Simple Micelles Research Articles

Lecithin Protects against Plasma Membrane Disruption by Bile Salts

Introduction.Detergent disruption of epithelial plasma membranes by bile salts may contribute to pathogenesis of cholestasis and gastroesophageal reflux disease. Bile, despite containing high concentrations of bile salts, normally is not toxic to biliary or intestinal epithelia. We hypothesize that lecithin in bile may protect cell membranes from disruption by bile salts.Methods.We studied the interactions of taurine conjugates of ursodeoxycholate (TUDCA), cholate (TCA), chenodeoxycholate (TCDCA), and deoxycholate (TDCA) with erythrocyte plasma membranes with or without large unilamellar egg lecithin vesicles for various times at 23°C. Release of hemoglobin was quantified spectrophotometrically. The concentration of bile salt monomers and simple micelles in the intermixed micellar aqueous phase (IMMC) was determined by centrifugal ultrafiltration.Results.The degree of hemolysis depended on the hydrophobicity of the bile salts and was progressive over time. Addition of lecithin reduced the hemolytic effects of 20 mM TCA or 2 mM TDCA in a concentration-dependent manner at both 30 min and 4 h. Increasing the concentration of lecithin progressively reduced the IMMC of TDCA. Hemolysis following addition of lecithin to 2 mM TDCA was comparable to hemolysis produced by lecithin-free TDCA solutions when diluted to similar IMMC values.Conclusion.We conclude that lecithin reduces plasma membrane disruption by hydrophobic bile salts. This protection may be attributable to association of bile salts with vesicles and mixed micelles, reducing the concentration of bile salt monomers and simple micelles available to interact with cell membranes. Lecithin may play a key role in preventing bile salt injury of biliary and gastrointestinal epithelia.

Investigation of the Interaction between Simple Micelles and Mixed Micelles in Taurocholate−Lecithin Solutions by Laser Light Scattering

A laser light-scattering technique was employed to investigate simple micelles and mixed micelles thermodynamically coexisting in various taurocholate (TC)−lecithin (L) solutions. When the nonmixed micellar TC (NMTC) concentrations were higher than 25 mM for L = 12 mM and 21 mM, the sizes of simple TC micelles (∼10 Å) and mixed TC−L micelles (∼24 Å) as well as the polydispersity (∼45%) of these micelles were nearly unchanged. At a constant concentration of L, the average size decreased and, more notably, the total scattered intensity also decreased with increasing concentrations of TC. A modified physical equation successfully modeled the scattered intensity versus different concentrations of simple TC micelles and mixed TC−L micelles. Moreover, a strong interaction between simple TC micelles and mixed TC−L micelles was found which appeared to be a process for the transfer of cholesterol molecules in the cholesterol gallstone dissolution.

Accurate separation of biliary lipid aggregates requires the correct intermixed micellar/intervesicular bile salt concentration.

The intermixed micellar/intervesicular bile salt (BS) concentration (IMC), composed of BS monomers and simple micelles, is in dynamic equilibrium with mixed micelles and vesicles. Accurate separation of biliary lipid aggregates is believed to depend on accurately measuring the IMC. Using centrifugal ultrafiltration, we measured the IMC of cholesterol-supersaturated model biles that were physiologically composed. Gel chromatography was performed using eluants containing the following: 1) the IMC; 2) the same BS composition as the IMC but higher or lower BS concentrations; 3) the same BS concentration as the IMC, but with more hydrophilic or hydrophobic BS; and 4) 10 mmol/L cholate. Compared with an eluant containing the same BS composition as the IMC, an eluant containing the same relative BS composition but 75% of the IMC increased the proportion of cholesterol in vesicles and decreased the vesicular cholesterol/egg yolk phosphatidylcholine (EYPC) ratio. In contrast, an eluant containing 150% of the IMC entirely transformed vesicles to micelles. Eluants containing slightly more hydrophobic or more hydrophilic BS eliminated or increased vesicular cholesterol content, respectively. An eluant of 10 mmol/L cholate overestimated vesicular cholesterol and in concentrated biles reproducibly produced an incompletely separated intermediate peak, possibly because of re-equilibration between mixed micelles and vesicles. Further, in concentrated biles, fractions eluting at volumes corresponding to mixed micelles were visibly turbid, irrespective of the eluant used. The correct IMC allows accurate separation of biliary lipid aggregates, but differences in BS concentration or composition substantially alter the vesicular percentage of cholesterol as well as the cholesterol/EYPC ratio. Elution with 10 mmol/L cholate may introduce artifactual gel-filtration peaks and inadequate separation of particles with widely differing molecular weights, both of which have confused previous analyses of biliary lipid aggregates.

Effect of Various Lipid-Bile Salt Mixed Micelles on the Intestinal Absorption of Amphotericin-B in Rat

Amphotericin-B (AmB), a lipophilic polyene antibiotic, is the drug of choice in the treatment of many serious mycotic diseases. The solubility and gastrointestinal membrane permeability (Papp) of AmB in mixed micellar systems were examined. Membrane permeability was determined using a rat gut perfusion method. The mixed micellar systems studied contained the bile salt in association with fatty acid. All mixed micellar systems enhanced the absorption of AmB relative to the simple micelle. These results have shown that mixed micelles can enhance the absorption of AmB to a greater extent relative to nonmicellar and simple micellar systems. Maximum enhancement (>20-fold) in the rate of AmB absorption was obtained with the sodium desoxycholate/soya lecithin 40:40 mM system. These results offer a possible explanation for the reported enhancement in gastrointestinal absorption of AmB when coadministered with lipid—bile salt mixed micelles, and these systems can be used as a vehicle for designing novel drug delivery systems for poorly absorbable drug(s).

Interactions and Molecular Weights of Simple Micelles and Mixed Micelles in Taurocholate and Taurocholate−Lecithin Solutions

A quasielastic light-scattering (QLS) technique was employed to investigate simple micelles and mixed micelles formed in taurocholate (TC) and TC−lecithin (L) solutions. When the size and polydispersity of simple TC micelles were nearly unchanged with total TC concentrations, the simple TC micelles were suggested to be all of a single size. According to the relationship between the total TC concentration and the TC monomer concentration, a monomer−micelle model predicted the aggregation number (n) and the critical micellar concentration (cmc) of simple TC micelles to be 5 and 10 mM. Based on Flory's theory, the scattered intensity, the TC monomer concentration, the molecular weights of simple TC micelles, and mixed TC−L micelles were estimated to be 2138 Da and 35 840 Da. Moreover, the interaction coefficients of simple TC micelles and mixed TC−L micelles were found to be 1.30 × 10-5 dL mol g-2 and 5.09 × 10-6 dL mol g-2. According to Flory's equation, the estimated interaction coefficients and the concen...

Cholesterol Binding to Simple Micelles in Aqueous Bile-Salt–Cholesterol Solutions

The true thermodynamic activity (AT) of cholesterol (Ch) in aqueous solutions containing taurocholate (TC)–Ch was determined by employing a direct assay of a 1 × 2-cm silicone polymer film with 0.025 cm thickness. Using theATdata, information on the nature of micellar species present in the TC–Ch system, and employing a binding-site model previously developed for tauroursodeoxycholate (TUDC)–Ch and taurochenodeoxycholate (TCDC)–Ch systems, it appeared that the Ch-binding affinity for simple bile-salt micelles corresponds precisely with the order of hydrophobicity, TUDC < TC < TCDC. Further, although simple TC micelles and simple TCDC micelles have similar binding capacities, the first Ch binding to a simple TC micelle may not significantly facilitate the second Ch binding, as occurs in simple TCDC micelles. For TUDC–Ch, TC–Ch, and TCDC–Ch systems, the concentration of bound simple micelles increased with increasingATvalues, whereas the unbound simple micelle concentration decreased proportionally. These results provide insights into the possible influence of bile-salt species on Ch-binding to simple micelles in bile-salt–Ch solutions.

Quantitative Examination of the Simple Micelle–Mixed Micelle Coexistence for the Taurocholate–Lecithin System by GP-HPLC

Gel-permeation, high-performance liquid chromatography (GP-HPLC) studies were conducted to quantify simple micelles and mixed micelles present in model biles. A TSK-gel G2000SW column was preequilibrated with a buffered mobile phase: phosphate buffer, pH 7.4, 0.1MNaCl, and 30 mM[14C]taurocholate (TC). Simulated biles with different TC and/or egg-yolk lecithin (L) levels were then introduced onto the GP column. The injected biles without L (TC alone) yielded positive peaks or negative valleys (relative to the baseline) at ∼9.5 min, depending on the injected TC concentration. A theoretical mass balance equation quantitatively accounted for the observed simple micelle peaks (or valleys). When TC-L coexisting simple micelle–mixed micelle model biles were injected onto the column, in addition to the simple micelle peaks at ∼9.5 min, mixed micelle peaks were observed at retention times in the region ∼7.5 to 8.0 min. Here, also, there was good mass balance between the expected and the observed TC peaks. The average molecular weights were estimated as ∼28,000–45,000 Da for the mixed micelles and ∼5500 Da for the simple micelles, respectively. The TC-to-L molar ratio (K) for the mixed micelle peak showed a curvilinear decrease as a function of L concentration and leveled off at around 1.5. These results are consistent with those obtained from previous equilibrium dialysis studies, although theKvalues from present studies are slightly lower (∼20%); a possible explanation for this discrepancy is presented.

The source of the bile: The legacy of Mathias Mokel

HepatologyVolume 24, Issue 1 p. 274-276 Hepatology ElsewhereFree Access The source of the bile: The legacy of Mathias Mokel D M Heuman M.D., D M Heuman M.D. Medical College of Virginia and McGuire Department of Veterans Affairs Medical CenterSearch for more papers by this author D M Heuman M.D., D M Heuman M.D. Medical College of Virginia and McGuire Department of Veterans Affairs Medical CenterSearch for more papers by this author First published: July 1996 https://doi.org/10.1002/hep.510240145Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat References 1 Somjen GJ, Gilat T. A non-micellar mode of cholesterol transport in human bile. FEBS Lett 1983; 156: 265– 268. 2 Somjen GJ, Gilat T. Contribution of vesicular and micellar carriers to cholesterol transport in human bile. J Lipid Res 1985; 26: 699– 704. 3 Hardison WG, Apter JT. Micellar theory of biliary cholesterol excretion. Am J Physiol 1972; 222: 61– 67. 4 Cohen DE. Studies of biliary lipid secretion and aggregation in model and native biles (Ph.D. dissertation). Cambridge, MA: Harvard University, 1986. 5 Cohen DE, Angelico M, Carey MC. Quasielastic light scattering evidence for vesicular secretion of biliary lipids. Am J Physiol 1989; 257: G1– G8. 6 Somjen GJ, Marikovsky Y, Lelkes P, Gilat T. Cholesterol-phospholipid vesicles in human bile: an ultrastructural study. Biochim Biophys Acta 1986; 879: 14– 21. 7 Ulloa N, Garrido J, Nervi F. Ultracentrifugal isolation of vesicular carriers of biliary cholesterol in native human and rat bile. Hepatology 1987; 70: 235– 244. 8 Amigo LC, Covarrubias C, Nervi F. Rapid isolation of vesicular and micellar carriers of biliary lipids by ultracentrifugation. J Lipid Res 1990; 31: 341– 347. 9 Cohen DE, Carey MC. Rapid (1 hour) high performance gel filtration chromatography resolves coexisting simple micelles, mixed micelles, and vesicles in bile. J Lipid Res 1990; 31: 2103– 2112. 10 Crawford JM, Mockel G-M, Crawford AR, Hagen SJ, Hatch VC, Barnes S, Godleski JJ, Carey MC. Imaging biliary lipid secretion in the rat. ultra-structural evidence for vesiculation of the hepatocyte canalicular membrane. J Lipid Res 1995; 36: 2147– 2163. 11 Möckel G-M, Gorti S, Tandon RK, Tanaka T, Carey MC. Microscope laser light-scattering spectroscopy of vesicles within canaliculi of rat hepatocyte couplets. Am J Physiol 1995; 269: G73– G84. Citing Literature Volume24, Issue1July 1996Pages 274-276 ReferencesRelatedInformation

Retinoid Absorption from Simple and Mixed Micelles in the Rat Intestine

▭ The absorption of three retinoid analogs etretinate (ET), acitretin (ETA), and motretinid (MOE) from two distinct micellar systems was studied in the rat intestine. Each of the three drugs was loaded into simple micelles consisting of 10 mM sodium taurocholate (NaTC) and mixed micelles consisting of 10 mM egg phosphatidylcholine (PC) and 10 mM NaTC. Following perfusion through the jejunum segments, both the fraction of drug disappearing from the segment and the permeability of the drug from the lumen into the gut wall (Peff) was greater with the mixed micelles as compared to the simple micelles. Perfusion flow rate had an influence on the Peff for ET and ETA. Similar trends as for the jejunum were seen in the ileum perfusions. The simultaneous uptake of PC and NaTC during the retinoid perfusions was monitored. There appeared to be a correlation between the Peff values for PC and that of the retinoids. The viability of the in-situ perfusion system was confirmed histologically. There is evidence to indicate that the permeability of the intestine is sensitive to subtle differences in the chemical structure of the retinoids.

Relation between micellar structure of model bile and activity of esterase

In a model bile solution composed of lecithin (L)-bile salt (B), the solubilization of lipid and the accessibility of enzyme to the lipid were examined by observation of EPR spectra and measurement of enzyme activity. The lifetime of the spin probe in the micellar phase was estimated to be approx. 1 μs by means of line shape analysis. Both population and lifetime increased with temperature and the molar ratio of lecithin to bile salt (L/B). The EPR data indicated that simple micelle of bile salt, mixed disk micelle of bile salt-lecithin, and multi-lamellar mixed disk micelle can exist in a model bile solution, depending on the L/B molar ratio across a range from 0 to 1.5. The maximal power of the mixed disk micelle to solubilize cholesteryl ester in the model bile at a L/B molar ratio of 1:1 was confirmed by EPR measurement of cholesteryl 12-DOXYL-stearate. Observation of the enzyme activity on a mixture of model bile and substrate at 37°C revealed selective accessibility of cholesterol esterase (bovine pancreas) to mixed disk micelle, of cholesterol oxidase ( Streptomyces cinnamomeus) to both simple and mixed disk micelle, and of pancreatic lipase (porcine pancreas) to both simple micelle and an oil droplet of substrate. The temperature-dependent activity of cholesterol oxidase to cholesterol in mixed disk micelle can be explained in terms of mesomorphic phase transition of lecithin side chains followed with fluidity of liquid crystal phase. Regarding phospholipase C from Bacillus cereus, though the selective accessibility to the micelles was not observed at 37°C, a decrease in activity for mixed disk micelle could be found at lower temperatures.

Calcium affinity for biliary lipid aggregates in model biles: Complementary importance of bile salts and lecithin

Background/Aims : Despite putative roles of calcium in biliary physiology and gallstone formation, quantitative aspects of calcium binding to bile salt (BS) monomers, simple micelles, mixed micelles, and vesicles, which constitute the lipid aggregates in bile, remain unexplored. Methods : Calcium activity was measured using the calcium electrode in pathophysiologically relevant model biles composed of either individual BS species or a physiological mixture of glycine and taurine conjugates, as functions of lecithin and cholesterol contents and total lipid concentration. Results : Calcium binding increased with increasing BS concentrations and lecithin contents and varied with species (dihydroxy > trihydroxy BS) and with conjugation (unconjugated > glycine conjugates > taurine conjugates). Although lecithin/cholesterol vesicles did not bind detectable calcium, when taurocholate was incorporated into membrane bilayers, calcium binding was substantially greater than with equimolar BS alone. Added cholesterol did not alter calcium binding, despite cholesterol saturation of biliary lipid aggregates and induction of liquid crystalline and solid crystalline-phase transitions. Conclusions : In model biles, most calcium is bound to mixed micelles, with minor contributions by BS monomers, simple micelles, and vesicles. It is proposed that BS-induced binding of calcium to vesicles and mixed micelles may be important in nucleation of cholesterol and bilirubinates from native bile.

The effect of mixed micellar systems, bile salt/fatty acids, on the solubility and intestinal absorption of clofazimine (B663) in the anaesthetised rat

Clofazimine (B663) is a highly lipophilic drug used in the treatment of leprosy. The solubility and gastrointestinal membrane permeability ( P app) of B663 in mixed micellar systems were examined. Membrane permeability was determined using a rat gut perfusion model and, in addition, these studies incorporated the hydrophilic marker PEG 4000. The mixed micellar systems studied contained the bile salt, sodium cholate (NaC), in association with different fatty acids including caprylic acid, oleic acid and linoleic acid. At a set concentration of NaC (40 mM) the solubility of B663 increased with increasing concentration of each fatty acid. Relative to NaC, the maximum enhancement in solubility (16-fold) was obtained with the NaC/linoleic acid (40:40 mM) system. An optimum bile salt/fatty acid ratio of 1:1 existed for maximum solubility enhancement. All mixed micellar systems enhanced the absorption of B663 relative to the simple micelle. The P app tended to increase with increasing fatty acid concentration, maximum enhancement being obtained with the NaC/linoleic acid 40:40 mM system. With each mixed micellar system a higher P app was obtained with lower drug loading. The effects of the mixed micellar systems on the absorption of PEG 4000 varied with fatty acid loading. These results have shown that mixed micelles can enhance the absorption of B663 to a greater extent relative to non-micellar and simple micellar systems. Maximum enhancement (> 800-fold) in the rate of B663 absorption was obtained with the NaC/linoleic acid 40:40 mM system. These results offer a possible explanation for the reported enhancement in gastrointestinal absorption of B663 when co-administered with fatty materials.

TEM investigations of self-assembly through physical synthesis

A novel way of enlarging the realm of surfactant solution microstructure is the "physical synthesis" of new microstructures and surfactant-templated materials by mixing simple surfactants or surfactant and inorganic and ionic species. Mixing can often produce properties not possessed by any one species alone. As we have recently shown, mixtures of single-tailed cationic and anionic surfactants associate in solution to form a quasi-double-tailed catanionic surfactant that forms bilayers rather than the simple micelles each species would form on its own. Mixed surfactant systems apparently produce new microstructures by altering the intermolecular and interaggregate forces in ways impossible for single component systems. In addition to spontaneous vesicles, we have found other new microstructures including dilute lamellar and L3 phases depending on the details of the surfactant mixture. Surfactant interactions with ionic species also play an important role in the in synthesis of a new M41S family of mesoporous silicas, and are likely to be important in the templating of biominerals at interfaces. By understanding the molecular and chemical basis of these interactions, we can begin to tailor new microstructured and/or biomimetic materials by controlling surfactant aggregation and phase behavior through physical synthesis rather than through much more elaborate chemical synthesis.

Electron spin resonance studies on tryptophan photolysis in frozen micellar systems of anionic surfactants

UV photolysis of tryptophan (Trp) was studied by the electron spin resonance method in frozen solutions of simple micelles (SMs) and reversed micelles (RMs) of anionic surfactants: sodium bis(2-ethylhexyl)sulphosuccinate, sodium dodecylsulphate and ammonium perfluoropolyether carbonate. The primary species stabilized at 77 K in protiated systems are Trp + , Trp and alkyl-type radicals while, in the perfluorinated RM system, only Trp + is observed. The yield of paramagnetic species is an order of magnitude higher in microheterogeneous systems than in homogeneous aqueous solutions. It is higher in RM than in SM samples, parallel to the fraction of Trp molecules bound to micellar interface. In the presence of oxygen the primary photoproducts transform into peroxy radicals on heating the sample above 100 K. In deaerated samples, Trp + survives up to temperatures of 190 K or higher (depending on the system).

Calorimetric study on the solubilization of butanol by alkylphosphate and alkylphosphate-lecithin systems.

Microcalorimetric studies on alkylphosphate-butanol and alkylphosphate-butanol-lecithin systems were carried out as a first step to study the role of butanol (used as cosurfactant) in the formation of microemulsions. The enthalpy of solubilization; delta H, of the same amount of butanol (1.806 mol kg-1) in aqueous sodium monoalkyl-phosphates, with a hydrocarbon chain varying from 6 to 10 carbon atoms, was investigated as a function of alkylphosphate concentration. Measurements were performed in the presence and in the absence of lecithin. The enthalpy values measured upon addition of butanol to the mixtures were negative in all cases. Below the critical micelle concentration (CMC) of the alkylphosphates, the heats of butanol dissolution were found to decrease upon increasing the concentration of the alkylphosphate. This behaviour was related to the dissolution of butanol in water and to the formation of mixed butanol-alkylphosphate and butanol-alkylphosphate-lecithin micelles. Above the CMC essentially constant delta H values were found. The delta H measured may be the sum of simultaneous effects--heat of alcohol dissolution and the simple alkylphosphate micelle dissociation or alkylphosphate-lecithin mixed aggregate dissociation and the heat of mixed micelle formation. The delta H at the plateaus were linearly related to the corresponding CMC values of the alkylphosphates.

Molecular species composition of inter-mixed micellar/vesicular bile salt concentrations in model bile: dependence upon hydrophilic-hydrophobic balance

We systematically studied the bile salt (BS) species composition of the intermixed micellar/vesicular (non-lecithin-associated) BS concentrations (IMC) of model biles using physiological biliary compositions prepared with mixtures of 12 common BS of humans. BS distributions in the IMC, which was composed of monomers plus simple micelles, were determined by rapid centrifugal ultrafiltration and/or modified equilibrium dialysis as functions of total lipid concentration, BS/lecithin ratio, cholesterol content, pH, and the weighted hydrophobic index of BS species. IMC values increased from 3 to 9.5 mM with increases in total lipid concentration and BS/lecithin ratio, but decreased appreciably as the overall BS composition of physiological BS mixtures became more hydrophobic. However, IMC values were not altered by increases in cholesterol content (0-10%) that induced a phase transition from a one-phase micellar system to a two-phase system of micelles and vesicles. As pH values were decreased (8 to 5), with partial protonation but not precipitation of glycine-conjugated BS (pK'a approximately 4.3 by titration), IMC values decreased slightly. For all model biles studied, IMC values of BS mixtures were markedly smaller than those previously found for model biles composed with taurocholate (Donovan, J. M. et al. 1991. J. Lipid Res. 32: 1501-1512). Hydrophilic BS were preferentially distributed in the IMC, whereas hydrophobic BS were preferentially associated with lecithin in mixed micelles and vesicles. Hence, BS composition, in addition to total lipid composition and BS/lecithin ratio, is a critical determinant of the relative and absolute concentrations of the BS species in the inter-mixed micellar/vesicular bile salt concentration.

Mechanisms of cholesterol monohydrate dissolution in aqueous taurocholate-, taurochenodeoxycholate-, and tauroursodeoxycholate-lecithin solutions—correlation between micellar species and dissolution rates

A complete mapping study of the cholesterol monohydrate (ChM) dissolution rates ( J) has been completed for the aqueous taurocholate (TC)-, taurochenodeoxycholate (TCDC)-, and tauroursodeoxycholate (TUDC)-lecithin (L) systems over a wide range of bile salt concentrations at seven levels of lecithin (0, 2, 4, 8, 16, 24, and 32 m M) and two levels of added NaCl (0.20 and 0.40 M). From equilibrium dialysis experiments, the simple micelle—mixed micelle coexistence equilibria were quantified and the micellar species concentrations determined. The rate constants for the simple micelle ( k s) and for the mixed micelle ( k M) at each level of lecithin were deduced from the J values and the micelle concentrations. The following were the important conclusions. First, these studies have confirmed our previous finding that the TC simple micelle is the principal species and the TC-L mixed micelle is much less important in the interfacially controlled ChM dissolution rates in the TC-L system. In the TCDC-L and TUDC-L systems, the relative importance of the simple and the mixed micelles depends on the lecithin and the total Na + concentrations in the dissolution medium. However, at physiological levels of lecithin (i.e., 24 m M and above), it is always the simple micelle which dominates the ChM dissolution kinetics in both systems. In the TCDC-L system, the k s value first increased dramatically with the addition of lecithin from 2 to 16 m M and then leveled off. In the TC-L system, the k s value remained relatively unchanged over a wide concentration range of lecithin. These results were interpreted to mean that surface-adsorbed lecithin facilitates the TCDC simple micelles, but not the TC simple micelles, in ChM dissolution. Lecithin facilitation of TUDC simple micelles was found to be intermediate. The mechanisms by which surface-adsorbed lecithin may facilitate the attack of ChM crystallites by simple micelles remain to be clarified. The k M values generally decreased with increasing lecithin concentration for the three bile salt-lecithin systems. This decrease in k M with the addition of lecithin may be related to the molecular composition (the K value) and the increasing stability of the mixed micelles at higher lecithin levels. As previously noted, the addition of lecithin to a constant level of TC results in a decrease in the ChM dissolution rate despite a linear increase in the equilibrium solubility. However, in the TCDC-L system, the ChM dissolution rate first rises with increasing lecithin to 16 m M lecithin and then levels off. These differing patterns for the TC-L and the TCDC-L systems are explained on the basis of the simple micelle-mixed micelle coexistence phenomenon and the relative effectiveness of the micellar species in dissolving the ChM crystallites.

Rapid (1 hour) high performance gel filtration chromatography resolves coexisting simple micelles, mixed micelles, and vesicles in bile.

We describe the use and validation of Superose 6, a high performance gel filtration medium for rapid, high resolution separation and sizing of coexisting simple micelles, mixed micelles, and vesicles in bile. We fractionated model biles (1.7-4.2 g/dl total lipid concentration, 0.15 M NaCl) composed of lecithin (L), cholesterol (Ch), and the common bile salt taurocholate (TC) using Superose 6 gel filtration columns (1.0 cm diameter, 30 cm length, 0.5 ml model bile application, 1.0 ml fractions) pre-equilibrated and eluted with 2.5-10.0 mM TC. Lipid particle sizes were determined by quasielastic light scattering and lipid compositions by conventional analyses. In the absence of L and Ch, pure TC "biles" (32.2 mM), when eluted in the presence of 7.5 mM TC, yielded a single peak of particles (mean hydrodynamic radii, Rh values of 11-15 A), consistent with simple TC micelles. Model biles containing L and TC ([L] = 13.8 mM, [TC] = 32.2 mM) were fractionated with baseline resolution into TC-L mixed micelles, (Rh values of 30-40 A) and simple TC micelles. In agreement with the ternary TC-L-H2O phase diagram (Mazer, N. A., et al. 1980. Biochemistry. 19: 601-615), the proportions of simple and mixed micelles were inversely related to L concentrations ([L] = 0-32.2 mM) and correlated positively with eluant TC concentration. Superose 6 gel fractionation of model biles "super-saturated" with Ch (TC:L:Ch molar ratio 27:63:10, total lipid concentration 3 g/dl) yielded high resolution separation of vesicles (Rh value of 320 A) from mixed micelles of TC-L-Ch (Rh values of 40-50 A) and simple TC micelles (Rh values of 11-15 A). At an eluant TC concentration of 7.5 mM, Ch-rich vesicles (Ch/L molar ratio = 1.6) separated that contained 40% of total Ch, 9% of total L, and no TC, accurately reflecting predictions of the quaternary L-Ch-TC-H2O metastable phase diagram (Mazer, N. A., and M. C. Carey. 1983 Biochemistry. 22: 426-442). This suggested that a 7.5 mM TC concentration approximated the intermicellar concentration under the experimental conditions. We also fractionated an identical model bile using conventional Sephacryl S-300, a medium generally used to study model and native biles. Compared with Superose 6, the Sephacryl S-300 column of equivalent size yielded particle separations with lower resolution and speed (30 h v l h).(ABSTRACT TRUNCATED AT 400 WORDS)

Small-angle neutron-scattering study of triglyceride solubilization by lecithin micelles: a direct observation of rod-to-sphere transition

The solubilization of tributyrin by rodlike diheptanoylphosphatidylcholine (diheptanoyl-PC) micelles has been studied by small-angle neutron-scattering techniques. A rod to sphere transition is observed from the measured angular distribution of neutron-scattering intensities. The transition is from the rodlike micelle to a globular mixed micelle which has tributyrin molecules at the core surrounded by an outer shell of diheptanoyl-PC molecules. When the molar ratio of tributyrin to diheptanoyl-PC is increased beyond 0.18, all rodlike diheptanoyl-PC micelles are transformed into globular mixed micelles. The size of these globular mixed micelles increases with increasing tributyrin concentration in the simple micelle region up to the solubility limit. The small-angle neutron scattering data were analyzed to obtain the structural parameters of the mixed micelles. It is found that the growth of the mixed micelles does not follow a constant available surface area model or a constant palisade layer thickness model. If the model is modified to include tributyrin molecules at the surface of the mixed micelle, the growth behavior can be explained. For the tributyrin/diheptanoyl-PC molar ratio less than 0.18, rodlike micelles are found to coexist with globular mixed micelles. By fitting the neutron scattering data in the middle Q range, it is possible to determinemore » the size and number density of the mixed micelles in the coexistence region. The results of analyzing the small-angle neutron scattering data in the coexistence region. The results of analyzing the small-angle neutron scattering data in the coexistence region show that some tributyrin molecules are solubilized by the rodlike dipheptanoyl-PC micelles and only part of the added tributyrin forms globular micelles.« less

Determination of antimony in waste water by micelle solubilization spectrophotometry

A simple, sensitive and selective micelle solubilization spectro-photometric method is described for the determination of micro amounts of antimony(III) in waste water with 2-(3,5-dibromo-2-pyri-dylazo)-5-diethylaminophenol (3,5-dibromo-PADAP) as colour reagent and emulsifier OP as solubilizing and sensitizing agent. In 0.15M HCl, antimony forms a blue-green ternary complex with the above-mentioned reagents. The complex exhibits maximum absorption at 630nm, and the value of the apparent molar absorption coefficient is found to be 5.2 × 1041 mol−1 cm−1. The complex is stable up to 7 days at room temperature. The system obeys Beer's Law in the range 0–25 μg Sb(III) per 10 ml. Most metal ions do not interfere with the determination of antimony.