Interactions of unconjugated bilirubin with vesicles, cyclodextrins and micelles: New modeling and the role of high pKa values

<p>Abstract</p> <p>Background</p> <p>Unconjugated bilirubin (UCB) is an unstable substance with very low aqueous solubility. Its aqueous pKa values affect many of its interactions, particularly their pH-dependence. A companion paper shows that only our prior solvent partition studies, leading to pKa values of 8.12 and 8.44, met all essential requirements for valid pKa determinations. Other published values, generally lower, some below 5.0, were shown to be invalid. The present work was designed to derive suitable models for interpreting published data on the pH-dependent binding of UCB with four agents, mentioned below, chosen because they are not, themselves, sensitive to changes in the pH range 4-10, and the data, mainly spectrometric, were of reasonable quality.</p> <p>Results</p> <p>These analyses indicated that the high pKa values, dianion dimerization constant and solubilities of UCB at various pH values, derived from our partition studies, along with literature-derived pH- and time-dependent supersaturation effects, were essential for constructing useful models that showed good qualitative, and sometimes quantitative, fits with the data. In contrast, published pKa values below 5.0 were highly incompatible with the data for all systems considered. The primary species of bound UCB in our models were: undissociated diacid for phosphatidylcholine, dianion for dodecyl maltoside micelles and cyclodextrins, and both monoanions and dianion for sodium taurocholate. The resulting binding versus pH profiles differed strikingly from each other.</p> <p>Conclusions</p> <p>The insights derived from these analyses should be helpful to explore and interpret UCB binding to more complex, pH-sensitive, physiological moieties, such as proteins or membranes, in order to understand its functions.</p

Review: Bilirubin pKa studies; new models and theories indicate high pKa values in water, dimethylformamide and DMSO

<p>Abstract</p> <p>Background</p> <p>Correct aqueous pKa values of unconjugated bilirubin (UCB), a poorly-soluble, unstable substance, are essential for understanding its functions. Our prior solvent partition studies, of unlabeled and [¹⁴C] UCB, indicated pKa values above 8.0. These high values were attributed to effects of internal H-bonding in UCB. Many earlier and subsequent studies have reported lower pKa values, some even below 5.0, which are often used to describe the behavior of UCB. We here review 18 published studies that assessed aqueous pKa values of UCB, critically evaluating their methodologies in relation to essential preconditions for valid pKa measurements (short-duration experiments with purified UCB below saturation and accounting for self-association of UCB).</p> <p>Results</p> <p>These re-assessments identified major deficiencies that invalidate the results of all but our partition studies. New theoretical modeling of UCB titrations shows remarkable, unexpected effects of self-association, yielding falsely low pKa estimates, and provides some rationalization of the titration anomalies. The titration behavior reported for a soluble thioether conjugate of UCB at high aqueous concentrations is shown to be highly anomalous. Theoretical re-interpretations of data in DMSO and dimethylformamide show that those indirectly-derived aqueous pKa values are unacceptable, and indicate new, high average pKa values for UCB in non-aqueous media (>11 in DMSO and, probably, >10 in dimethylformamide).</p> <p>Conclusions</p> <p>No reliable aqueous pKa values of UCB are available for comparison with our partition-derived results. A companion paper shows that only the high pKa values can explain the pH-dependence of UCB binding to phospholipids, cyclodextrins, and alkyl-glycoside and bile salt micelles.</p

Revalidation and rationale for high pKa values of unconjugated bilirubin

<p>Abstract</p> <p>Background</p> <p>Our prior solvent partition analysis, published in 1992, yielded pKa values for unconjugated bilirubin of about 8.1 and 8.4, but these results have been challenged and studies by other methods have suggested pKa values below 5.0.</p> <p>Methods</p> <p>We repeated our published solvent partition studies, using ¹⁴C-unconjugated bilirubin highly purified by extraction of residual labeled impurities from CHCl₃into an aqueous buffer, pH 7.0. Partition ratios at six pH values from 5.0 to 9.0 were determined by radioassay and compared with our prior values obtained by diazo assay.</p> <p>Results</p> <p>At pH values ranging from 4.8 to 9.2, stable aqueous/chloroform ¹⁴C-partition ratios did not differ significantly from our published partition ratios based on diazo assay.</p> <p>Conclusion</p> <p>These results support the high pKa values of unconjugated bilirubin, above 8.0, derived from our earlier solvent partition study. In both studies, our measurements were based on the rapid analysis of clearly under-saturated solutions of highly-purified bilirubin over a wide pH range, using properly purified and preserved solvents. No previous direct estimate of the aqueous pKa values of unconjugated bilirubin meets all these preconditions. Three theoretical factors acting in combination, each related to the unique, extensive internal H-bonding of the -COOH groups, are proposed to support high pKa values of unconjugated bilirubin in water: a) donation of an H-bond from the -OH moiety of the -COOH group, which is broken on ionization; b) hindered solvation of the -COO^-group after ionization; and c) restricted rotation of the -COO^-and -COOH groups. Our findings and rationale rebut methodological and theoretical criticisms leveled against our prior work. High pKa values for unconjugated bilirubin dictate that: a) bilirubin diacid, which readily diffuses across membranes and can cause neurotoxicity, is the dominant unbound bilirubin species of unconjugated bilirubin in plasma at physiological pH; b) at the near-neutral pH range of gallbladder bile, the monoanion is the major unconjugated bilirubin anion present, concordant with the finding that the calcium bilirubinate precipitated in gallstones is the monoanion salt. Our conclusions are thus relevant to understanding bilirubin-induced neurological disease in severely jaundiced neonates and the precipitation of calcium bilirubinate salts in gallstones.</p

Affinity of human serum albumin for bilirubin varies with albumin concentration and buffer composition: results of a novel ultrafiltration method.

Albumin binding is a crucial determinant of bilirubin clearance in health and bilirubin toxicity in certain disease states. However, prior attempts to measure the affinity of albumin for bilirubin have yielded highly variable results, reflecting both differing conditions and the confounding influence of impurities. We therefore have devised a method based on serial ultrafiltration that successively removes impurities in [(14)C]bilirubin until a stable binding affinity is achieved, and then we used it to assess the effect of albumin concentration and buffer composition on binding. The apparent binding affinity of human serum albumin for [(14)C]bilirubin was strongly dependent on assay conditions, falling from (5.09 +/- 0.24) x 10(7) liters/mol at lower albumin concentrations (15 microm) to (0.54 +/- 0.05) x 10(7) liters/mol at higher albumin concentrations (300 microm). To determine whether radioactive impurities were responsible for this change, we estimated impurities in the stock bilirubin using a novel modeling approach and found them to be 0.11-0.13%. Formation of new impurities during the study and their affinity for albumin were also estimated. After correction for impurities, the binding affinity remained heavily dependent on the albumin concentration (range (5.37 +/- 0.26) x 10(7) liters/mol to (0.65 +/- 0.03) x 10(7) liters/mol). Affinities decreased by about half in the presence of chloride (50 mm). Thus, the affinity of human albumin for bilirubin is not constant, but varies with both albumin concentration and buffer composition. Binding may be considerably less avid at physiological albumin concentrations than previously believed

Brown pigment gallstones: The role of bacterial hydrolases and another missed opportunity

The bile acids in brown pigment stones and gallbladder bile were fractionated into free acids, glycine and taurine conjugates, and sulfates, using diethylamino-hydroxypropyl-Sephadex LH-20 (DEAPLH-20) column chromatography, and were quantitated by gas chromatography. Twenty-eight cases of brown pigment stones were studied and divided into two groups: those with and those without bacteria possessing bile acid-deconjugating activity. In the former, free bile acid amounted to 62 ± 34% of the total bile acid, while in the latter, only 0.1% of total bile acid was free bile acid. The fraction of total bile acid made up of free bile acids was found to be consistently higher in brown pigment stones than in the corresponding bile, irrespective of the presence or absence of biliary infection. Free bile acid is present in negligible amounts in normal bile. Total bile acid concentration in the bile of patients with brown pigment stones was significantly less than that of controls (13 vs. 50 mg/ml). Biliary infection is almost always present in cases with brown pigment stones. These findings suggest that bacterial infection is present at the initiation of brown pigment stone formation as well as during the period of ensuing stone growth.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38351/1/1840130335_ftp.pd

Modulation of Mrp1 (ABCc1) and Pgp (ABCb1) by Bilirubin at the Blood-CSF and Blood-Brain Barriers in the Gunn Rat

Accumulation of unconjugated bilirubin (UCB) in the brain causes bilirubin encephalopathy. Pgp (ABCb1) and Mrp1 (ABCc1), highly expressed in the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) respectively, may modulate the accumulation of UCB in brain. We examined the effect of prolonged exposure to elevated concentrations of UCB on expression of the two transporters in homozygous, jaundiced (jj) Gunn rats compared to heterozygous, not jaundiced (Jj) littermates at different developmental stages (2, 9, 17 and 60 days after birth). BBB Pgp protein expression was low in both jj and Jj pups at 9 days (about 16–27% of adult values), despite the up-regulation in jj animals (2 and 1.3 fold higher than age matched Jj animals at P9 and P17–P60, respectively); Mrp1 protein expression was barely detectable. Conversely, at the BCSFB Mrp1 protein expression was rather high (60–70% of the adult values) in both jj and Jj at P2, but was markedly (50%) down-regulated in jj pups starting at P9, particularly in the 4th ventricle choroid plexuses: Pgp was almost undetectable. The Mrp1 protein down regulation was accompanied by a modest up-regulation of mRNA, suggesting a translational rather than a transcriptional inhibition. In vitro exposure of choroid plexus epithelial cells obtained from normal rats to UCB, also resulted in a down-regulation of Mrp1 protein. These data suggest that down-regulation of Mrp1 protein at the BSCFB, resulting from a direct effect of UCB on epithelial cells, may impact the Mrp1-mediated neuroprotective functions of the blood-cerebrospinal fluid barrier and actually potentiate UCB neurotoxicity

Effect of bilirubin on cytochrome c oxidase activity of mitochondria from mouse brain and liver

<p>Abstract</p> <p>Background</p> <p>The unbound, free concentration (B_f) of unconjugated bilirubin (UCB), and not the total UCB level, has been shown to correlate with bilirubin cytotoxicity, but the key molecular mechanisms accounting for the toxic effects of UCB are largely unknown.</p> <p>Findings</p> <p>Mouse liver mitochondria increase unbound UCB oxidation, consequently increasing the apparent rate constant for unbound UCB oxidation by HRP (Kp), higher than in control and mouse brain mitochondria, emphasizing the importance of determining Kp in complete systems containing the organelles being studied. The <it>in vitro </it>effects of UCB on cytochrome <it>c </it>oxidase activity in mitochondria isolated from mouse brain and liver were studied at B_franging from 22 to 150 nM. The results show that UCB at B_fup to 60 nM did not alter mitochondrial cytochrome <it>c </it>oxidase activity, while the higher concentrations significantly inhibited the enzyme activity by 20% in both liver and brain mitochondria.</p> <p>Conclusions</p> <p>We conclude that it is essential to include the organelles being studied in the medium used in measuring both Kp and B_f. A moderately elevated, pathophysiologically-relevant B_fimpaired the cytochrome <it>c </it>oxidase activity modestly in mitochondria from mouse brain and liver.</p