title: Metastable and equilibrium phase diagrams of unconjugated bilirubin IXα as functions of pH in model bile systems: Implications for pigment gallstone formation.
abstract: Metastable and equilibrium phase diagrams for unconjugated bilirubin IXα (UCB) in bile are yet to be determined for understanding the physical chemistry of pigment gallstone formation. Also, UCB is a molecule of considerable biomedical importance because it is a potent antioxidant and an inhibitor of atherogenesis. We employed principally a titrimetric approach to obtain metastable and equilibrium UCB solubilities in model bile systems composed of taurine-conjugated bile salts, egg yolk lecithin (mixed long-chain phosphatidylcholines), and cholesterol as functions of total lipid concentration, biliary pH values, and CaCl2 plus NaCl concentrations. Metastable and equilibrium precipitation pH values were obtained, and average pKa values of the two carboxyl groups of UCB were calculated. Added lecithin and increased temperature decreased UCB solubility markedly, whereas increases in bile salt concentrations and molar levels of urea augmented solubility. A wide range of NaCl and cholesterol concentrations resulted in no specific effects, whereas added CaCl2 produced large decreases in UCB solubilities at alkaline pH values only. UV-visible absorption spectra were consistent with both hydrophobic and hydrophilic interactions between UCB and bile salts that were strongly influenced by pH. Reliable literature values for UCB compositions of native gallbladder biles revealed that biles from hemolytic mice and humans with black pigment gallstones are markedly supersaturated with UCB and exhibit more acidic pH values, whereas biles from nonstone control animals and patients with cholesterol gallstone are unsaturated with UCB.
authors.affiliation: Department of Medicine, Harvard Medical School, Boston, Massachusetts; Division of Gastroenterology, Brigham and Women's Hospital, Boston, Massachusetts; Harvard Digestive Diseases Center, Boston, Massachusetts.
authors.forename: Marvin D
authors.initials: MD
authors.lastname: Berman
authors.affiliation: Department of Medicine, Harvard Medical School, Boston, Massachusetts; Division of Gastroenterology, Brigham and Women's Hospital, Boston, Massachusetts; Harvard Digestive Diseases Center, Boston, Massachusetts mcarey@partners.org.
authors.forename: Martin C
authors.initials: MC
authors.lastname: Carey
mesh_terms: D000818:Animals; D001646:Bile / Q000737:chemistry* / Q000378:metabolism; D001663:Bilirubin / Q000737:chemistry* / Q000378:metabolism; D002122:Calcium Chloride / Q000737:chemistry; D002784:Cholesterol / Q000378:metabolism; D003414:Crigler-Najjar Syndrome / Q000235:genetics / Q000378:metabolism; D004195:Disease Models, Animal; D042882:Gallstones / Q000737:chemistry* / Q000378:metabolism; D006461:Hemolysis; D006801:Humans; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D054709:Lecithins / Q000737:chemistry; D051379:Mice; D008823:Micelles; D008956:Models, Chemical; D011911:Rats, Gunn; D017207:Rats, Sprague-Dawley; D012965:Sodium Chloride / Q000737:chemistry; D012995:Solubility; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D014508:Urea / Q000737:chemistry
chemical_list: D054709:Lecithins; D008823:Micelles; D012965:Sodium Chloride; D014508:Urea; D002784:Cholesterol; D002122:Calcium Chloride; D001663:Bilirubin
keywords: cholesterol; egg yolk lecithin; lecithin; mean pK′a values; metastable and equilibrium solubilities; native biles; pathophysiological correlations with unconjugated bilirubin; taurine-conjugated bile salts