Bilirubin Chemistry, Ionization and Solubilization by Bile Salts

Abstract

Bilirubin is a linear tetrapyrrole whose conformation is affected by internal hydrogen bonds formed between the carboxyl side chains and dipyrromethenone rings. Structural variations include: constitutional isomerism of the vinyl or carboxyethyl side chains, geometric isomerism of the methene bridges, tautomerism of the lactam groups, conformational rotations about the central methylene bridge and ionization of one or both carboxyl groups. Aggregation of the dianion into dimers and multimers may occur. The pK,' values of the two carboxyl groups are affected greatly by the environment and may differ widely in micellar solutions like bile. Solubility of bilirubin in water is less than 1 nM at pH = 7 and about 0.1 at pH = 8. Nonetheless, it dissolves poorly in most lipid solvents, except for asymmetrical chloroalkanes. Hydrogen bond-breaking solvents, especially dimethyl sulfoxide, are most effective in solubilizing bilirubin. In bile salt solutions, solubility of bilirubin is well above the concentrations of unconjugated bilirubin found in normal human gallbladder bile, and is impaired by lecithin but unaffected by cholesterol. At physiological pH in bile salt solutions, bilirubin is predominantly in its monoanion form that binds readily to the micelles. In such solutions, addition of physiological concentrations of calcium precipitates calcium bilirubinate, leaving residual bilirubin concentrations of up to 15 rc.M in 50 mM taurocholate or close to the maximum bilirubin concentrations in normal bile. Studies in which disodium bilirubinate is dissolved in bile salt solutions and pH is adjusted to the physiological range reveal that metastable supersaturation with bilirubin may occur and that a mesophase may also form in the presence of lecithin, akin to that seen with cholesterol. These results form a basis for understanding the precipitation of calcium bilirubinate from bile during pigment gallstone formation.Bilirubin is a linear tetrapyrrole of the biladiene-a,c type, consisting of two asymmetrical dipyrromethenones connected by a methylene bridge (1) (Figure 1). In the natural 1x0-Z,Z isomer that predominates in uiuo, each dipyrrolic half-molecule carries a carboxyethyl side chain that might be expected to render the pigment a highly polar, ionized and water-soluble molecule at physiological pH values. However, in its fully protonated (diacid) form, each of the two carboxyl groups forms internal hydrogen bonds with the -C=O and two >NH in the dipyrromethenone rings in the opposite half of the molecule (2). Collectively, these two trios of hydrogen bonds stabilize the bilirubin molecule in a rigid conformation that resembles a ridge tile or partially folded book (2), in

Coadministration of Atazanavir-Ritonavir and Zinc Sulfate: Impact on Hyperbilirubinemia and Pharmacokinetics

“…In our study, 14-day intake of ZnSO 4 has shown to decrease unconjugated bilirubin exposure measured over 24 h by 23%. This is probably a consequence of ZnSO 4 chelating unconjugated bilirubin after biliary elimination (15).…”

New pathophysiological concepts underlying pathogenesis of pigment gallstones

“…However, at pH values of human hepatic (7.3–8.7) and gallbladder (6.2–7.8) biles [2], UCB is a monoacidic species (HUCB − ) principally because of the presence of high concentrations of bile salts. Nevertheless, it is exquisitely sensitive to precipitation as the monoacid salt with typical biliary concentrations of ionized Ca [2,38]. In contrast, mono- and bisglucuronosyl bilirubins, the predominant conjugated bile pigment species in human and most laboratory animal biles, bind ionized calcium as soluble salts and are not precipitated with physiological calcium concentrations [39].…”

Can pigment gallstones be induced by biliary stricture and prevented by medicine in Guinea pigs

“…Although content of TBA in S+M group was not higher than that in S group (P > 0.05, Table 4), the content of Ca ++ in S+M group was significantly less than in S group (P < 0.01, Table 4), and concentrations of TCa in both S+M and S groups were almost the same (P > 0.05, Table 4). It was indicated that Ca ++ buffer capacity of bile in S+M group was enhanced by chenodeoxycholic acid administration [8][9][10] . GP content in the bile of S+M group was slightly less than that of S group, but had no significance (P > 0.05, Table 4).…”

“…They contained chenodeoxycholic acid, glucurolactone, and aspirin. Chenodeoxycholic acid was reported to reduce pigment gallstone formation in hamster by preventing precipitation of calcium bilirubinate [8][9][10] . Glucurolactone could inhibit β-G activity, and decrease the precipitation of Ca ++ and bilirubin [6,7] .…”