Subunit interactions and physical properties of bovine gallbladder mucin

Role of Secretory Mucins in the Occurrence and Development of Cholelithiasis

Cholelithiasis is a common biliary tract disease. However, the exact mechanism underlying gallstone formation remains unclear. Mucin plays a vital role in the nuclear formation and growth of cholesterol and pigment stones. Excessive mucin secretion can result in cholestasis and decreased gallbladder activity, further facilitating stone formation and growth. Moreover, gallstones may result in inflammation and the secretion of inflammatory factors, which can further increase mucin expression and secretion to promote the growth of gallstones. This review systematically summarises and analyses the role of mucins in gallstone occurrence and development and its related mechanisms to explore new ideas for interventions in stone formation or recurrence.

Acidic pH increases airway surface liquid viscosity in cystic fibrosis

Cystic fibrosis (CF) disrupts respiratory host defenses, allowing bacterial infection, inflammation, and mucus accumulation to progressively destroy the lungs. Our previous studies revealed that mucus with abnormal behavior impaired mucociliary transport in newborn CF piglets prior to the onset of secondary manifestations. To further investigate mucus abnormalities, here we studied airway surface liquid (ASL) collected from newborn piglets and ASL on cultured airway epithelia. Fluorescence recovery after photobleaching revealed that the viscosity of CF ASL was increased relative to that of non-CF ASL. CF ASL had a reduced pH, which was necessary and sufficient for genotype-dependent viscosity differences. The increased viscosity of CF ASL was not explained by pH-independent changes in HCO3- concentration, altered glycosylation, additional pH-induced disulfide bond formation, increased percentage of nonvolatile material, or increased sulfation. Treating acidic ASL with hypertonic saline or heparin largely reversed the increased viscosity, suggesting that acidic pH influences mucin electrostatic interactions. These findings link loss of cystic fibrosis transmembrane conductance regulator-dependent alkalinization to abnormal CF ASL. In addition, we found that increasing Ca2+ concentrations elevated ASL viscosity, in part, independently of pH. The results suggest that increasing pH, reducing Ca2+ concentration, and/or altering electrostatic interactions in ASL might benefit early CF.

Cyclic AMP potentiates Ca2+-dependent exocytosis in pancreatic duct epithelial cells

Exocytosis is evoked by intracellular signals, including Ca²⁺ and protein kinases. We determined how such signals interact to promote exocytosis in exocrine pancreatic duct epithelial cells (PDECs). Exocytosis, detected using carbon-fiber microamperometry, was stimulated by [Ca²⁺]_i increases induced either through Ca²⁺ influx using ionomycin or by activation of P2Y2 or protease-activated receptor 2 receptors. In each case, the exocytosis was strongly potentiated when cyclic AMP (cAMP) was elevated either by activating adenylyl cyclase with forskolin or by activating the endogenous vasoactive intestinal peptide receptor. This potentiation was completely inhibited by H-89 and partially blocked by Rp-8-Br-cAMPS, inhibitors of protein kinase A. Optical monitoring of fluorescently labeled secretory granules showed slow migration toward the plasma membrane during Ca²⁺ elevations. Neither this Ca²⁺-dependent granule movement nor the number of granules found near the plasma membrane were detectably changed by raising cAMP, suggesting that cAMP potentiates Ca²⁺-dependent exocytosis at a later stage. A kinetic model was made of the exocytosis stimulated by UTP, trypsin, and Ca²⁺ ionophores with and without cAMP increase. In the model, without a cAMP rise, receptor activation stimulates exocytosis both by Ca²⁺ elevation and by the action of another messenger(s). With cAMP elevation the docking/priming step for secretory granules was accelerated, augmenting the releasable granule pool size, and the Ca²⁺ sensitivity of the final fusion step was increased, augmenting the rate of exocytosis. Presumably both cAMP actions require cAMP-dependent phosphorylation of target proteins. cAMP-dependent potentiation of Ca²⁺-induced exocytosis has physiological implications for mucin secretion and, possibly, for membrane protein insertion in the pancreatic duct. In addition, mechanisms underlying this potentiation of slow exocytosis may also exist in other cell systems.

Gallbladder inflammation is associated with increase in mucin expression and pigmented stone formation

Mucin is a high molecular weight glycoprotein that plays an important role in protecting the gallbladder epithelium from the detergent effect of bile. However, it also participates in gallstone formation. There is little information about a possible relationship between gallbladder inflammation and mucin expression or gallbladder stones' characteristics. The aims of this study were to investigate stone characteristics and patterns of mucin expression in the gallbladder epithelium and bile of gallstone patients, in relation to inflammation. Gallbladder bile and tissue samples from 21 patients were obtained at surgery. Mucin content was evaluated by gel filtration on a Sepharose CL-4B column. Dot blot for bile mucin apoproteins and immunohistochemistry staining for gallbladder mucosal mucin apoproteins were performed with antibodies to MUC2, MUC3, MUC5AC, MUC5B and MUC6. Staining intensity score (0-3) was used for assessment of antigen expression and the level of inflammation. Gallstone cholesterol content was determined in 16 patients. MUC 5AC and MUC 5B were demonstrated in 95.4 and 100% of gallbladder bile samples, respectively. Immunohistochemistry staining with antibodies to MUC 2, MUC 3, MUC 5AC, MUC 5B and MUC 6 were positive in 0, 100, 85.7, 100 and 95.4% of the gallbladder mucosal samples, respectively. Pigmented brown stones were associated with a higher level of gallbladder inflammation. Mucin species expressed in gallbladder epithelium are MUC3, MUC5AC, MUC5B and MUC6. MUC5AC and MUC5B are secreted into bile. Inflammation of the gallbladder is accompanied by a higher level of MUC5AC expression and is associated with pigmented brown stones.

Pattern of Ca2+ increase determines the type of secretory mechanism activated in dog pancreatic duct epithelial cells

Intracellular calcium concentration ([Ca(2+)](i)) is a key factor controlling secretion from various cell types. We investigated how different patterns of [Ca(2+)](i) signals evoke salt secretion via ion transport mechanisms and mucin secretion via exocytosis in dog pancreatic duct epithelial cells (PDEC). Activation of epithelial P2Y(2) receptors by UTP generated two patterns of [Ca(2+)](i) change: 2-10 microm UTP induced [Ca(2+)](i) oscillations, whereas 100 microm UTP induced a sustained [Ca(2+)](i) increase, both in the micromolar range. As monitored by carbon-fibre amperometry, the sustained [Ca(2+)](i) increase stimulated a larger increase in exocytosis than [Ca(2+)](i) oscillations, despite their similar amplitude. In contrast, patch-clamp recordings revealed that [Ca(2+)](i) oscillations synchronously activated a K(+) current as efficiently as the sustained [Ca(2+)](i) increase. This K(+) current was mediated by intermediate-conductance Ca(2+)-activated K(+) channels (32 pS at -100 mV) which were sensitive to charybdotoxin and resistant to TEA. Activation of these Ca(2+)-dependent K(+) channels hyperpolarized the plasma membrane from a resting potential of -40 mV to -90 mV, as monitored in perforated whole-cell configuration, in turn enhancing Na(+)-independent, Cl(-)-dependent and DIDS-sensitive HCO(3)(-) secretion, as monitored through changes in intracellular pH. PDEC therefore encode concentrations of purinergic agonists as different patterns of [Ca(2+)](i) changes, which differentially stimulate K(+) channels, the Cl(-)-HCO(3)(-) exchanger, and exocytosis. Thus, in addition to amplitude, the temporal pattern of [Ca(2+)](i) increases is an important mechanism for transducing extracellular stimuli into different physiological effects.

Human tear viscosity: An interactive role for proteins and lipids

Human tear viscosity is poorly understood. Tears need to remain on the ocular surface for lubrication without causing damage to the surface epithelia due to drag when blinking. Whole tears are shear-thinning (non-Newtonian), which cannot be explained by the amount of mucin present, nor by individual proteins. Whole tears minus lipids become Newtonian. Though no free lipids had previously been found in collected tears, tear lipocalin (TL), a major tear protein, is known to bind lipids. In this study, we aimed to confirm whether there are any free lipids in collected tears, and to clarify the combined contribution of tear proteins to viscosity, including experiments on recombinant TL, both without (apo-TL) and with (holo-TL) bound lipid. We also investigated possible oligomer formation by holo- and apo-TL as a mechanism for viscosity using SDS-PAGE and analytical ultracentrifugation (AU). For comparison, we have included results for beta-lactoglobulin, a well-characterised lipocalin protein. No free lipids were detected in whole tears. Rheology showed that any protein combination that included lysozyme or lactoferrin was shear-thinning, as was apo-TL, though holo-TL was Newtonian (linear). Results from SDS-PAGE and AU showed apo-TL to be entirely monomeric, but holo-TL showed some dimerization. Both apo- and holo-beta-lactoglobulin exhibited a monomer-dimer equilibrium. We conclude that hetero-protein interactions, possibly electrostatic, involving lipid-binding-induced structural changes to TL, significantly contribute to the viscosity of human tears.

Calcium-dependent protein interactions in MUC5B provide reversible cross-links in salivary mucus

The macromolecular organization within saliva was investigated by tracer diffusion measurements of fluorescent polystyrene microspheres by fluorescence recovery after photobleaching using a confocal microscope (confocal-FRAP). There was a concentration-dependent reduction in microsphere diffusion; this was much greater in the presence of calcium (10 mm) and was reduced by the addition of EGTA (10 mm). These effects on tracer diffusion showed that native saliva contained a macromolecular organization that was sensitive to free calcium concentrations. This was supported by a major increase in the weight average molecular weight of the high molecular weight mucin fraction in saliva (10-62 x 106) and an increase in intrinsic viscosity of saliva (733 to 1203 ml/g) both caused by calcium. Analysis of the change in tracer diffusion in saliva showed a 20-fold increase in the apparent pore size (from 130 nm in 10 mm CaCl2 to 2600 nm in 10 mm EGTA at physiological concentration). The effect was specific for calcium and was unaffected by up to 2 m NaCl. The calcium binding activity was contained in a high buoyant density fraction of saliva excluded from Sepharose CL-2B. Calcium binding to this fraction gave an approximate Kd of 7 x 10-6 m, and the binding was irreversibly destroyed by treatment with 6 m guanidinium chloride and by mild reduction, suggesting it to be to a protein site. This fraction of saliva was shown to contain MUC5B as the single major protein species by positive ion electrospray ionization-tandem mass spectrometry analysis. The results suggested that oligomeric MUC5B in saliva is assembled into much larger linear or branched assemblies through calcium-mediated protein cross-links.

Ca2+ activates cystic fibrosis transmembrane conductance regulator- and Cl- -dependent HCO3 transport in pancreatic duct cells

Pancreatic duct cells secrete bicarbonate-rich fluids, which are important for maintaining the patency of pancreatic ductal trees as well as intestinal digestive function. The bulk of bicarbonate secretion in the luminal membrane of duct cells is mediated by a Cl(-)-dependent mechanism (Cl(-)/HCO(3)(-) exchange), and we previously reported that the mechanism is CFTR-dependent and cAMP-activated (Lee, M. G., Choi, J. Y., Luo, X., Strickland, E., Thomas, P. J., and Muallem, S. (1999) J. Biol. Chem. 274, 14670-14677). In the present study, we provide comprehensive evidence that calcium signaling also activates the same CFTR- and Cl(-)-dependent HCO(3)(-) transport. ATP and trypsin evoked intracellular calcium signaling in pancreatic duct-derived cells through the activation of purinergic and protease-activated receptors, respectively. Cl(-)/HCO(3)(-) exchange activity was measured by recording pH(i) in response to [Cl(-)](o) changes of the perfusate. In perfusate containing high concentrations of K(+), which blocks Cl(-) movement through electrogenic or K(+)-coupled pathways, ATP and trypsin highly stimulated luminal Cl(-)/HCO(3)(-) exchange activity in CAPAN-1 cells expressing wild-type CFTR, but not in CFPAC-1 cells that have defective (DeltaF508) CFTR. Notably, adenoviral transfection of wild-type CFTR in CFPAC-1 cells completely restored the stimulatory effect of ATP on luminal Cl(-)/HCO(3)(-) exchange. In addition, the chelation of intracellular calcium by 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid (BAPTA) treatment abolished the effect of calcium agonists on luminal Cl(-)/HCO(3)(-) exchange. These results provide a molecular basis for calcium-induced bicarbonate secretion in pancreatic duct cells and highlight the importance of CFTR in epithelial bicarbonate secretion induced by various stimuli.

Mucin and phospholipids determine viscosity of gallbladder bile in patients with gallstones

AIM: An increased viscosity of gallbladder bile has been considered an important factor in the pathogenesis of gallstone disease. Besides lipids and proteins, mucin has been suggested to affect the viscosity of bile. To further clarify these issues we compared mucin, protein and the lipid componEnts of hepatic and gallbladder bile and its viscosity in patients with gallstones.

METHODS: Viscosity of bile (mPa.s) was measured using rotation viscosimetry in regard to the non Newtonian property of bile at low shear rates.

RESULTS: Biliary viscosity was markedly higher in gallbladder bile of patients with cholesterol (5.00 ± 0.60 mPa.s, mean ± SEM, n = 28) and mixed stones (3.50 ± 0.68 mPa.s; n = 8) compared to hepatic bile (0.92 ± 0.06 mPa.s, n = 6). A positive correlation between mucin and viscosity was found in gallbladder biles (r = 0. 65; P < 0.001) but not in hepatic biles. The addition of physiologic and supraphysiologic amounts of mucin to gallbladder bile resulted in a dose dependent non linear increase of its viscosity. A positive correlation was determined between phospholipid concentration and viscosity (r = 0.34, P < 0.005) in gallbladder biles. However, no correlation was found between total protein or the other lipid concentrations and viscosity in both gallbladder and hepatic biles.

CONCLUSION: The viscosity of gallbladder bile is markedly higher than that of hepatic bile in patients with gallstones. The concentration of mucin is the major determinant of biliary viscosity and may contribute by this mechanism to the role of mucin in the pathogenesis of gallstones.

Cystic fibrosis mutations and genetic predisposition to idiopathic chronic pancreatitis

Idiopathic chronic pancreatitis is a leading cause of chronic pancreatitis. Work from this and other groups has shown that idiopathic chronic pancreatitis is associated with mutations of the cystic fibrosis gene (CFTR). Many idiopathic pancreatitis patients have compound heterozygote genotypes in which both copies of the CFTR gene are abnormal. In these patients, the pancreatic disease can be viewed as a mild variant of cystic fibrosis, in which there is sufficient residual CFTR function to prevent lung disease. This article summarizes the evidence associating these abnormal CFTR genotypes with idiopathic chronic pancreatitis and reviews the implications of this association for the pathogenesis, classification, and prevention of pancreatitis.

Characterization of glycoproteins in the epithelial cells of human and other mammalian gallbladder. A review

The mammalian gallbladder mucosa is lined by a simple columnar epithelium. Typical surface epithelial cells (principal cells) contain short microvilli, secretory granules, dense bodies, mitochondria and Golgi apparatus. Dense bodies are thought to be lysosomes. Secretory granules contain mucous glycoproteins which are released to the lumen by exocytosis. Oligosaccharide side chains of mucous glycoproteins may provide a favorable environment for nucleation of cholesterol in gallstone formation; therefore they have been studied during the past decades. Histochemical techniques allow the in situ identification of carbohydrates at both the cellular and subcellular levels. The oligosaccharide chains of principal cell mucous glycoproteins have been studied by classical histochemical techniques (PAS, alcian blue, HID, etc). These techniques indicate that mammalian gallbladder mucous glycoproteins are heavily sulphated, whereas sialic acid residues are scarce. Neutral mucins have not been described in the mammalian gallbladder. Electron microscopic studies have located the oligosaccharide chains in secretory granules and Golgi apparatus. More recently, lectins (molecules which specifically recognize and bind with different saccharides or saccharide sequences) have been applied for the intracellular localization of carbohydrate residues. Lectin histochemistry has detected fucose, galactose, N-acetylglucosamine, N-acetylgalactosamine and N-acetylneuraminic acid residues in mucous granules, Golgi apparatus and apical membrane of human principal cells. Mannose residues were observed only in dense bodies. The combined use of deglycosylation procedures and lectin histochemistry has revealed a variety of terminal sequences in oligosaccharide chains of gallbladder mucous glycoproteins: Neu5Ac(alpha 2-3)Gal(beta 1-3)GalNAc, Neu5Ac(alpha 2-3)Gal(beta 1-4)GlcNAc and Gal(beta 1-4)GlcNAc. This technology also suggested the occurrence of N-linked oligosaccharides in the dense bodies of principal cells. Mucous granules mainly contained mucin-type O-linked oligosaccharides although some N-linked chains have also been detected. Gallstone formation is probably a complex process depending on multiple factors. Mucous glycoproteins are one of the factors involved in this process. Histochemical methods offer an excellent research tool for the characterization of glycoproteins in the epithelial cells of the gallbladder, thus contributing to the elucidation of the pathophysiology of gallstone formation.

Dissolution of human cholesterol gallstones in bile salt/lecithin mixtures: effect of bile salt hydrophobicity and various pHs

BACKGROUND

Unconjugated bile salts currently available for gallstone dissolution are poorly effective. We evaluated in vitro the litholytic potency of taurine-amidated bile salts against human cholesterol gallstones.

METHODS

Seventy radiolucent gallstones with similar size and composition (cholesterol content, 70.1 +/- 0.9%) from a single patient were incubated in model biles composed of 100 mM of either taurochenodeoxycholate (TCDC), taurocholate (TC), taurohyodeoxycholate (THDC) or tauroursodeoxycholate (TUDC) and of 45 mM egg yolk lecithin in saline buffered with tris/HCl (at pHs 7 and 8) or phosphate (at pHs 4 and 6). Biles (total lipids, 10 g/dl; cholesterol saturation, 99%) were incubated at 37 degrees C for 40 days. Gallstones were periodically weighed and returned to the dissolution vials, and the biliary cholesterol concentration was monitored.

RESULTS

Model biles remained optically clear during the initial 48 h of incubation. Then, biles containing THDC and TUDC, but not those with TC and TCDC, became progressively turbid until, after several days, a white precipitate surrounded the residual stone. Abundant liquid crytalline droplets were observed at polarizing microscopy in biles containing TUDC and THDC. Gallstone dissolution was closely related to cholesterol solubilization and decreased in the order TCDC > THDC > or = TC > TUDC, being highest at pH 8. At the physiologic pH of 7 THDC was more litholythic than TC.

CONCLUSIONS

In vitro, the litholytic potency of bile salts on cholesterol gallstones primarily depends on their hydrophobicity. THDC is a new potential gallstone-dissolving agent, deserving in vivo studies.

Subcellular characterization of glycoproteins in the principal cells of human gallbladder. A lectin cytochemical study

Gallbladder mucus is mainly composed of glycoproteins, which seem to play a critical role in cholesterol nucleation during gallstone formation. The biosynthetic pathway and sequential processing as well as the characterization of the oligosaccharide side-chains of human gallbladder secretory glycoproteins have not been completely defined. The aim of the present study is the subcellular characterization of the glycoproteins in the principal cells of human gallbladder. Principal cells of normal human gallbladder were studied by means of a variety of cytochemical techniques, including lectin histochemistry, enzyme and chemical treatments, immunocytochemistry and lectin-gold technology. Fucose, galactose, N-acetylglucosamine, N-acetylgalactosamine and N-acetylneuraminic acid residues were detected in mucous granules, Golgi apparatus and apical membrane of principal cells. Mannose residues were only observed in dense bodies. Oligosaccharide side-chains of the glycoproteins contained in the biliary mucus are synthesized in the Golgi apparatus of the principal cells of the gallbladder epithelium and are also contained in the mucous granules of these cells. Terminal N-acetylneuraminic acid(alpha 2-3)galactose(beta 1-3)N-acetylgalactosamine, N-acetylneuraminic acid(alpha 2-3)galactose(beta 1-4)N-acetylglucosamine and galactose(beta 1-4)N-acetylglucosamine sequences are contained in the oligosaccharide chains of gallbladder mucus glycoproteins. The dense bodies detected in the cytoplasm of the principal cells contained N-linked glycoproteins. Mucin-type O-linked glycoproteins were the main components of the mucous granules although some N-linked chains were also detected.

Indomethacin decreases viscosity of gallbladder bile in patients with cholesterol gallstone disease

There is experimental evidence that inhibition of cyclooxygenase with nonsteroidal anti-inflammatory drugs may decrease cholesterol gallstone formation and mitigate biliary pain in gallstone patients. The mechanisms by which NSAIDs exert these effect are unclear. In a prospective, controlled clinical trial we examined the effects of oral indomethacin on the composition of human gallbladder bile. The study included 28 patients with symptomatic cholesterol or mixed gallstones. Of these, 8 were treated with 3 x 25 mg indomethacin daily for 7 days prior to elective cholecystectomy while 20 received no treatment and served as controls. Bile and tissue samples from the gallbladder were obtained during cholecystectomy. Indomethacin tissue levels in the gallbladder mucosa, as assessed by HPLC, were 1.05 +/- 0.4 ng/mg wet weight, a concentration known to inhibit effectively cyclooxygenase activity. Nevertheless, no differences between the treated and untreated groups were found in the concentrations of biliary mucus glycoprotein (0.94 +/- 0.27 versus 0.93 +/- 0.32 mg/ml) or total protein (5.8 +/- 0.9 versus 6.4 +/- 1.3 mg/ml), cholesterol saturation (1.3 +/- 0.2 versus 1.5 +/- 0.2), or nucleation time (2.0 +/- 3.0 versus 1.5 +/- 2.0 days). However, biliary viscosity, measured using a low-shear rotation viscosimeter, was significantly lower in patients receiving indomethacin treatment (2.9 +/- 0.6 versus 5.6 +/- 1.2 mPa.s; P < 0.02). In conclusion, in man oral indomethacin decreases bile viscosity without alteration of bile lithogenicity or biliary mucus glycoprotein content.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine gallbladder mucin accelerates cholesterol monohydrate crystal growth in model bile

BACKGROUND

Gallbladder mucin accelerates cholesterol crystal nucleation, an early step in the pathogenesis of gallstones. To examine the role of gallbladder mucin in postnucleation gallstone maturation, the influence of mucin on cholesterol monohydrate crystal growth was studied in a novel model system.

METHODS

Cholesterol crystals of a uniform size were incubated in model biles at 37 degrees C with varying cholesterol saturation indices. Crystal size was quantitated by measuring the width and length of individual crystals under polarizing light microscopy and calculating average crystal area.

RESULTS

Crystal growth was dependent on the degree of cholesterol supersaturation of bile. Bovine gallbladder mucin (0.5-8 mg/mL) accelerated crystal growth in supersaturated model bile in a concentration- and time-dependent fashion compared with control incubations with bovine serum albumin or model bile alone (P < 0.05). Cholesterol crystal growth was accompanied by a progressive decrease in cholesterol saturation and an increase in total cholesterol crystal mass. Crystal growth was also accompanied by a decrease in total crystal number, suggesting net transfer of cholesterol to larger crystals.

CONCLUSIONS

The acceleration of cholesterol crystal growth by gallbladder mucin may be of pathophysiological importance in the postnucleation maturation of cholesterol gallstones.

Mucin-like high molecular mass protein fractions from total pig gallbladder bile mucus, pig gallbladder wall mucus, and total human gallbladder bile mucus

Native mucin-like complexes were obtained from both pig gallbladder bile and pig gallbladder wall mucus by precipitation, centrifugation, and gel permeation chromatography. Crude preparations by either dialysis (native mucus from bile, and native gallbladder wall mucus) or by precipitation (crude total bile mucus, and lipomucoid) were purified by gel permeation chromatography on Sephacryl S-300HR and Sephacryl S-500HR (Pharmacia). The elution profiles obtained with a reversibly denaturing and detergent-containing eluent showed the same pattern for all samples, although the amounts of the four main fractions differed somewhat. The excluded fraction with the highest carbohydrate portion had an apparent M(r) > 10(7). This fraction and the following included lipomucoid (in physiological solution tightly bound to fraction I), and an eluent-insoluble mucus portion from all samples were characterized by determination of the protein concentration, carbohydrates, sialic acids, and lipids, using standard methods. Sugar analysis was performed by gas-liquid chromatography. Human gallbladder bile was subjected to the same procedures of mucus precipitation and separation. Human gallbladder bile mucus showed identical behaviour to that of pig gallbladder bile mucus, and showed a very similar elution pattern in gel chromatography.

Changes in gallbladder bile composition following gallstone formation and weight reduction

Changes in gallbladder bile composition that occurred in patients who developed gallstones during weight reduction were evaluated. Bile was sampled directly from the gallbladder in 11 morbidly obese patients with no gallstones at the time of gastric bypass surgery and after gallstones had formed at cholecystectomy. Bile salt concentration ([BS]) increased significantly from a mean of 82.7-157.7 mmol/L (P less than 0.05). The concentration of cholesterol in gallbladder bile increased slightly and cholesterol saturation declined slightly with weight reduction and gallstone formation. Gallbladder mucin concentration increased 18-fold from a mean of 62 to 1110 micrograms/mL (P less than 0.001). Both free [Ca2+] and total calcium [Ca] increased 40% from mean values of 1.12 and 5.05 mmol/L at gastric bypass to 1.86 and 8.60 mmol/L after gallstone formation (P less than 0.05). The increase in [Ca2+] observed after gallstone formation was much greater than anticipated from changes in [BS] alone. This excess [Ca2+] in gallbladder bile increased curvilinearly with increasing mucin concentration. These results show that both gallbladder mucin and [Ca2+] increase with gallstone formation in humans and that mucin may modulate [Ca2+] in gallbladder bile.

Mucin complexes: characterization of the "link" component of submandibular mucus glycoprotein

1. Analysis of the submandibular saliva revealed that the secretion consists of mucin complexed with 150 kDa fibronectin fragment and DNA. 2. The kallikreins, secreted by the submandibular gland, appear to be responsible for the fibronectin fragmentation, since an identical peptide was also generated when fibronectin was subjected to incubation with the submandibular saliva or the purified enzyme. 3. The results provide evidence that the 150 kDa glycopeptide so-called salivary mucin "link" component is neither an integral part of the mucin molecule, nor linked to mucin subunits by disulfide bonds, but is a fibronectin fragment which associates with mucin. 4. Using mucin monoclonal antibody (3G12), it was revealed that the nonglycosylated (naked) 8-12 kDa fragment of the mucin molecule is responsible for the interaction of mucin with other components of saliva. 5. Under physiological conditions, the interaction of mucin with fibronectin on the luminal surfaces may be relevant in building mucous barrier and protection of the delicate oral epithelium from damage.

Identity of mucin's "118-kDa link protein" with fibronectin fragment

Human and rat intestinal mucin was purified by equilibrium density gradient centrifugation and Sepharose 2B chromatography according to M. Mantle, D. Mantle, and A. Allen (1981, Biochem. J. 195, 277-285) and analyzed using mucin, DNA, and fibronectin-specific antibodies in dot-blot, ELISA, and Western blotting. The 118-kDa component of the mucins and the 118-kDa fragment of fibronectin from the same source displayed affinity for concanavalin A and immunoreacted with fibronectin antibodies. The amino acid and carbohydrate compositions of the 118-kDa peptide electroeluted by gel electrophoresis of mucin and fibronectin preparations were identical within each pair of glycopeptides and closely resembled the "link protein component" of human and rat intestinal mucin preparations of R. E. F. Fahim, R. D. Specian, G. G. Forstner, and J. F. Forstner (1987, Biochem. J. 243, 631-640) and M. Mantle and G. Stewart (1989, Biochem. J. 259, 631-640). We therefore conclude that the "link protein" claimed to be an integral part of mucus glycoproteins in actuality is the 118-kDa fragment of fibronectin.

Characterization of bovine gallbladder mucin. Amino acid sequences of tryptic peptides from the glycosylated domain of the protein core

Gallbladder mucin is a densely glycosylated macro-molecule that promotes cholesterol gallstone formation in experimental animals and in humans. Bovine gallbladder mucin structure was studied after chemical deglycosylation by treatment with anhydrous hydrogen fluoride at 23 degrees C for 3 hours. Deglycosylated mucin contained less than 5% of the amino sugar and neutral hexose content of native mucin. Electrophoretic and molecular sieve chromatographic analyses indicated that significant cleavage of the mucin polypeptide core had occurred during deglycosylation. Deglycosylated mucin was separated into three major fractions by reverse-phase chromatography, one of which was enriched with respect to threonine and proline. Tryptic peptides prepared from this fraction were purified by molecular sieve and reverse-phase chromatography, and the amino acid sequences (8-20 residues) of the four principal tryptic peptides were determined. These peptides contained 65%-75% threonine and proline residues and demonstrated 80%-100% sequence similarity. These data provide the first information on the primary structure of gallbladder mucin and suggest that repeating amino acid sequences occur in this protein. Comparison of gallbladder mucin peptide structure with the consensus repeat sequence of human intestinal mucin showed approximately 60% sequence similarity. It was concluded that mammalian gastrointestinal mucins may be derived from a common ancestral gene.

Evidence of hydrophobic domains in human respiratory mucins. Effect of sodium chloride on hydrophobic binding properties

Hydrophobic binding properties of purified human respiratory mucins were studied by the fluorescence probe technique using mansylphenylalanine (Mns-Phe) as the fluorescent probe. Mucins were purified from tracheobronchial secretions of cystic fibrosis (CF) and asthmatic patients, as well as from individuals with normal lungs, according to a protocol earlier established in our laboratory. Purified mucins were subjected to reduction-alkylation and Pronase digestion to study the effects of these treatments on the hydrophobic properties of the mucins. In addition, the effects of increased NaCl concentration on the hydrophobic properties of native and reduced-alkylated mucins were also investigated. Native mucins showed evidence of a large number of low-affinity (KD approximately 10(-5) M) binding sites for the hydrophobic ligand Mns-Phe and had between 40 and 50 binding sites/mg of mucin. Reduction of mucin using dithiothreitol in the presence of 6 M guanidine hydrochloride and subsequent alkylation with iodoacetamide apparently caused marked conformational changes in the mucin molecules as revealed by the presence of both high-affinity (KD approximately 10(-6) M) and low-affinity (KD approximately 10(-5) M) binding sites for the probe and an increase in the number of probe binding sites. Pronase digestion of the native and reduced-alkylated mucins almost completely eliminated binding of the fluorescent probe to the mucins, showing that the binding sites are on the nonglycosylated, Pronase-sensitive portion of the mucin molecules. Increasing NaCl concentrations (0.03-1.0 M) did not appreciably alter the native mucin-induced Mns-Phe fluorescence, while that of the reduced-alkylated mucin-induced Mns-Phe fluorescence was progressively increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of peptides from the protein core of bovine gallbladder mucin

Gallbladder mucin may promote cholesterol gallstone formation by accelerating cholesterol monohydrate crystal nucleation in supersaturated bile. In this study, peptides were isolated from the mucin protein core by protease digestion and molecular-sieve high-performance liquid chromatography. Tryptic peptides were purified by anion exchange or reverse-phase high-performance liquid chromatography, and amino acid compositions were determined. Tryptic peptides were (a) nonglycosylated, (b) selectively enriched in serine, glutamic acid plus glutamine, and glycine, and (c) depleted in threonine and proline compared with native gallbladder mucin. Bilirubin derivatized with Woodward's reagent K covalently bound to purified mucin. Tryptic digestion of the mucin-bilirubin complex yielded low-molecular-weight nonglycosylated peptides with covalently bound bilirubin. These data indicate that the mucin protein core contains at least two distinct domains. One domain is rich in threonine and proline and contains the majority of covalently bound carbohydrate. A second domain, possibly internally located, is nonglycosylated, enriched in serine, glutamic acid plus glutamine, and glycine, and binds hydrophobic ligands such as bilirubin and 1-anilino-8-naphthalene sulfonate. Hydrophobic domains on the mucin protein core may contribute to the pathogenesis of cholesterol cholelithiasis.

Addition of N-acetylcysteine to aqueous model bile systems accelerates dissolution of cholesterol gallstones

The organic matrix of cholesterol gallstones contains a macromolecular complex of mucin and bilirubin that may inhibit stone dissolution by limiting contact of desaturated bile with crystalline cholesterol. The goal of this study was to determine if the mucolytic agent N-acetylcysteine could accelerate gallstone dissolution in vitro. Paired gallstones were dissolved in either pure taurocholate (140 mM) or ursodeoxycholate (100 mM), or in bovine bile supplemented with either taurocholate or ursodeoxycholate to achieve the same respective bile-salt concentrations. N-acetylcysteine was added to 1 stone from each pair at a concentration of 500 mM in pure bile salts and 100 mM in supplemented bile. Gallstones dissolved significantly faster in bovine bile supplemented with taurocholate or ursodeoxycholate than in pure solutions of the respective bile salts (n = 30, p less than 0.001). N-acetylcysteine significantly accelerated gallstone dissolution in pure solutions of bile acids (n = 30, p less than 0.001 for each) and in supplemented bovine biles (n = 30, p less than 0.001). N-acetylcysteine also significantly increased the frequency of complete gallstone dissolution in taurocholate-supplemented (66.6% vs. 40.0%) and ursodeoxycholate-supplemented (76.6% vs. 50.0%) bile. These results indicate that the mucolytic agent N-acetylcysteine significantly accelerates in vitro gallstone dissolution. We speculate that adjuvant therapy with an appropriate mucolytic agent may potentially increase the efficacy of clinical gallstone dissolution.