Self-assembly of aqueous bilirubin ditaurate, a natural conjugated bile pigment, to contraposing enantiomeric dimers and M(-) and P(+) tetramers and their selective hydrophilic disaggregation by monomers and micelles of bile salts.

The solution behavior of bilirubin ditaurate (BDT), the first naturally occurring conjugated bile pigment to be physically and chemically characterized, was assessed in aqueous solution and in monomeric and micellar solutions of common taurine-conjugated bile salts (BS). Analytical ultracentrifugation revealed that BDT self-associates in monomer-dimer equilibria between 1 and 500 µM, forming limiting tetramers at low millimolar concentrations. Self-association was enthalpically driven with ΔG values of ≈5 kcal/mol, suggesting strong hydrophobic interactions. Added NaCl and decreases in temperature shifted the oligomerization to lower BDT concentrations. On the basis of circular dichroism spectra and the limiting size of the self-aggregates, we infer that the tetramers are composed of 2P(+) and 2M(-) enantiomeric BDT pairs in "ridge-tile" conformations interacting in a "double-bookend" structure. With added monomeric BS, blue shifts in the UV-vis spectra and tight isosbestic points revealed that BDT/BS heterodimers form, followed by BDT "decorating" BS micelles mostly via hydrophilic interactions. Conformational enantiomerism, fluorescence intensities, and anisotropy, as well as resistance of the hybrid particles to disaggregation in 6 M urea, suggested that two or three hydrogen-bonding sites bound BDT monomers to the hydroxyl groups of BS, possibly via pyrrole-π-orbital-OH interactions. BDT stabilized these interactions by enveloping the BS in its "ridge-tile" pincers with variable strain that maximized van der Waals interactions. Possibly because the BDT molecule becomes highly strained with BS subtending a 7ß-hydroxyl group, BDT became totally resistant to oxidation in air. This work predicts that, because of BS dissolution of the BDT self-aggregates, BS/bilirubin hybrid particles, which are stabilized hydrophilically, are likely to be the dominant mode of transport for all conjugated bilirubins in bile.

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

Recently [Neubrand, M. W., et al. (2015) Biochemistry 54, 1542-1557], we determined a concentration-dependent monomer-dimer-tetramer equilibrium in aqueous bilirubin ditaurate (BDT) solutions and explored the nature of high-affinity binding of BDT monomers with monomers and micelles of the common taurine-conjugated bile salts (BS). We now investigate, employing complementary physicochemical methods, including fluorescence emission spectrophotometry and quasi-elastic light scattering spectroscopy, the influence of phosphatidylcholine (PC), the predominant phospholipid of bile and calcium, the major divalent biliary cation, on these self-interactions and heterointeractions. We have used short-chain, lyso and long-chain PC species as models and contrasted our results with those of parallel studies employing unconjugated bilirubin (UCB) as the fully charged dianion. Both bile pigments interacted with the zwitterionic headgroup of short-chain lecithins, forming water-soluble (BDT) and insoluble ion-pair complexes (UCB), respectively. Upon micelle formation, BDT monomers apparently remained at the headgroup mantle of short-chain PCs, but the ion pairs with UCB became internalized within the micelle's hydrophobic core. BDT interacted with the headgroups of unilamellar egg yolk (EY) PC vesicles; however, with the simultaneous addition of CaCl2, a reversible aggregation took place, but not vesicle fusion. With mixed EYPC/BS micelles, BDT became bound to the hydrophilic surface (as with simple BS micelles), and in turn, both BDT and BS bound calcium, but not other divalent cations. The calcium complexation of BDT and BS was enhanced strongly with increases in micellar EYPC, suggesting calcium-mediated cross-bridging of hydrophilic headgroups at the micelle's surface. Therefore, the physicochemical binding of BDT to BS in an artificial bile medium is influenced not only by BS species and concentration but also by long-chain PCs and calcium ions that exert a specific rather than a counterion effect. This work should serve as a physicochemical template for studies with other conjugated bilirubins, including bilirubin diglucuronoside (BDG), the principal bilirubin conjugate (cBR) in human bile.