Hemin and bile pigments are the secondary structure regulators of intrinsically disordered antimicrobial peptides

The interaction of protoporphyrin compounds of human origin with the major bee venom component melittin (26 a.a., Z +6) and its hybrid derivative (CM15, 15 a.a., Z +6) were studied by a combination of various spectroscopic methods. Throughout a two-state, concentration-dependent process, hemin and its metabolites (biliverdin, bilirubin, bilirubin ditaurate) increase the parallel β-sheet content of the natively unfolded melittin, suggesting the oligomerization of the peptide chains. In contrast, α-helix promoting effect was observed with the also disordered but more cationic CM15. According to fluorescence quenching experiments, the sole Trp residue of melittin is the key player during the binding, in the vicinity of which the first pigment molecule is accommodated presumably making indole-porphyrin π-π stacking interaction. As circular dichroism titration data suggest, cooperative association of additional ligands subsequently occurs, resulting in multimeric complexes with an apparent dissociation constant ranged from 20 to 65 μM. Spectroscopic measurements conducted with the bilirubin catabolite urobilin and stercobilin refer to the requirement of intact dipyrrinone moieties for inducing secondary structure transformations. The binding topography of porphyrin rings on a model parallel β-sheet motif was evaluated by absorption spectroscopy and computational modeling showing a slipped-cofacial binding mode responsible for the red shift and hypochromism of the Soret band. Our results may aid to recognize porphyrin-responsive binding motifs of biologically relevant, intrinsically disordered peptides and proteins, where transient conformations play a vital role in their functions.

Thymoquinone, an active constituent of Nigella sativa seeds, binds with bilirubin and protects mice from hyperbilirubinemia and cyclophosphamide-induced hepatotoxicity

Some reports indicate that thymoquinone (TQ), the main constituent of Nigella sativa seeds, is hepatoprotective. The aim of this study was to determine whether TQ is able to bind directly to bilirubin, and whether TQ or liposomal formulation of TQ (Lip-TQ) can reduce cyclophosphamide (CYP)-induced liver toxicity, serum bilirubin level in mice. The binding of TQ with bilirubin was studied by UV-VIS, fluorescence and Near-UV CD spectroscopy. Inhibition of binding of bilirubin to erythrocytes by TQ was also examined. To increase the in vivo efficacy, Lip-TQ was prepared and used against CYP-induced toxicity. The protective role of TQ or Lip-TQ against CYP-induced toxicity was assessed by determining the liver function parameters, the levels of superoxide dismutase (SOD) and catalase (CAT), and histological studies. It was found that TQ binds to bilirubin and significantly inhibits the binding of bilirubin to erythrocytes. Lip-TQ (10 mg/kg) significantly reduced the levels of aspartate transaminase (AST) from 254 ± 48 to 66 ± 18 IU/L (P < 0.001), alanine transaminase (ALT) from 142 ± 28 to 47.8 ± 16 IU/L (P < 0.05) and serum bilirubin from 2.8 ± 0.50 to 1.24 ± 0.30 mg/dl (P < 0.05). Treatment with Lip-TQ reduced the CYP-induced inflammation and hemorrhage in liver tissues. Moreover, treatment with free or Lip-TQ protected the activity of SOD and CAT in CYP-injected mice. Therefore, TQ can reduce the level of bilirubin in systemic circulation in disease conditions that lead to hyperbilirubinemia and liver toxicity and hence may be used as a supplement in the treatment of liver ailments.

On the interaction of polypeptides with bile salts or bilirubin-IX alpha

Aqueous solutions formed by polypeptides, simple models of proteins, and bile salts (sodium cholate and deoxycholate, NaC and NaDC, respectively) or bilirubin-IX alpha (BR) have been studied by CD measurements. They could mimic more complicated biliary systems, thus supplying a possible interpretation of the behavior of some amino acid residues in the biliary proteins. The aggregation of NaDC and NaC in water can be monitored by CD measurements. Bile salts, in submicellar and micellar form, stabilize poly(L-Lys) (PLL) in alpha-helical conformation. The alpha-helix content increases with increasing bile salt concentration and ionic strength. NaDC seems to be a slightly better stabilizing agent of the alpha-helix conformation than NaC. Models characterized by hydrogen bonds between bile salts and PLL are proposed, also resorting to previous data available on the systems formed by NaDC and poly(L-Leu-L-Leu-L-Lys) (PLLL) or poly(L-Leu-L-Leu-L-Asp) (PLLA). Binding of BR to PLL, poly(D-Lys), poly(L-Glu), PLLL, and PLLA in water has been investigated by CD spectra in order to clarify the nature of the association complexes and the mechanism of the BR enantioselective complexation. Potential energy calculations provide binding models capable of explaining the enantioselective ability of the PLL and PLLL alpha-helices toward the left- and right-handed enantiomer of BR, respectively. BR is bound to -NH2 groups of PLL and PLLL lying on a right- and left-handed spiral, respectively. These results, together with those formerly obtained for some bile salts-BR systems, indicate that the selectivity originates from a binding that involves large regions of the BR molecule and gives rise, very probably, to moderate conformational changes from the "ridge tile" structure observed in the crystals. In some cases van der Waals forces can play a crucial role in the chiral recognition of bilirubin. Moreover, possible interaction models of BR with human serum albumin are proposed on the basis of a recent x-ray crystal structure of the protein.

The induced circular dichroism of bilirubin complexed with the alpha-helix form of poly(L-lysine)

Binding of bilirubin by the alpha-helix conformation of poly(L-lysine) in water induces optical activity. The bisignate circular dichroism spectrum exhibits exciton bands centred at 444 nm, negative, and at 525 nm, positive. The magnitude of the induced circular dichroism depends on the concentration of total bilirubin and total lysine residues, the molar ratio of total lysine residues-to-total bilirubin molecules, the pH and the degree of polymerization of poly(L-lysine). Although bilirubin binds to the random coil conformation of poly(L-lysine), as evidence by the absorption spectrum, the complex is optically inactive. The results suggest that bilirubin binds to the poly(L-lysine) in the form of dimers and oligomers.

Circular dichroism of bilirubin-amine association complexes: insights into bilirubin-albumin binding

Bichromophoric (4Z, 15Z)-bilirubin-IX alpha, the yellow-orange cytotoxic pigment of jaundice, adopts either of two intramolecularly hydrogen-bonded enantiomeric conformations that are in dynamic equilibrium in solution. The addition of optically active amines induces the pigment solutions to exhibit intense bisignate circular dichroism in the region of the bilirubin long wavelength uv-visible absorption band. The most intense circular dichroism Cotton effects, (delta epsilon) approximately equal to 130, are induced by beta-arylamines and are comparable to those exhibited by bilirubin complexes with serum albumin and other proteins. Like serum albumin and other proteins, the optically active base acts as a chiral complexation agent to induce an asymmetric transformation of bilirubin, whose induced bisignate circular dichroism Cotton effect is characteristic of exciton splitting of the component pyrromethenone chromophores. The amines thus serve as chiral templates for molecular recognition, and the complementary action of the amine complexation sites provides insight into the binding forces important in protein-bilirubin heteroassociation.