sulfatides, together with other sphingolipids, take part in lateral domain formation in biological membranes. This study was conducted to characterize the lateral phase behavior of N-palmitoyl-sulfatide in mixed bilayer membranes. Going from simple lipid mixtures with sulfatide as the only sphingolipid in a fluid matrix of POPC, to more complex membranes including other sphingolipids, we have examined 1) ordered domain formation with sulfatide, 2) sterol enrichment in such domains and 3) stabilization of the domains against temperature by the addition of calcium. Using two distinct phase selective fluorescent probes, trans-parinaric acid and cholestatrienol, together with a quencher in the fluid phase, we were able to distinguish between ordered domains in general and ordered domains enriched in sterol. We found that N-palmitoyl-sulfatide formed ordered domains when present as the only sphingolipid in a fluid phospholipid bilayer, but these domains did not contain sterol and their stability was unaffected by calcium. However, at low, physiologically relevant concentrations, sulfatide partitioned favorably into domains enriched in other sphingolipids and cholesterol. These domains were stabilized against temperature in the presence of divalent cations. We conclude that sulfatides are likely to affect the lateral organization of biomembranes.     

Domain formation and stability in complex lipid bilayers as reported by cholestatrienol

In this study, we used cholestatrienol (CTL) as a fluorescent reporter molecule to study sterol-rich L(o) domains in complex lipid bilayers. CTL is a fluorescent cholesterol analog that mimics the behavior of cholesterol well. The ability of 12SLPC to quench the fluorescence of cholestatrienol gives a measure of the amount of sterol included in L(o) domains in mixed lipid membranes. The stability of sterol-rich domains formed in complex lipid mixtures containing saturated sphingomyelins, phosphatidylcholines, or galactosylceramide as potential domain-forming lipids were studied. The amount of sterol associated with sterol-rich domains seemed to always increase with increasing temperature. The quenching efficiency was highly dependent on the domain-forming lipid present in complex lipid mixtures. Sphingomyelins formed stable sterol-enriched domains and were able to shield CTL from quenching better than the other lipids included in this study. The saturated phosphatidylcholines also formed sterol-rich domains, but the quenching efficiency in membranes with these was higher than with sphingomyelins and the domains melted at lower temperatures. PGalCer was not able to form sterol-enriched domains. However, we found that PGalCer stabilized sterol-rich domains formed in PSM-containing bilayers. Using a fluorescent ceramide analog, we also demonstrated that N-palmitoyl-ceramide displaced the sterol from sphingolipid-rich domains in mixed bilayer membranes.     

Inhibition of cholesterol absorption in rats by plant sterols

The extent and site(s) of inhibition of cholesterol absorption by plant sterols, sitosterol and fucosterol, were studied in rats. The intragastric administration of a single emulsified lipid meal containing 25 mg [3H]cholesterol and 25 mg of either sitosterol or fucosterol inhibited the lymphatic absorption of cholesterol by 57% and 41%, respectively, in 24 hr. Less than 2% of each plant sterol was absorbed in the 24-hr period. In contrast, neither plant sterol (50 microM) inhibited cholesterol absorption when co-administered with equimolar amounts of cholesterol in phospholipid-bile salt micelles nor was either absorbed from the micellar solution. A series of in vitro studies was conducted to identify the site(s) of plant sterol inhibition of cholesterol absorption and to account for the difference in inhibitory effectiveness of sitosterol and fucosterol. A comparison of the micellar solubility of each sterol alone and in equimolar binary mixtures (to 2.0 mM) revealed that the solubility of individual sterols decreased in the following order: cholesterol, fucosterol, sitosterol, and that in binary mixtures cholesterol solubility was decreased by sitosterol and, to a lesser extent, by fucosterol relative to its solubility alone. A comparison between micellar-solubilized cholesterol and either sitosterol or fucosterol for binding to isolated brush border membranes, intestinal mucin, or for esterification by either cholesterol esterase or acyl coenzyme A:cholesterol acyltransferase revealed moderate to no competition. The data suggest that plant sterols displace cholesterol from bile salt (taurocholate) micelles and that sitosterol is more effective than fucosterol in this capacity.     

Sterol utilization in honey bees fed a synthetic diet: Analysis of prepupal sterols

The sterols of prepupal honey bees, Apis mellifera L., from brood reared by workers fed chemically-defined synthetic diets containing cholesterol, campesterol, sitosterol, stigmasterol, 24-methylenecholesterol, or no sterol over a 12-week period were isolated, identified, and quantified. The major sterol present in each prepupal sample was 24-methylenecholesterol, but significant levels of sitosterol and isofucosterol were also present in every case, as was a very small percentage of desmosterol (usually < 1%). This is the first report of isofucosterol being identified in the sterols of the honey bee. A considerably larger percentage of each dietary sterol was found in prepupae reared by workers fed that particular sterol in the diet. This was most dramatic in the case of the cholesterol diet in which case cholesterol content increased to as much as 17.2% of the prepupal sterols, whereas cholesterol had not exceeded 2.2% in samples from other diet regimens. However, stigmasterol comprised no more than 6.3% of the total sterols in any sample from prepupae fed the stigmasterol diet. The preponderance of 24-methylenecholesterol in all prepupae, regardless of the dietary sterol provided to the workers, as well as the lesser quantities of sitosterol and isofucosterol present in all samples, suggest a unique system of utilization and metabolism of these dietary sterols by the worker bees. Apparently they make available to the brood varying amounts of unchanged dietary sterol plus considerable and fairly constant portions of 24-methylenecholesterol, sitosterol, and isofucosterol drawn from their own sterol pools.     

Sterols in hardening winter wheat

The main sterols in winter wheat crowns and roots were sitosterol and campesterol, with significant amounts of stigmasterol and traces of cholesterol. The main groups of sterol-containing lipids were free sterols, steryl glucosides, steryl esters and esterified steryl glucosides. Sterol analysis within each group showed little difference between them. Steryl esters were relatively rich in cholesterol and poor in stigmasterol. Free sterols were rich in stigmasterol. Low temperature caused an increase in sterol content but had little effect on sterol composition and sterol to lipid P ratio. There was some increase in steryl esters and some decrease in free sterols. Cholesterol and stigmasterol decreased in the steryl ester and free sterol fractions, respectively. There was little evidence for involvement of sterols in winter wheat frost hardening.     

Sphingolipids and the formation of sterol-enriched ordered membrane domains

This review is focused on the formation of lateral domains in model bilayer membranes, with an emphasis on sphingolipids and their interaction with cholesterol. Sphingolipids in general show a preference for partitioning into ordered domains. One of the roles of cholesterol is apparently to modulate the fluidity of the sphingolipid domains and also to help segregate the domains for functional purposes. Cholesterol shows a preference for sphingomyelin over phosphatidylcholine with corresponding acyl chains. The interaction of cholesterol with different sphingolipids is largely dependent on the molecular properties of the particular sphingolipid in question. Small head group size clearly has a destabilizing effect on sphingolipid/cholesterol interaction, as exemplified by studies with ceramide and ceramide phosphoethanolamine. Ceramides actually displace sterol from ordered domains formed with saturated phosphatidylcholine or sphingomyelin. The N-linked acyl chain is known to be an important stabilizer of the sphingolipid/cholesterol interaction. However, N-acyl phosphatidylethanolamines failed to interact favorably with cholesterol and to form cholesterol-enriched lateral domains in bilayer membranes. Glycosphingolipids also form ordered domains in membranes but do not show a strong preference for interacting with cholesterol. It is clear from the studies reviewed here that small changes in the structure of sphingolipids alter their partitioning between lateral domains substantially.     

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.     

Bilayer polarity and its thermal dependency in the ?o and ?d phases of binary phosphatidylcholine/cholesterol mixtures

Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (?_o) and liquid-disordered (?_d) phases display significantly different polarities. Moreover, in the ?_o phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 °C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.     

Sterol chemical configuration and conformation influence the thermotropic phase behaviour of dipalmitoylphosphatidylcholine mixtures containing 5β-cholestan-3β- and -3α-ol

We report here our differential scanning calorimetry measurements investigating the thermotropic phase behaviour of binary dipalmitoylphosphatidylcholine (DPPC)/sterol mixtures containing two saturated sterols with different ring configurations (5β-H and either 3α-OH or 3β-OH). These measurements differ in the proportions of sharp and broad components in the heating endotherms, representing the melting of the sterol-poor and sterol-rich lipid micro-domains of the DPPC bilayer, respectively. Our results suggest that the 5,10-cis ring configuration of both saturated sterols and the ring A conformations have the greatest influence on DPPC bilayer properties, most likely by inducing small increases in the mean area/molecule as compared to cholesterol. However, the C3-OH orientation also influences sterol miscibility, likely due to variations in the strength and number of interfacial H-bonds with changes in molecular area, which in turn probably reflect the depth of the sterol in the DPPC bilayer. This influence of C3-OH orientation is significantly greater than was observed in our earlier study of cholesterol/- and epicholesterol/DPPC mixtures. Overall, our results show that both saturated and unsaturated 3α-ols are less miscible than the corresponding 3β-ols, but that the presence of a Δ⁵ double bond can improve the sterol miscibility in the DPPC bilayer at high sterol concentrations.     

Domain-formation in DOPC/SM bilayers studied by pfg-NMR: effect of sterol structure

Pulsed field gradient (pfg)-NMR spectroscopy was utilized to determine lipid lateral diffusion coefficients in oriented bilayers composed of 25 mol % sterol and equimolar amounts of dioleoylphosphatidylcholine and sphingomyelin. The occurrence of two lipid diffusion coefficients in a bilayer was used as evidence of lateral phase separation into liquid ordered and liquid disordered domains. It was found that cholesterol, ergosterol, sitosterol, and lathosterol induced domains, whereas lanosterol, stigmasterol, and stigmastanol resided in homogeneous membranes in the temperature interval of 24-70 degrees C. Among the domain-forming sterols, differences in the upper miscibility temperature indicated that the stability of the liquid ordered phase could be modified by small changes in the sterol structure. The domain-forming capacity for the different sterols is discussed in terms of the ordering effect of the sterols on the lipids, and it is proposed that the driving force for the lateral phase separation is the reduced solubility of the unsaturated lipid in the highly ordered phase.     

Optimisation of plant sterols incorporation in human keratinocyte plasma membrane and modulation of membrane fluidity.

The in vitro effects of plant sterols were investigated with regard to their uptake and membrane lipid fluidity in human keratinocytes. Among the different media tested to transport sterols (liposomes, micelles and organic solvents), the best results in terms of incorporation and viability were obtained by the use of the organic solvents dimethylsulfoxide and ethanol. After 48 h incubation exogenous sterol can account for about 30% of the total cell sterol content. The total sterol amount in plasma membranes increased 2-fold after incubation with cholesterol, whereas it was not altered when phytosterols were incorporated. The incorporation of cholesterol, sitosterol and stigmasterol led to an increase in the percent of unsaturated fatty acid C18:1 in the plasma membrane. The effect of this uptake on membrane fluidity was studied by means of fluorescence polarisation using DPH and TMA-DPH as fluorescent probes. Whereas cholesterol and sitosterol had no significant effect on the DPH fluorescence anisotropy (rs), the presence of stigmasterol induced a 12% decrease of rs reflecting an increase in membrane fluidity. We can conclude from this study that in the presence of sitosterol, the mean fluidity of the membrane is regulated whereas stigmasterol triggers a looseness of molecular packing of phospholipids acyl chains, in accordance with previous results obtained on purely lipid model membranes.     

Steroid structural requirements for interaction of ostreolysin, a lipid-raft binding cytolysin, with lipid monolayers and bilayers

Ostreolysin, a cytolytic protein from the edible oyster mushroom (Pleurotus ostreatus), recognizes and binds specifically to membrane domains enriched in cholesterol and sphingomyelin (or saturated phosphatidylcholine). These events, leading to permeabilization of the membrane, suggest that a cholesterol-rich liquid-ordered membrane phase, which is characteristic of lipid rafts, could be its possible binding site. In this work, we present effects of ostreolysin on membranes containing various steroids. Binding and membrane permeabilizing activity of ostreolysin was studied using lipid mono- and bilayers composed of sphingomyelin combined, in a 1/1 molar ratio, with natural and synthetic steroids (cholesterol, ergosterol, β-sitosterol, stigmasterol, lanosterol, 7-dehydrocholesterol, cholesteryl acetate, and 5-cholesten-3-one). Binding to membranes and lytic activity of the protein are both shown to be dependent on the intact sterol 3β-OH group, and are decreased by introducing additional double bonds and methylation of the steroid skeleton or C17-isooctyl chain. The activity of ostreolysin mainly correlates with the ability of the steroids to promote formation of liquid-ordered membrane domains, and is the highest with cholesterol-containing membranes. Furthermore, increasing the cholesterol concentration enhanced ostreolysin binding in a highly cooperative manner, suggesting that the membrane lateral distribution and accessibility of the sterols are crucial for the activity of this new member of cholesterol-dependent cytolysins.     

Bioconversion and binding of sterols by thermophilic moulds

None of the fourteen thermophilic moulds was able to break down the aliphatic side chain of sterols,viz. cholesterol, lanosterol, sitosterol, and stigmasterol so as to yield 4-androstene-3, 17-dione, 1,4-androstadiene-3, 17-dione and progesterone. InAcremonium alabamensis and.Talaromyces emersonii, cholestenone was detected as a product of fermentation of cholesterol whereas the former yielded stigmastadienone from stigmasterol and sitosterol. Lanosterol appeared to be resistant to fungal bioconversion. All the thermophilic moulds exhibited avidity for binding sterols to the mycelium, but the ability to bind sterol seemed to depend upon the nature of the organism and the sterol.     

A comparison of the packing behavior of egg phosphatidylcholine with cholesterol and biogenically related sterols in Langmuir monolayer films

Cholesterol and selected derivatives were studied as mixed Langmuir monolayers with egg phosphatidylcholine (PC). As an extension of our earlier work, which employed binary sterol/PC mixtures, here we examined ternary mixed monolayers containing cholesterol along with an alternate sterol and PC in different molar ratios, using pressure-area isotherms. The ternary systems behaved similarly to the binary sterol/PC systems reported previously, with similar condensation noted for the sterol/PC films. To better understand how variations in sterol structure affect sterol packing in such membrane monolayers, binary mixtures containing cholestenone, cholestanol, and lanosterol with PC were also studied. Cholestanol behaved similarly to cholesterol when incorporated with PC, while cholestenone and lanosterol did not cause as much film condensation. The observed differences in molecular packing, and attributed sterol structural differences, are considered within the context of sterol/phospholipid mixtures in biological membranes.