Effects of Weak Nonspecific Interactions with ATP on Proteins

Adenosine triphosphate (ATP) is an immensely well-studied metabolite serving multiple key biochemical roles as the major chemical energy currency in living systems, a building block of ribonucleic acids, and a phosphoryl group donor in kinase-mediated signaling. Intriguingly, ATP has been recently proposed to act as a hydrotrope that inhibits aggregation of amyloidogenic proteins; however, the underlying mechanism and the general physicochemical effect that coexistence with ATP exerts on proteins remain unclear. By combining NMR spectroscopy and MD simulations, here we observed weak but unambiguously measurable and concentration-dependent noncovalent interactions between ATP and various proteins. The interactions were most pronounced for an intrinsically disordered protein (α-synuclein) and for residues in flexible regions (e.g., loops or termini) of two representative folded proteins (ubiquitin and the dimeric ubiquitin-binding domain of p62). As shown by solution NMR, a consequence of the ATP-protein interaction was altered hydration of solvent-exposed residues in the protein. The observation that ATP interacted with all three proteins suggests that ATP is a general nonspecific binder of proteins. Several complementary biophysical methods further confirmed that, at physiological concentrations of ∼5-10 mM, ATP starts to form oligomeric states via magnesium-chelating and chelation-independent mechanisms, in agreement with previous studies. Although the observed ATP-protein interaction was relatively weak overall, the high ratio of ATP (monomeric free ATP, mono- and divalent ion-bound ATP, oligomeric and chelated ATP) to proteins in cells suggests that most proteins are likely to encounter transient interactions with ATP (and chemically similar metabolites) that confer metabolite-mediated protein surface protection.

Three Generations of Bile Acid Sequestrants

Cholestyramine, the first bile acid sequestrant to be marketed, has been in use for over 20 years. Despite its low potency, requiring 16-24 g of polymer to achieve 20% LDL cholesterol reduction in hypercholesterolemic individuals, only one other sequestrant, colestipol, has come to market in the ensuing period. GelTex Pharmaceuticals has been involved for over six years in the discovery and development of new, more potent polymeric sequestrants. Two binding mechanisms are presented — one that operates via an aggregate binding structure and one that is effective via a defined site binding structure. These two binding mechanisms are compared and contrasted through bile acid binding isotherms. The best of these new sequestrants bind bile acids through a combination of hydrophobicity and ion exchange. Optimization and balancing of each of these interactions led us to more potent materials. The first of these, colesevelam hydrochloride is expected to be three to four times more potent than cholestyramine. A third generation product is still in research at GelTex. With another twofold increase in potency possible, single tablet therapy may become a reality.

Influence of La3+ ions on the egg-yolk phosphatidylcholine and sodium taurocholate self-assemblies in aqueous suspension

In this study, the influence of La3+ on the mixed egg-yolk phosphatidylcholine (EYPC)-sodium taurocholate (TC) aggregates was investigated by using turbidity, quasi-elastic light-scattering (QELS) technique, Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). La3+ ions exert different effects on different EYPC-TC aggregates. The EYPC-TC mixed spheroidal micelles are stable in the presence of La3+ ions. The EYPC-TC vesicles are aggregated by the interaction of La3+ with the EYPC phosphate group. Particularly, La3+ causes structural transition of the EYPC-TC mixed disk micelles by changing the colloidal properties of TC. This process includes two steps, destruction of mixed disk micelles and formation of mixed spheroidal micelle. Several intermediate structures such as multi-lamellar vesicles and cylindrical micelles were observed during the transition. The experimental data and analysis in the current study suggest that metal ion is one of the important factors to control the structure of the EYPC-TC self-assemblies. And the possible influence of metal ions on the properties of bile biocolloid has been discussed.

Phosphatidylcholine-enriched diet prevents gallstone formation in mice susceptible to cholelithiasis

Cholesterol gallstones affect approximately 10-15% of the adult population in North America. Phosphatidylcholine (PC) is considered to be the main cholesterol solubilizer in bile. This study examined the effect of a PC-enriched diet on gallstone incidence in mice susceptible to cholelithiasis. The result obtained showed that the feeding of a lithogenic (LG) diet for 4 weeks or 8 weeks resulted in cholesterol gallstone incidences of 47% and 89%, respectively. These gallstone incidences were either reduced or prevented when the LG diet was enriched with 2% or 6% PC, respectively. The cholesterol saturation index (CSI) was reduced only in mice fed with LG + 6% PC diet as compared with mice fed the LG diet alone. However, in all groups, the CSI was significantly higher than in mice fed Purina chow diet. The biliary anionic polypeptide fraction (APF) was significantly increased in mice fed the LG + 2% PC diet and was reduced in those fed with LG + 6% PC diet. In conclusion, prevention or delay of gallstone formation was not due to a consistent effect on biliary lipid composition, suggesting a direct effect of PC on cholesterol solubilization and/or the effect of an additional nonlipid biliary component such as APF.

Mdr P-glycoproteins are not essential for biliary excretion of the hydrophobic heme precursor protoporphyrin in a griseofulvin-induced mouse model of erythropoietic protoporphyria

Hepatic complications in erythropoietic protoporphyria (EPP) have been attributed to toxic actions of accumulated protoporphyrin (PP). PP can only be removed via the bile but transport systems involved have not been defined. The aim of this study was to gain insight in the mode of biliary PP excretion, with emphasis on the potential contribution of the Mdr1 P-glycoprotein export pump and biliary lipids as PP carriers. Control mice and mice homozygous for Mdr1a/b (Abcb1) or Mdr2 (Abcb4) gene disruption, the latter unable to secrete phospholipids and cholesterol into bile, were treated with griseofulvin to chemically induce protoporphyria. All groups showed dramatically increased PP levels in erythrocytes and liver after griseofulvin treatment. Histologically, massive PP deposits were found in livers of control and Mdr1a/b(-/-) mice but not in those of Mdr2(-/-) mice. Serum unesterified cholesterol and phospholipids were increased by griseofulvin because of formation of lipoprotein-X in control and Mdr1a/b(-/-) mice only. Yet, bile flow was not impaired in griseofulvin-treated mice, and biliary bile salt, phospholipid, and cholesterol secretion rates were significantly increased. Surprisingly, biliary PP excretion was similar in all 3 groups of griseofulvin-treated mice: the observed linear relationship between hepatic and biliary PP concentrations and identical liver-to-bile concentration ratios in treated and untreated mice suggest a passive mode of excretion. In conclusion, the data show that Mdr P-glycoproteins are not critically involved in biliary removal of excess PP and indicate that the presence of biliary lipids is required for formation of intrahepatic PP deposits.

Review: pathogenesis of gallstones

The aim of this article is to review selected aspects of the pathogenesis of cholesterol-rich, gall-bladder stones (GBS)--with emphasis on recent developments in biliary cholesterol saturation, cholesterol microcrystal nucleation, statis within the gall-bladder and, particularly, on the roles of intestinal transit and altered deoxycholic acid (DCA) metabolism, in GBS development. In biliary cholesterol secretion, transport and saturation, recent developments include evidence in humans and animals, that bile lipid secretion is under genetic control. Thus in mice the md-2 gene, and in humans the MDR-3 gene, encodes for a canalicular protein that acts as a 'flippase' transporting phospholipids from the inner to the outer hemi-leaflet of the canalicular membrane. In the absence of this gene, there is virtually no phospholipid or cholesterol secretion into bile. Furthermore, when inbred strains of mice that have 'lith genes' are fed a lithogenic diet, they become susceptible to high rates of GBS formation. The precipitation/nucleation of cholesterol microcrystals from supersaturated bile remains a critical step in gallstone formation. methods of studying this phenomenon have now been refined from the original 'nucleation time' to measurement of cholesterol appearance/detection times, and crystal growth assays. Furthermore, the results of recent studies indicate that, in addition to classical Rhomboid-shape monohydrate crystals, cholesterol can also crystallize, transiently, as needle-, spiral- and tubule-shaped crystals of anhydrous cholesterol. A lengthy list of promoters, and a shorter list of inhibitors, has now been defined. There are many situations where GB stasis in humans is associated with an increased risk of gallstone formation--including iatrogenic stone formation in acromegalic patients treated chronically with octreotide (OT). As well as GB stasis, however, OT-treated patients all have 'bad' bile which is supersaturated with cholesterol, has excess cholesterol in vesicles, rapid microcrystal mulceation times and a two-fold increase in the percentage DCA in bile. This increase in the proportion of DCA seems to be due to OT-induced prolongation of large bowel transit time (LBTT). Thus LBTT is linearly related to (i) the percentage of DCA in serum; (ii) the DCA pool size; and (III) the DCA input or 'synthesis' rate. Furthermore, the intestinal prokinetic, cisapride, counters the adverse effects of OT on intestinal transit, and 'normalizes' the percentage of DCA in serum/bile. Patients with spontaneous gallstone disease also have prolonged LBTTs, more colonic gram-positive anaerobes, increased bile acid metabolizing enzymes and higher intracolonic pH values, than stone-free controls. Together, these changes lead to increased DCA formation, solubilization and absorption, Thus, in addition to the 'lithogenic liver' and 'guilty gall-bladder' one must now add the 'indolent intestine' to the list of culprits in cholesterol gallstone formation.

Microstructural evolution of lipid aggregates in nucleating model and human biles visualized by cryogenic transmission electron microscopy

Obtaining reliable information on the physical state and ultrastructure of bile is difficult because of its mixed aqueous-lipid composition and thermodynamic metastability. We have used time-lapse cryogenic transmission electron microscopy (cryo-TEM) combined with video-enhanced light microscopy (VELM) to study microstructural evolution in nucleating bile. A well-characterized model bile and gallbladder biles from cholesterol and pigment gallstone patients were studied sequentially during cholesterol nucleation and precipitation. In model bile, cholesterol crystallization was preceded by the appearance of the following distinct microstructures: spheroidal micelles (3-5 nm), discoidal membrane patches (50-150 nm) often in multiple layers (2-10), discs (50-100 nm), and unilamellar (50-200 nm) and larger multilamellar vesicles (MLVs). The membrane patches and discs appeared to be short-lived intermediates in a micelle-to-vesicle transition. Vesicular structures formed by growth and closure of patches as well as by budding off from vesicles with fewer bilayers. MLVs became more abundant, uniform, and concentric as a function of time. In native bile, all the above microstructures, except discoidal membrane patches, were observed. However, native MLVs were more uniform and concentric from the beginning. When cholesterol crystals appeared by light microscopy, MLVs were always detected by cryo-TEM. Edges of early cholesterol crystals were lined up with micelles and MLVs in a way suggesting an active role in feeding crystal growth from these microstructures. These findings, for the first time documented by cryo-TEM in human bile, provide a microstructural framework that can serve as a basis for investigation of specific factors that influence biliary cholesterol nucleation and crystal formation.

Cryoelectron microscopy of a nucleating model bile in vitreous ice: formation of primordial vesicles

Because gallstones form so frequently in human bile, pathophysiologically relevant supersaturated model biles are commonly employed to study cholesterol crystal formation. We used cryo-transmission electron microscopy, complemented by polarizing light microscopy, to investigate early stages of cholesterol nucleation in model bile. In the system studied, the proposed microscopic sequence involves the evolution of small unilamellar to multilamellar vesicles to lamellar liquid crystals and finally to cholesterol crystals. Small aliquots of a concentrated (total lipid concentration = 29.2 g/dl) model bile containing 8.5% cholesterol, 22.9% egg yolk lecithin, and 68.6% taurocholate (all mole %) were vitrified at 2 min to 20 days after fourfold dilution to induce supersaturation. Mixed micelles together with a category of vesicles denoted primordial, small unilamellar vesicles of two distinct morphologies (sphere/ellipsoid and cylinder/arachoid), large unilamellar vesicles, multilamellar vesicles, and cholesterol monohydrate crystals were imaged. No evidence of aggregation/fusion of small unilamellar vesicles to form multilamellar vesicles was detected. Low numbers of multilamellar vesicles were present, some of which were sufficiently large to be identified as liquid crystals by polarizing light microscopy. Dimensions, surface areas, and volumes of spherical/ellipsoidal and cylindrical/arachoidal vesicles were quantified. Early stages in the separation of vesicles from micelles, referred to as primordial vesicles, were imaged 23-31 min after dilution. Observed structures such as enlarged micelles in primordial vesicle interiors, segments of bilayer, and faceted edges at primordial vesicle peripheries are probably early stages of small unilamellar vesicle assembly. A decrease in the mean surface area of spherical/ellipsoidal vesicles was correlated with the increased production of cholesterol crystals at 10-20 days after supersaturation by dilution, supporting the role of small unilamellar vesicles as key players in cholesterol nucleation and as cholesterol donors to crystals. This is the first visualization of an intermediate structure that has been temporally linked to the development of small unilamellar vesicles in the separation of vesicles from micelles in a model bile and suggests a time-resolved system for further investigation.

The morphology and composition of cholesterol, protein, and bilirubin deposits in dried human bile: cathodoluminescence and backscattered electron imaging

This work is the first to deal with the application of color cathodoluminescence scanning electron microscopy (CCL SEM) and a novel version of combined imaging with backscattered electrons (CCL + BSE SEM) for the study of the composition of bile and its precipitation mechanisms. The present study demonstrates cholesterol, protein, and bilirubin distribution in deposits of normal and abnormal human bile after solution evaporation to full dryness. Qualitative CCL SEM analysis showed that dried bile remnants include different proportions of the above components. Three types of deposits were observed: Arborescent crystals, typical cholesterol crystals, and amorphous bilirubin particles. The selection of crystalline or amorphous precipitate phases is determined by the dehydration/concentration process. The findings may explain key features in lithogenesis.

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.

Imaging supramolecular aggregates in bile models and human bile

Investigation of cholesterol crystallization is essential for the understanding of gallstone formation. Previous work has revealed a variety of aggregates of different sizes and shapes prior to the appearance of "classical" plate-like cholesterol monohydrate crystals both in native biles and model systems. In this article, we review existing data based on various microscopic techniques and present data on microstructural pathways leading to cholesterol crystal formation in two different bile models and in native bile. In continuation of our recent investigation of microstructures in nucleating human bile, we now present data suggesting that polymorphism is not limited to complex native bile, but also appears in two, simplified model systems. These studies employed cryo-transmission electron microscopy (cryo-TEM) and video-enhanced light microscopy, using Nomarski optics (VELM). Only the combined use of these two complementary, non-perturbing direct methods can cover the whole range of microstructures ranging from a few nanometers to several microns. Concentrated isotropic solutions of bile models, composed of cholesterol, lecithin and taurocholate, were diluted to induce cholesterol supersaturation and start an evolution of microstructures, leading to cholesterol crystallization. Initially, small spheroidal micelles were observed by cryo-TEM. Subsequently, uni-, oligo- and multilamellar vesicles, compatible with structures seen at the same time by VELM, appeared in coexistence with micelles. Thereafter, during a dynamic phase of cholesterol crystallization, filaments, tubular and helical microstructures, as well as classical plate-like cholesterol monohydrate crystals were noted by light microscopy. Eventually, large plate-like crystals were observed by VELM, while cryo-TEM revealed only small spheroidal micelles. The crystallization process in native human bile during ex vivo incubation was found to bear close resemblance to the findings in the model systems, further supporting the applicability of these systems to the exploration of microstructural aspects of nucleating human bile.

An improved ultracentrifugation method for the separation of cholesterol carriers in bile

Vesicles and micelles, the major carriers of cholesterol in bile, play a role in the formation of cholesterol gallstones. A simple and rapid ultracentrifugation method was developed to isolate these biliary cholesterol carriers when only microliter amounts of bile were available. The proposed method employs a 46 to 0% sucrose density gradient, a NVT90 near-vertical rotor, and a centrifugation time of one hour. As little as 25 microL of bile can be used with no disruption of the carriers. The method was validated by comparison with gel filtration column chromatography using 6 mM taurocholate in the elution buffer. The sucrose linear density gradient ultracentrifugation procedure described here is simple, fast, and compares favorably with the gel filtration chromatography method for the separation of cholesterol carriers from bile.

The effects of dietary phospholipids enriched with phosphatidylethanolamine on bile and red cell membrane lipids in humans

The role of phospholipids in biliary cholesterol solubilization and crystallization has only recently begun to be appreciated. Phospholipid vesicles are believed to be the metastable carrier from which cholesterol nucleates. Cholesterol crystallization is influenced by the phospholipid species in bile. Feeding rats and hamsters with diets enriched in phospholipids or their precursors, especially ethanolamine, resulted in reduced cholesterol saturation of bile. Although whole phospholipids are normal dietary constituents, the effects and safety of phospholipid components have not been tested in humans. In the present study, we have evaluated the effects of a dietary phospholipid mixture, enriched with phosphatidylethanolamine, on human bile and red blood cell membrane lipid composition. Five ambulatory volunteers having a chronic indwelling T-tube, with an intact enterohepatic circulation, were investigated. Thirty-six grams of phospholipids (54% phosphatidylethanolamine, 54% linoleyl acyl chains) were added to their daily diet for fourteen days. Biliary nucleation time, cholesterol carriers, as well as plasma, red blood cell membrane, and bile lipid compositions, were monitored. Following phospholipid supplementation, the proportion of linoleyl chains (18:2) in biliary phospholipids increased significantly from 31.1 +/- 1.2 to 37.7 +/- 5.3%, while that of oleyl chains (18:1) decreased from 11.4 +/- 1.6 to 9.6 +/- 1.1%. These changes were accompanied by an increase of linoleate and its metabolite, arachidonate, in red cell membranes. Phospholipid feeding did not cause any side effects, and no significant changes in biliary nucleation time, cholesterol, phospholipid, or bile salt concentrations, or in the distribution of cholesterol within micelles or vesicles. We conclude that phospholipid feeding is safe, and can be effective as a vehicle for lecithin fatty acyl chain modulation of bile and lipid membranes. These findings may provide a basis for a controlled modulation of biliary phospholipids to increase cholesterol solubility in bile.

Relationship between biliary lipid and protoporphyrin secretion; potential role of mdr2 P-glycoprotein in hepatobiliary organic anion transport

BACKGROUND/AIMS

Erythropoietic protoporphyria, caused by ferrochelatase deficiency, leads to protoporphyrin accumulation in the liver. Therapeutic attempts to increase the secretion of this hydrophobic organic anion into bile are hampered by a lack of understanding of the secretory mechanism(s) involved. We have investigated biliary secretion of protoporphyrin in rats and mice, primarily targeted on the role of biliary lipids in this process.

METHODS

Gel permeation chromatography was applied to investigate the association of porphyrins with lipid fractions in bile. Secretion of endogenous porphyrins was studied in (GY mutant) rats and mdr2 P-glycoprotein deficient mice, under conditions of widely varying biliary lipid secretion rates.

RESULTS

Gel permeation chromatography revealed that, in native human and rat bile, protoporphyrin associated with cholesterol/phospholipid vesicles upon elution with bile salt-free buffer. In contrast, the more hydrophilic coproporphyrin isomers I and III were found only in bile salt/organic anion hybrid particles and smaller aggregates. Interruption of the enterohepatic circulation in normal Wistar rat resulted in parallel decrease of endogenous protoporphyrin-, lipid-, and bile salt secretion, but did not alter the secretion of coproporphyrin I and III. Uncoupling of lipid- from bile salt secretion by sulfated taurolithocholate resulted in impaired secretion into bile of protoporphyrin only. Conversely, secretion of coproporphyrin I and III, but not that of protoporphyrin, was impaired in mutant Groningen Yellow rats with defective ATP-dependent hepatobiliary organic anion transport. In mice homozygous for a disruption of the mdr2 P-glycoprotein gene, resulting in complete absence of phospholipids in bile and strongly reduced cholesterol output, secretion of protoporphyrin was reduced by 90%, whereas that of coproporphyrin I and III was affected to a much lesser extent.

CONCLUSIONS

Our data demonstrate a close association between protoporphyrin and lipid secretion into bile, indicating that these processes are, at least functioning coupled. This finding implicates a role of mdr2 P-glycoprotein activity in hepatobiliary removal of the hydrophobic organic anion protoporphyrin. Hence, it may be speculated that protoporphyrin secretion can be influenced by drugs, diet or other means that affect biliary lipid secretion.

Analysis of micellar and vesicular lecithin and cholesterol in model bile using 1H- and 31P-NMR

The distribution of phospholipid and cholesterol between the vesicular and micellar phases in bile plays an important role in the formation of cholesterol gallstones. Conventional analytical procedures to determine the distribution are potentially unreliable because they disturb the distribution of these compounds between the two phases. In this work, we circumvent this problem by using NMR. 31P-NMR is used to quantify directly the micellar and the vesicular amount of lecithin. The previously described 1H-NMR method to determine directly the micellar lecithin (Groen et al., J Lipid Res 31: 1315-1321) has been optimized by the implementation of a spectral quantification method. The agreement between the 31P and 1H methods was excellent. In our hands, the method of Ellul et al. (FEBS Lett 300: 30-32) did not allow quantification of micellar cholesterol, although our fitting procedure offered the possibility of quantifying overlapping spectral peaks by the use of prior knowledge about all the parameters of the compounds visible in the spectrum. The micellar cholesterol concentration was so low compared to the overlapping lecithin peaks that no reliable quantification was possible. The same problem was encountered when using other characteristic cholesterol resonances for quantification. Our data suggest that cholesterol present in the vesicular phases is too immobile to give rise to high-resolution peaks and the amount of cholesterol present in the micellar phase is too low to allow quantitation by NMR. We conclude that 1H-NMR can be used to determine micellar lecithin, and 31P-NMR to determine micellar as well as vesicular lecithin in model bile.

In vitro determination by 1H-NMR studies that bile with shorter nucleation times contain cholesterol-enriched vesicles

Although biliary vesicles are considered to be the primary source of cholesterol found in cholesterol gallstones, difficulties in quantitatively separating the different cholesterol transport modes in bile still remain. Proton nuclear magnetic resonance spectroscopy (1H-NMR) offers an alternative approach. Investigations were carried out on both model biles and human gallbladder bile samples: (i) to follow the effect of increasing sodium glycocholate concentrations on the 1H-NMR spectra of arachidonic acid rich-phospholipid, and cholesterol-lecithin vesicles, (ii) to compare the concentrations of total phospholipids in bile determined enzymatically with those obtained by integration of the phospholipid choline head group resonance peak, and (iii) to examine the relationship between biliary cholesterol nucleation time (NT) and the areas of the biliary lipid 1H-NMR peaks. It was found that the molecular motions of vesicle phospholipid, as determined by 1H-NMR, were restricted by saturation with cholesterol. In bile from patients with cholesterol gallstones, the reduced NMR fluidity of the phospholipid choline-head group indicated that the proportion of cholesterol-phospholipid vesicles containing more than 50% cholesterol, on a molar basis, was increased. The ratios of the N+(CH3)3 and = CH proton resonance peaks showed no overlap between samples with cholesterol gallstones and shorter NT and those with either no gallstones or pigment stones and longer NT. 1H-NMR spectroscopy indicates in a non-invasive manner those biles which are prone to cholesterol crystal formation.

Lamellar bodies coexist with vesicles and micelles in human gallbladder bile. Ursodeoxycholic acid prevents cholesterol crystal nucleation by increasing biliary lamellae

The aggregative forms of lipids in human gallbladder bile and their relation to cholesterol crystallization are controversial. Using combined chemical, gel-chromatographic, optical/electron microscopic and quasielastic light-scattering methods, we investigated this issue in native gallbladder bile obtained from nine untreated cholesterol gallstone patients and eight cholesterol gallstone patients treated for 1 week with 600 mg/day of ursodeoxycholic acid. Bile obtained at cholecystectomy was ultracentrifuged for 2 h at 150,000 g to obtain isotropic samples. The conventional cholesterol crystal observation time was 3.1 +/- 4.1 (SD) days in controls and 19.0 +/- 1.9 days in the ursodeoxycholic acid-treated group (p < 0.001). Bile was analyzed by high-resolution gel-chromatography using 7 mM sodium taurocholate in the elution buffer. Biliary lipids eluted in four chromatographic zones: zone #I, corresponding to the column void volume, contained only minimal amounts of lipids; zone #II (apparent m.w. 100-220 kDa) comprised 29.1 +/- 12.4% of biliary cholesterol in the untreated group and 8.3 +/- 4.3% in the ursodeoxycholic acid-group (p < 0.001). At negative staining electron microscopy, this region was composed of roundish vesicles ranging from 7 to 20 nm in diameter. Zone #III (apparent m.w. 50-100 kDa) carried 59.1 +/- 2.1% of cholesterol in untreated patients and 81.2 +/- 9.5% in ursodeoxycholic acid-rich biles, respectively (p < 0.001). At negative staining electron microscopy, this region was composed of lamellar stacks of variable length, usually with 5 nm interspaces and up to 30 nm in width. In ursodeoxycholic acid-rich biles, lamellae often appeared in the form of concentric fingerprint-like images. Quasielastic light-scattering measurements in this region were compatible with the size estimates obtained at electron microscopy. Zone #IV (apparent m.w. 6-50 kDa) carried 11.8 +/- 9.4% and 11.6 +/- 9.0% of cholesterol, respectively (not significant). Since this region comprised a considerable fraction of endogenous bile salts and had no distinct morphological structures, it was interpreted as mixed micelles. The cholesterol crystal observation time showed a significant inverse correlation (r = -0.85, p < 0.001) with percent cholesterol carried by vesicles (zone #II) and a direct correlation (r = 0.86, p < 0.001) with percent cholesterol carried by lamellar bodies (zone #III). Vesicles and lamellae identical to those observed in isolated gel-chromatographic fractions were observed also on direct electron microscopic examination of unfractionated isotropic native biles. Similar findings were observed also in matched model biles.(ABSTRACT TRUNCATED AT 400 WORDS)

Degree of fatty acyl chain unsaturation in biliary lecithin dictates cholesterol nucleation and crystal growth

To clarify factors involved in the formation of cholesterol gallstones, we studied the relationship between the degree of fatty acyl chain unsaturation of biliary lecithin and bile metastability. We used supersaturated model bile solutions (molar taurocholate/lecithin/cholesterol ratio (73:19.5:7.5), total lipid concentration 9 g/dl) that contained equimolar egg yolk or soybean lecithins or a sn-1 palmitoyl, sn-2 linoleoyl phosphatidylcholine. Gel permeation chromatographic studies showed that the vesicular cholesterol distribution and dimension were inversely related to the degree of unsaturation of the lecithin species, estimated by reverse phase, high-performance liquid chromatography. Differential interference contrast microscopy and assay of cholesterol crystal growth showed that a higher degree of fatty acyl chain unsaturation of the lecithin species was associated with a faster nucleation time and rate of crystal growth. Our results suggest that vesicular lecithins containing more unsaturated fatty acyl chains bind less tightly to cholesterol than lecithins containing predominantly saturated fatty acids, and that the biliary lecithin species dictates, in part, the nucleation and growth of cholesterol crystals in bile.