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.

Physico-chemical characterization of bilirubin-10-sulfonate and comparison of its acid–base behavior with unconjugated bilirubin

“…Interestingly, the p K a * value of UCB did substantially differ from that of bilirubin-10-sulfonate, and it was in the alkaline region, supporting previous data by some other authors 4 , 5 . Our data are also consistent with the recent observation of Berman and Carey, who described similar pH relationships of UCB in model bile systems, with implications for pigment gallstone formation 28 . In addition, similar conclusions were made by Mukerjee and Ostrow in their comprehensive paper reviewing available data on p K a values of bilirubin in non-aqueous media 17 .…”

Influence of Phosphatidylcholine and Calcium on Self-Association and Bile Salt Mixed Micellar Binding of the Natural Bile Pigment, Bilirubin Ditaurate

“…47,48 In studies 39 of the NaTDC/ EYPC systems (3 g/dL, 0.15 M NaCl, 23 °C, pH 7.0) with 200 μM BDT, extending to equimolar NATDC:EYPC ratios, i.e., close to the mixed micellar phase limit, 46 the hypsochromic λ max at 417 nm is more pronounced but no shift occurred with increasing EYPC:NaTDC ratios. Hence, with increasing EYPC:BS ratios in mixed micelles with NaTDC 38,39 or with NaTC as in the work presented here (Figure 9a), there is a larger relative distribution of BDT onto polar domains of the mixed micelles as indicated by a higher ratio of bound BDT (A 419 ) to unbound BDT (A 450 ), principally as aqueous monomers.…”

“…Therefore, these results taken together suggest that the strong affinity of membrane lipids, principally long-chain PCs, 43 for binding ionized UCB molecules is driven principally by interactions between membrane lipid headgroups and the anionic groups of UCB. In striking contrast, when UCB is essentially undissociated at pH 7.4, 36,38,44 the nonionic species is dissolved hydrophobically within the bilayers of long-chain PCs. 45 Interactions of BDT with BS (NaTC)/EYPC micelles (Figure 9a) were dominated by partitioning of BDT monomers onto a hydrophilic environment of the mixed micelles.…”

Anti-inflammatory and hepatoprotective effects of polyherbal composition in patients with chronic cholecystitis

“…Increased ALT and AST levels are also indicators of cholestasis [13], which along with elevated cholesterol serum level promote cholelithiasis and gallbladder inflammation. Additional risk factor of cholelithiasis is elevated serum level of unconjugated bilirubin in all study participants, since presence of unconjugated bilirubin plays an important part in the formation of cholesterol gallstones [14]. Treatment with polyherbal tea resulted in the normalization of mentioned biochemi cal constants in all patients with chronic…”