Lecithin hydrophobicity modulates the process of cholesterol crystal nucleation and growth in supersaturated model bile systems

Identification of changes in bile composition in pancreaticobiliary reflux based on liquid chromatography/mass spectrometry metabolomics

INTRODUCTION

Pancreaticobiliary reflux (PBR) can induce gallstone formation; however, its pathogenic mechanism remains unclear. In this study, we explored the mechanism of PBR by the non-targeted metabolomic analysis of bile in patients with PBR.

OBJECTIVE

The aim of this study was to investigate the pathogenic mechanism in PBR by the non-targeted metabolomic analysis of bile collected during surgery.

METHODS

Sixty patients who underwent gallstone surgery at our center from December 2020 to May 2021 were enrolled in the study. According to the level of bile amylase, 30 patients with increased bile amylase ( > 110 U/L) were classified into the PBR group, and the remaining 30 patients were classified into the control group (≤ 110 U/L). The metabolomic analysis of bile was performed.

RESULTS

The orthogonal projections to latent structure-discriminant analysis of liquid chromatography mass spectrometry showed significant differences in bile components between the PBR and control groups, and 40 metabolites were screened by variable importance for the projection value (VIP > 1). The levels of phosphatidylcholine (PC) and PC (20:3(8Z,11Z,14Z)/14:0) decreased significantly, whereas the levels of lysoPC (16:1(9z)/0:0), lysoPC (15:0), lysoPC (16:0), palmitic acid, arachidonic acid, leucine, methionine, L-tyrosine, and phenylalanine increased.

CONCLUSIONS

Significant differences in bile metabolites were observed between the PBR and control groups. Changes in amino acids and lipid metabolites may be related to stone formation and mucosal inflammation.

Free fatty acids and triglyceride change in the gallbladder bile of gallstone patients with pancreaticobiliary reflux

Background

Pancreaticobiliary reflux (PBR) causes chronic inflammation of the gallbladder mucosa and changes in the bile components, which are known to promote gallstone formation. This study aimed to investigate the bile biochemistry changes in gallstone patients with PBR and provide new clues for research on the involvement of PBR in gallstone formation.

Methods

Patients undergoing surgery for gallstones between December 2020 and May 2021 were eligible for inclusion. The bile biochemistry (including amylase, lipase, triglyceride, cholesterol, free fatty acids [FFAs], alanine aminotransferase [ALT], aspartate aminotransferase [AST], alkaline phosphatase [ALP], and γ-glutamyl transferase [γ-GT]) of the included gallstone patients was analysed to determine correlations with PBR.

Results

In this study, 144 gallstone patients who underwent surgery were enrolled. Overall, 15.97 % of the patients had an increased bile amylase level, which was associated with older age and significantly higher bile levels of ALP, lipase, triglyceride, and FFAs. Positive correlations were observed between amylase and lipase, triglyceride, FFAs levels in the gallbladder bile. However, the bile levels of triglyceride, FFAs, and lipase were positively correlated with each other only in the PBR group and showed no significant correlation in the control (N) group. In addition, elevated bile FFAs levels were found to be an independent risk factor for gallbladder wall thickening.

Conclusions

In conclusion, PBR-induced increase in FFAs and triglyceride in the gallbladder bile is a cause of gallstone formation, and an increase in bile ALP suggests the presence of cholestasis in PBR.

Nutritional factors (nutritional aspects) in biliary disorders: bile acid and lipid metabolism in gallstone diseases and pancreaticobiliary maljunction

Nutritional factors play a key role in the pathogenesis of biliary diseases such as gallstones and pancreaticobiliary maljunction. Gallstones are primarily classified into cholesterol stone and pigment stone according to the major composition. Cholesterol gallstone formation is very likely based upon supersaturated bile formation, and pigment stones are formed in bile rich in bilirubin. Thus, defects of hepatic metabolism of lipids and organic anions lead to biliary stones. Here, the recent understanding of cholesterol gallstone pathogenesis is elaborated. On the other hand, there is another important link of biliary lipid degradation to serious biliary disease, namely pancreaticobiliary maljunction. Lysophosphatidylcholine (lysoPC), a derivative of phosphatidylcholine hydrolysis by phospholipase A2, is a highly abundant bioactive lipid mediator present in circulation as well as in bile. Increases in bile of lysoPC and phospholipase A2 have been reported in pancreaticobiliary maljunction and considered to be the major risk factor for biliary tract cancers. Further, oxidized fatty acids have been established as a potent ligand for G2A, a member of G protein-coupled receptor family that mediates a diverse array of biological processes including cell growth and apoptosis. Thus, both of lysoPC and free fatty acids are supposed to play an important role through G2A in biliary inflammation and carcinogenesis of pancreaticobiliary maljunction. Taken together, nutritional factors, especially lipid compounds, are seemingly crucial in the pathogenesis of biliary diseases, and such a causal relationship is reviewed by mainly authors' previous publications.

Evidence of domain formation in cardiolipin-glycerophospholipid mixed monolayers. A thermodynamic and AFM study

The interaction of the three main components of the mitochondrial membrane, namely cardiolipin, phosphatidylcholine, and phosphatidylethanolamine, has been studied investigating mixed cardiolipin-phosphatidylcholine and cardiolipin-phosphatidylethanolamine monolayers at different cardiolipin molar fractions. The thermodynamic behavior of the mixed monolayers was investigated by means of surface pressure and surface potential measurements, and atomic force microscopy was employed to characterize the morphology of the monolayers. Langmuir isotherms and surface potential curves show a regular behavior with a progressive transition toward the isotherm of the pure component. Positive deviations from ideality in the excess Gibbs energies of mixing suggest the presence of repulsive interactions in both systems. Analysis of partial molecular dipole moment indicates a discontinuity at a definite cardiolipin/phosphatidylethanolamine molar fraction, suggesting the formation of a stoichiometric complex; as a consequence, in mixed cardiolipin-phosphatidylethanolamine monolayers, a phase separation is observed at phosphatidylethanolamine excess. AFM measurements indicate the presence of two domains: one made by phosphatidylethanolamine and the other by a regular arrangement of phosphatidylethanolamine and cardiolipin at a fixed molecular ratio.

Pseudo-ternary phase diagrams of aqueous mixtures of Quil A, cholesterol and phospholipid prepared by the lipid-film hydration method

Pseudo-ternary phase diagrams of the polar lipids Quil A, cholesterol (Chol) and phosphatidylcholine (PC) in aqueous mixtures prepared by the lipid film hydration method (where dried lipid film of phospholipids and cholesterol are hydrated by an aqueous solution of Quil A) were investigated in terms of the types of particulate structures formed therein. Negative staining transmission electron microscopy and polarized light microscopy were used to characterize the colloidal and coarse dispersed particles present in the systems. Pseudo-ternary phase diagrams were established for lipid mixtures hydrated in water and in Tris buffer (pH 7.4). The effect of equilibration time was also studied with respect to systems hydrated in water where the samples were stored for 2 months at 4 degrees C. Depending on the mass ratio of Quil A, Chol and PC in the systems, various colloidal particles including ISCOM matrices, liposomes, ring-like micelles and worm-like micelles were observed. Other colloidal particles were also observed as minor structures in the presence of these predominant colloids including helices, layered structures and lamellae (hexagonal pattern of ring-like micelles). In terms of the conditions which appeared to promote the formation of ISCOM matrices, the area of the phase diagrams associated with systems containing these structures increased in the order: hydrated in water/short equilibration period<hydrated in buffer/short equilibration period<hydrated in water/prolonged equilibration period. ISCOM matrices appeared to form over time from samples, which initially contained a high concentration of ring-like micelles suggesting that these colloidal structures may be precursors to ISCOM matrix formation. Helices were also frequently found in samples containing ISCOM matrices as a minor colloidal structure. Equilibration time and presence of buffer salts also promoted the formation of liposomes in systems not containing Quil A. These parameters however, did not appear to significantly affect the occurrence and predominance of other structures present in the pseudo-binary systems containing Quil A. Pseudo-ternary phase diagrams of PC, Chol and Quil A are important to identify combinations which will produce different colloidal structures, particularly ISCOM matrices, by the method of lipid film hydration. Colloidal structures comprising these three components are readily prepared by hydration of dried lipid films and may have application in vaccine delivery where the functionality of ISCOMs has clearly been demonstrated.

Inhibition of cholesterol crystallization under bilirubin deconjugation: partial characterization of mechanisms whereby infected bile accelerates pigment stone formation

Pigment gallstones have been reported to be closely associated with biliary tract infection. We previously reported that addition of unconjugated bilirubin (UCB), which is deconjugated by beta-glucuronidase in infected bile, could enhance cholesterol crystal formation in supersaturated model bile (MB). The present study evaluated the effect of beta-glucuronidase on the processes of pigment gallstone formation and cholesterol crystallization. Supersaturated MB (taurocholate/lecithin/cholesterol at 71:18:11, a total lipid concentration of 10.0 g/dl and a cholesterol saturation index (CSI) of 2.0) and native rat bile were mixed at a ratio of 3:1. Then, mixed bile was incubated with or without beta-glucuronidase and changes of the following parameters were investigated over time: (1) the UCB/total bilirubin ratio; (2) cholesterol crystal formation; (3) the precipitate weight and the cholesterol concentration in the precipitate and supernatant; and (4) the lipid distribution of vesicles in the supernatant. Compared with beta-glucuronidase-free bile, (1) beta-glucuronidase-containing bile showed a significant increase of the UCB/total bilirubin ratio, (2) as well as a significantly longer nucleation time (96+/-17.0 vs. 114+/-20.0) and fewer cholesterol crystals. (3) The precipitate weight and the cholesterol concentration in the precipitate were significantly increased, while the cholesterol concentration in supernatant was decreased. (4) When mixed bile was incubated with beta-glucuronidase, the cholesterol concentration in the vesicles was lower than in bile without beta-glucuronidase. The precipitate weight and the cholesterol concentration in the precipitate was increased by incubation with beta-glucuronidase, while cholesterol concentration was decreased in the supernatant (especially in the vesicles). This means that bile vesicles were more stable and it was more difficult for cholesterol crystals to form. Thus, the presence of beta-glucuronidase may inhibit the formation of pure cholesterol stones even in the presence of cholesterol supersaturation.

Less hydrophobic phosphatidylcholine species simplify biliary vesicle morphology, but induce bile metastability with a broad spectrum of crystal forms

Cholesterol crystallization in bile is affected by phosphatidylcholine (PtdCho) hydrophobicity. The aim of the present study was to determine whether PtdCho species modulate the metastable-labile limit and equilibrium solubility of cholesterol in the micellar phase of bile, thereby altering the distribution of cholesterol to biliary lipid carriers and thus influencing cholesterol crystallization. Supersaturated model bile (with a cholesterol saturation index of 2.0 and a total lipid concentration of 10 g/dl) was prepared with various PtdCho/(bile salt+PtdCho) ratios (0.1-0.5) using egg yolk or soya bean PtdCho. Subsequently, the following features were determined: metastable-labile limit, equilibrium solubility of cholesterol, metastable zone, and cholesterol crystallization process. Less hydrophobic PtdCho species destabilized bile cholesterol to induce rapid crystallization, because of a broad integrated metastable zone, whereas more hydrophobic species stabilized bile cholesterol with a less integrated metastable zone and thus retarded cholesterol crystallization. Cholesterol crystallization was accelerated by a decrease in the PtdCho/(bile salt+PtdCho) ratio, whereas the final nucleated crystal mass was increased by an increase in this ratio. With decreasing hydrophobicity of the PtdCho species, the intermixed micellar/vesicular concentration of bile salts decreased in association with less formation of vesicles and increased formation of micelles, and a variety of crystal forms were detected. In conclusion, PtdCho species directly influenced the cholesterol crystallization process in model bile by remodelling the bile mesophase, and also had an indirect influence by altering the balance between bile salt micelles and vesicles.

Less hydrophobic phosphatidylcholine species simplify biliary vesicle morphology, but induce bile metastability with a broad spectrum of crystal forms

Cholesterol crystallization in bile is affected by phosphatidylcholine (PtdCho) hydrophobicity. The aim of the present study was to determine whether PtdCho species modulate the metastable-labile limit and equilibrium solubility of cholesterol in the micellar phase of bile, thereby altering the distribution of cholesterol to biliary lipid carriers and thus influencing cholesterol crystallization. Supersaturated model bile (with a cholesterol saturation index of 2.0 and a total lipid concentration of 10 g/dl) was prepared with various PtdCho/(bile salt+PtdCho) ratios (0.1-0.5) using egg yolk or soya bean PtdCho. Subsequently, the following features were determined: metastable-labile limit, equilibrium solubility of cholesterol, metastable zone, and cholesterol crystallization process. Less hydrophobic PtdCho species destabilized bile cholesterol to induce rapid crystallization, because of a broad integrated metastable zone, whereas more hydrophobic species stabilized bile cholesterol with a less integrated metastable zone and thus retarded cholesterol crystallization. Cholesterol crystallization was accelerated by a decrease in the PtdCho/(bile salt+PtdCho) ratio, whereas the final nucleated crystal mass was increased by an increase in this ratio. With decreasing hydrophobicity of the PtdCho species, the intermixed micellar/vesicular concentration of bile salts decreased in association with less formation of vesicles and increased formation of micelles, and a variety of crystal forms were detected. In conclusion, PtdCho species directly influenced the cholesterol crystallization process in model bile by remodelling the bile mesophase, and also had an indirect influence by altering the balance between bile salt micelles and vesicles.

Does bilirubin play a role in the pathogenesis of both cholesterol and pigment gallstone formation? Direct and indirect influences of bilirubin on bile lithogenicity

Bilirubin is found in the center of cholesterol gallstones, but its pathogenic role in their formation is unknown. Bilirubin causes a disproportionate reduction of biliary lipid secretion without affecting bile salt secretion in association with a change of biliary lecithin species, which modulates the cholesterol crystallization process. Therefore, the present study investigated whether bilirubin can influence the cholesterol crystallization procedure, and the mechanism(s) of any such action. Supersaturated model bile was prepared (taurocholate/lecithin/cholesterol at 71:18:11, a total lipid concentration of 9.0 g/dl, and cholesterol saturation index of 1.8), and cholesterol crystallization was monitored over time using a spectrophotometer and video-enhanced differential contrast microscopy in the absence or presence of bilirubin (at a final concentration of 10 microM, 20 microM, 40 microM, and 100 microM). Bilirubin enhanced the onset of cholesterol crystallization by 50%, whereas the crystal growth rate and final crystal mass were reduced at a high concentration of bilirubin. Taken together, these results suggest that bilirubin influences the cholesterol crystallization process, by either a direct interaction with biliary lipids that alters metastability, an indirect alteration of the bile salt-micellar lipid holding capacity, or both. Thus, bilirubin may play a role in the pathogenesis of both cholesterol and pigment gallstones.

Octreotide increases the proportions of arachidonic acid-rich phospholipids in gall-bladder bile

BACKGROUND AND AIMS

Octreotide treatment of acromegalic patients induces cholesterol gallstone formation, in part by impairing cholecystokinin release and gall-bladder contraction. However, there are few data on the effect of octreotide on biliary arachidonic acid-rich phospholipids or mucin glycoprotein, factors which also influence cholesterol gallstone formation.

METHODS

In acromegalic patients studied before and during 3 months of octreotide treatment, we measured mucin glycoprotein concentrations and the molecular species of phosphatidylcholine, and related the results to the cholesterol saturation and percentage of deoxycholic acid in gall-bladder bile.

RESULTS

The relative proportions of the major arachidonic acid-rich phosphatidylcholine species, PC 16:0-20:4 and PC 18:0-20:4, increased significantly during octreotide treatment. These changes were associated with a rise in the cholesterol saturation index and a non-significant twofold increase in mucin glycoprotein concentration. There were significant correlations between PC 16:0-20:4 and the cholesterol saturation index, percentage of vesicular cholesterol and percentage of deoxycholic acid in gall-bladder bile.

CONCLUSIONS

In acromegalic patients, octreotide increases the proportions of arachidonic acid-rich phospholipids, with associated rises in: (a) the cholesterol saturation index and percentage of vesicular cholesterol, and (b) the percentage of deoxycholic acid in gall-bladder bile-changes similar to those found in patients with cholesterol-rich gall-bladder stones.

Is a role of phospholipase A(2) in cholesterol gallstone formation phospholipid species-dependent?

Phospholipase A(2) plays a role in cholesterol gallstone formation by hydrolyzing bile phospholipids into lysolecithin and free fatty acids. This study investigated its effects on cholesterol crystallization in model bile systems. Supersaturated model bile solutions with different cholesterol saturation indexes (1.2, 1.4, and 1.6) were prepared using cholesterol, taurocholate, and egg yolk phosphatidylcholine, soybean phosphatidylcholine, palmitoyl-oleoyl phosphatidylcholine, or palmitoyl-linoleoyl phosphatidylcholine. Then the effect of digestion of phosphatidylcholine by phospholipase A(2) on bile metastability was assessed by spectrophotometry and video-enhanced differential contrast microscopy. Addition of phospholipase A(2) caused the release of free fatty acids in a time-dependent manner. Cholesterol crystallization was enhanced by an increased crystal growth rate in model bile containing hydrophilic species such as soybean or palmitoyl-linoleoyl phosphatidylcholine, consisting predominantly of polyunsaturated fatty acids. Because phospholipase A(2) enhanced cholesterol crystallization in bile containing hydrophilic phosphatidylcholine species, but not hydrophobic phosphatidylcholine species, release of polyunsaturated fatty acids by hydrolysis may be responsible for such enhancement. Therefore, the role of phospholipase A(2) in cholesterol gallstone formation depends on the phospholipid species present in bile, so that phospholipid species selection during hepatic excretion is, in part, crucial to the cholesterol stone formation.

Gallbladder dysfunction enhances physical density but not biochemical metastability of biliary vesicles

The gallbladder role in cholesterol gallstone pathogenesis occurs through modulation of bile cholesterol metastability. The present study characterized the effects of concentrating bile on cholesterol crystallization through vesicle transformation, crystal habits, and potentiation of effector substances. Supersaturated model biles with total lipid concentrations of 12, 9, 6, and 3 g/dl were prepared with identical molar ratios (taurocholate-egg yolk phosphatidylcholine-cholesterol: 71:18:11). Bile metastability was assessed spectrophotometrically, and morphology of vesicle and crystal was sequentially scanned by video-enhanced differential contrast microscopy. The effects of replacing 30% of egg yolk phosphatidylcholine with soy bean phosphatidylcholine, 30% of taurocholate with taurodeoxycholate or tauroursodeoxycholate, and addition of concanavalin A-binding glycoprotein on each model bile were examined. By lowering total lipid concentration, cholesterol crystallization was retarded with less fusion and aggregation of vesicles. The effects of substances promoting cholesterol crystallization were enhanced with lesser bile. By replacing 30% of taurocholate with tauroursodeoxycholate, cholesterol crystallization was markedly inhibited in all concentrations, forming stable liquid-crystals. Impaired water absorption by the gallbladder may stabilize vesicles and inhibit rapid cholesterol crystallization, but the potential of cholesterol crystallization effector substances must be modified to alter bile cholesterol metastability.

Role of phospholipase A2 in cholesterol gallstone formation is associated with biliary phospholipid species selection at the site of hepatic excretion: indirect evidence

Phospholipase A2 plays a role in cholesterol gallstone development by hydrolyzing bile phospholipids into lysolecithin and free fatty acids. Lysolecithin and polyunsaturated free fatty acids are known to stimulate the synthesis and/or secretion of gallbladder mucin via a prostanoid pathway, leading to enhancing cholesterol crystal nucleation and growth, and therefore, the action of phospholipase A2 is associated, in part, with bile phospholipid fatty acid. To clarify this hypothesis, we evaluated the effect on bile lipid metastability in vitro of replacing phospholipids with lysolecithin and various free fatty acids. Supersaturated model biles were created with an identical composition (cholesterol saturation index, 1.8; egg yolk lecithin, 34 mM; taurocholate, 120 mM; cholesterol, 25 mM) except for 5%, 10%, or 20% replacement of egg yolk lecithin with a combination of palmitoyl-lysolecithin and a free fatty acid (palmitate, stearate, oleate, linoleate, or arachidonate), followed by time-sequentially monitoring of vesicles and cholesterol crystals using spectrophotometer and video-enhanced differential contrast microscopy. Replacement with hydrophilic fatty acids (linoleate and arachidonate) reduced vesicle formation and promoted cholesterol crystallization, whereas an enhanced cholesterol-holding capacity was evident after replacement with hydrophobic fatty acids (palmitate and stearate). These results indicate that the effect of phospholipase A2 on bile lithogenecity is modulated by the fatty acid species in bile phospholipids, and therefore, that the role of phospholipase A2 in cholesterol gallstone formation is dependent, in part, on biliary phospholipid species selection at the site of hepatic excretion.

Genetic determinants of variation in gallbladder disease in the Mexican-American population

Since there have not been any studies that quantify the influence of genetic factors on gallbladder disease (GBD) in humans using information from families, we utilized pedigree data to explore the genetic control of variation in liability to GBD. Using an extension of a variance components approach, we performed genetic analyses of GBD using information from 32 low-income Mexican-American families with two slightly different general models incorporating several sex-specific GBD risk factors. After evaluating the relative magnitudes of the covariate effects from these two models, we identified a parsimonious model including only significant predictors of GBD. According to this model, heritability for GBD was high (h2 = 0.44+/-0.18), after accounting for the significant effects of age, leptin in both sexes, total cholesterol, and HDL cholesterol in males only. We have shown quantitatively that variation in GBD is under strong genetic control. However, there are two major limitations to our findings: (1) since GBD was defined by a self-reported clinical history rather than an ultrasound examination, the prevalence of GBD could have been underestimated; and (2) since our design did not allow for shared environmental effects, our estimate of heritability may have been inflated.

Transcytotic vesicle fusion is reduced in cholestatic rats: redistribution of phospholipids in the canalicular membrane

Cholestasis, which affects phospholipid trafficking, therefore would be expected to alter canalicular membrane phospholipid composition and fluidity, as well as fatty acid composition of membrane phospholipid. These alterations may affect transcytotic vesicle fusion and would be expected to cause variety of cholestatic phenomena. The aim of this study was to determine the effect of cholestasis on transcytotic vesicle fusion. Sprague-Dawley rats with extrahepatic and intrahepatic cholestasis were prepared by bile duct ligation (6 hr or three days) and phalloidin infusion (0.4 mg/kg body weight), respectively. Liposomes of phosphatidylserine/phosphatidylcholine were labeled with octadecyl rhodamine B chloride. Fusion of liposomes to canalicular membrane vesicle preparations from cholestatic and control rats was induced by the addition of calcium. The degree of fusion was evaluated by measuring the increase in rhodamine fluorescence. Membrane phospholipid content also was analyzed. Rates of liposomal fusion to membranes from cholestatic rats were decreased compared to controls. The saturated/unsaturated and saturated/polyunsaturated fatty acid ratios were increased in membrane phosphatidylcholine and decreased in membrane sphingomyelin from cholestatic rats. Cholesterol/phospholipid ratios were increased. Thus, in the presence of cholestasis, a redistribution of phospholipid species within canalicular membranes is associated with decreased transcytotic vesicle fusion. Cholestasis likely decreases membrane fluidity. The regulation of phospholipid species within hepatocellular membranes may play an important role in intrahepatic lipid transport.

Partial replacement of bile salts causes marked changes of cholesterol crystallization in supersaturated model bile systems

Cholesterol crystallization is a key step in gallstone formation and is influenced by numerous factors. Human bile contains various bile salts having different hydrophobicity and micelle-forming capacities, but the importance of lipid composition to bile metastability remains unclear. This study investigated the effect of bile salts on cholesterol crystallization in model bile (MB) systems. Supersaturated MB systems were prepared with an identical composition on a molar basis (taurocholate/phosphatidylcholine/cholesterol, 152 mM:38 mM: 24 mM), except for partial replacement of taurocholate (10, 20, and 30%) with various taurine-conjugated bile salts. Cholesterol crystallization was quantitatively estimated by spectrophotometrically measuring crystal-related turbidity and morphologically scanned by video-enhanced microscopy. After partial replacement of taurocholate with hydrophobic bile salts, cholesterol crystallization increased dose-dependently without changing the size of vesicles or crystal morphology and the rank order of crystallization was deoxycholate>chenodeoxycholate>cholate (control MB). All of the hydrophilic bile salts (ursodeoxycholate, ursocholate and beta-muricholate) inhibited cholesterol precipitation by forming a stable liquid-crystal phase, and there were no significant differences among the hydrophilic bile-salt species. Cholesterol crystallization was markedly altered by partial replacement of bile salts with a different hydrophobicity. Thus minimal changes in bile-salt composition may dramatically alter bile lipid metastability.

Self-assembly of helical ribbons

The self-assembly of helical ribbons is examined in a variety of multicomponent enantiomerically pure systems that contain a bile salt or a nonionic detergent, a phosphatidylcholine or a fatty acid, and a steroid analog of cholesterol. In almost all systems, two different pitch types of helical ribbons are observed: high pitch, with a pitch angle of 54 +/- 2 degrees, and low pitch, with a pitch angle of 11 +/- 2 degrees. Although the majority of these helices are right-handed, a small proportion of left-handed helices is observed. Additionally, a third type of helical ribbon, with a pitch angle in the range 30-47 degrees, is occasionally found. These experimental findings suggest that the helical ribbons are crystalline rather than liquid crystal in nature and also suggest that molecular chirality may not be the determining factor in helix formation. The large yields of helices produced will permit a systematic investigation of their individual kinetic evolution and their elastic moduli.

Partial replacement of bile salts causes marked changes of cholesterol crystallization in supersaturated model bile systems

Cholesterol crystallization is a key step in gallstone formation and is influenced by numerous factors. Human bile contains various bile salts having different hydrophobicity and micelle-forming capacities, but the importance of lipid composition to bile metastability remains unclear. This study investigated the effect of bile salts on cholesterol crystallization in model bile (MB) systems. Supersaturated MB systems were prepared with an identical composition on a molar basis (taurocholate/phosphatidylcholine/cholesterol, 152 mM:38 mM: 24 mM), except for partial replacement of taurocholate (10, 20, and 30%) with various taurine-conjugated bile salts. Cholesterol crystallization was quantitatively estimated by spectrophotometrically measuring crystal-related turbidity and morphologically scanned by video-enhanced microscopy. After partial replacement of taurocholate with hydrophobic bile salts, cholesterol crystallization increased dose-dependently without changing the size of vesicles or crystal morphology and the rank order of crystallization was deoxycholate>chenodeoxycholate>cholate (control MB). All of the hydrophilic bile salts (ursodeoxycholate, ursocholate and beta-muricholate) inhibited cholesterol precipitation by forming a stable liquid-crystal phase, and there were no significant differences among the hydrophilic bile-salt species. Cholesterol crystallization was markedly altered by partial replacement of bile salts with a different hydrophobicity. Thus minimal changes in bile-salt composition may dramatically alter bile lipid metastability.

Role of bile salt hydrophobicity in distribution of phospholipid species to carriers in supersaturated model bile solutions

BACKGROUND

Phospholipid species modulate cholesterol-holding capacity and, therefore, regulate bile metastability.

METHODS

In this study, we investigated the effect of bile salt hydrophobicity on the distribution of phospholipids among lipid particles in supersaturated model bile solutions (total lipid concentration, 9 g/dL; taurocholate/phospholipid ratio 3.0, cholesterol saturation index 1.3), by using gel permeation chromatography.

RESULTS

With an increase of bile salt hydrophobicity in the elution buffer, the uptake of cholesterol and phospholipids into bile salt micelles was increased, associated with an increased cholesterol/phospholipid molar ratio of the vesicles. In contrast, there was an inverse correlation between the hydrophobicity of the phospholipid species in the vesicles and that of bile salts in the elution buffer, suggesting that hydrophobic bile salts induced preferential uptake of hydrophobic phospholipids into bile salt micelles, while less hydrophobic phospholipids, with a relatively low cholesterol-holding capacity, remained in the vesicles.

CONCLUSIONS

These data indicate that bile salt hydrophobicity regulates vesicular cholesterol metastability by modulating the hydrophobicity of phospholipids in vesicles, as well as the lipid distribution among various biliary lipid particles.

The comparative potency of cholesterol crystallization-effector proteins in supersaturated model bile systems: association with vesicle transformation

Various proteins which affect cholesterol crystallization are known to be present in bile, although the relative potency of their action is yet to be established. In this study, we evaluated the comparative potency of nucleating-effector proteins using a recently developed method for quantitative assessment of vesicle transformation in supersaturated model bile systems, to partially characterize mechanisms of their action. Concanavalin A-bound glycoproteins isolated from human gall-bladder bile shortened cholesterol crystallization time by 40% and increased cholesterol growth rate and final crystal mass by 161 and 19%, respectively, when compared to the control. In addition, immunoglobulins isolated from human gall-bladder bile increased cholesterol growth rate by 9%, but showed no significant effect on cholesterol crystallization time and final crystal mass. In contrast, human serum apolipoproteins A-I and B reduced cholesterol growth rate by 26 and 31% and reduced final crystal mass by 12 and 21%, but did not affect cholesterol crystallization time. Gel permeation chromatography revealed that proteins were distributed to both vesicles and bile salt micelles, but that no marked redistribution of lipids was caused by addition of these proteins. Furthermore, no significant difference in crystal structure was observed by video-enhanced contrast microscopy. These results indicate that nucleating-effector substances tested in this study may modulate vesicular cholesterol-holding capacity, thus affecting cholesterol crystallization. Such modulation is based upon the protein-vesicle association which defines the physico-chemical metastability of vesicular cholesterol.

Quantitative assessment of comparative potencies of cholesterol-crystal-promoting factors: relation to mechanistic characterization

The crystallization of cholesterol is affected by various factors in bile. The present study evaluated the relative importance of cholesterol-nucleation-promoting factors and partially characterized the mechanisms of their action. Model biles with an identical relative composition of cholesterol, egg-yolk phosphatidylcholine and taurocholate, except for replacing phosphatidylcholine (5-20%) with dilinoleoyl-phosphatidylcholine or taurocholate (10-30%) with taurodeoxycholate. Cholesterol crystallization was quantitatively assessed spectrophotometrically and morphologically estimated by the laser-scattering diffraction analyser and video-enhanced microscopy in the absence and presence of concanavalin A-binding glycoprotein isolated from human bile. In a series of experiments, lipid distribution among particulate species was determined after isolation by FPLC. In all experiments, cholesterol crystallization was dose-dependently enhanced with a rank order of: concanavalin A-binding glycoprotein > dilinoleoyl - phosphatidyl choline> taurodeoxycholate. No morphological alteration was evident for vesicles and crystals, but the cholesterol/phospholipid ratio in vesicles was increased significantly by replacement with dilinoleoyl-phosphatidylcholine and excess cholesterol. A high proportion of relatively hydrophilic phosphatidylcholine species such as dilinoleoyl-phosphatidylcholine and excess cholesterol in bile cause a redistribution of cholesterol to increase a vesicular cholesterol/phospholipid ratio, eventually promoting cholesterol crystallization, whereas concanavalin A-binding glycoprotein acts via differing mechanisms.

Transcytotic vesicle fusion with canalicular membranes is modulated by phospholipid species: implications for biliary lipid secretion

Phospholipid species modulate bile metastability and the subselection of such species for biliary secretion occurs at the canalicular membrane. In this study, the role of phospholipid head groups and hydrophobic indices in transcytotic vesicle fusion with the canalicular membrane inner leaflet was investigated using rat canalicular membrane vesicles (CMV) and liposomes. The CMV were purified from Sprague-Dawley rat liver, and small unilamellar vesicles (SUV) of phosphatidylserine (PS), phosphatidylcholine (PC) and mixtures of PS/PC (1:1, 2:1 and 4:1) were labelled with 8 mol% of octadecyl rhodamine B chloride (R18). The PC species used in this study were egg yolk PC (EYPC), soybean PC (SBPC), dipalmitoyl PC (DPPC) and dilinoleoyl PC (DLPC). Fusion of SUV with CMV was initiated by the addition of a millimolar concentration of Ca2+ and the degree of fusion was estimated by the increase of R18 fluorescence. Ca(2+)-dependent fusion of SUV consisting of PS, and PS/PC (4:1) with CMV was observed (PS > PS/PC; 4:1), whereas no detectable fusion was evident between CMV and SUV of PC alone or PS/PC (1:1 or 2:1). The rank order of fusibility between CMV and SUV of PS/PC (4:1) containing various PC species was PS/DLPC > PS/SBPC > PS/EYPC > PS/DPPC. The hydrophobic index of PC as determined by high performance liquid chromatography (HPLC) was related closely to liposome fusibility (r = -0.88). These results suggest that transcytotic vesicle fusion with the canalicular membrane inner leaflet is regulated by the phospholipid hydrophobicity of the vesicles.

Influence of bile salts on molecular interactions between sphingomyelin and cholesterol: relevance to bile formation and stability

Bile salts enhance secretion of cholesterol into bile and its subsequent solubilization with phosphatidylcholine in mixed micelles. Sphingomyelin, a major structural lipid of the hepatocyte canalicular membrane, and disaturated phosphatidylcholines are known to impede nucleation of solid cholesterol crystals in supersaturated model systems. To understand these effects physico-chemically, we compared the influence of bile salts on interactions of cholesterol with natural sphingomyelins, as well as with dipalmitoyl and egg yolk phosphatidylcholines using various in vitro systems. Submicellar bile salts enhanced significantly bidirectional transfer of dehydroergosterol (a fluorescent cholesterol analog) between sphingomyelin and egg yolk phosphatidylcholine vesicles in the rank order taurocholate < tauroursodeoxycholate < taurodeoxycholate. Quasielastic light scattering of serially diluted sphingomyelin-taurocholate mixtures (1:1 molar ratio, 3 g/dl) revealed metastable temperature-dependent transitions between globular micelles, rod-shaped micelles and vesicles, suggesting that phase transitions under these experimental conditions were metastable only at temperatures below 37 degrees C. Ternary phase diagrams of all sphingomyelins and dipalmitoyl phosphatidylcholine with cholesterol and taurocholate (37 degrees C, 3 g/dl, 0.15 M NaCl) were identical. Compared to systems containing egg yolk phosphatidylcholine, the 1-phase micellar zone and 2- and 3-phase solid cholesterol crystal-containing zones were reduced markedly while the 2-phase zone with stable cholesterol-sphingomyelin liquid crystals was greatly expanded. Our results suggest that the high affinity of cholesterol for sphingomyelin is lost in the presence of bile salts. Our findings may be relevant to secretion of cholesterol into bile and to its inability to crystallize in the hepatocyte canalicular lumen or its surrounding membranes.