Dansylation of human serum albumin in the study of the primary binding sites of bilirubin and L-tryptophan.
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ABSTRACT:
SUBMITTER: Jacobsen C
PROVIDER: S-EPMC1161149 | biostudies-other | 1979 Jul
REPOSITORIES: biostudies-other
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Project description:A one-step procedure has been developed for the preparation of [3H]bilirubin IX-alpha in good yield from unlabelled bilirubin. Irradiation of an aqueous solution of [3H]bilirubin IX-alpha in the presence of human serum albumin results in the covalent attachment of the bilirubin to the protein. Preliminary degradation studies have been carried out to locate the site of attachment of the bilirubin to the albumin.
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Project description:Binding of L-tryptophan, diazepam, salicylate and Phenol Red to defatted human serum albumin was studied by ultrafiltration at pH 7.0. All ligands bind to one high-affinity binding site with association constants of the order of 10(4)-10(5)M-1. The number of secondary binding sites was found to vary from zero to five, with association constants about 10(3)M-1. Competitive binding studies with different pairs of the ligands were performed. Binding of both ligands was determined simultaneously. L-Tryptophan and diazepam were found to compete for a common high-affinity binding site on albumin. The following combinations of ligands do not bind competitively to albumin: L-tryptophan-Phenol Red, L-tryptophan-salicylate and Phenol Red-salicylate. On the other hand, high-affinity bindings of the three ligands do not take place independently but in such a way that binding of one of the ligands results in a decrease in binding of the other ligands. The decreases in binding are reciprocal and can be accounted for by introducing a coupling constant. The magnitude of the constant is dependent on the ligands being bound. In the present study, the mutual decrease in binding was more pronounced with L-tryptophan-salicylate and Phenol Red-salicylate than with L-tryptophan-Phenol Red.
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Project description:Bilirubin, an insoluble yellow-orange pigment derived from heme catabolism, accumulates to toxic levels in individuals with impaired or immature liver function. The resulting jaundice may be managed with phototherapy to isomerize the biosynthetic 4Z,15Z-bilirubin-IXalpha to more soluble and excretable isomers, such as 4Z,15E-bilirubin. Bilirubin and its configurational isomers are transported to the liver by human serum albumin (HSA) but their precise binding location(s) on the protein have yet to be determined. To investigate the molecular details of their interaction, we co-crystallised bilirubin with HSA. Strikingly, the crystal structure--determined to 2.42 A resolution--revealed the 4Z,15E-bilirubin-IXalpha isomer bound to an L-shaped pocket in sub-domain IB. We also determined the co-crystal structure of HSA complexed with fusidic acid, an antibiotic that competitively displaces bilirubin from the protein, and showed that it binds to the same pocket. These results provide the first crystal structure of a natural bilirubin pigment bound to serum albumin, challenge some of the present conceptions about HSA-bilirubin interactions, and provide a sound structural framework for finally resolving the long-standing question of where 4Z,15Z-bilirubin-IXalpha binds to the protein.
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Project description:Human serum albumin (HSA) is the main plasma protein responsible for a distribution of drugs in the human circulatory system. The binding to HSA is one of the factors that determines both the pharmacological actions and the side effects of drugs. The derivative of heme, protoporphyrin IX (PpIX), is a hydrophobic photosensitizer widely used in photodynamic diagnosis and therapy of various malignant disorders. Using absorption and fluorescence spectroscopy, it has been demonstrated that PpIX forms complexes with HSA. Its binding sites in the tertiary structure of HSA were found in the subdomains IB and IIA. PpIX binds to HSA in one class of binding sites with the association constant of 1.68 × 10? M-1 and 2.30 × 10? M-1 for an excitation at wavelength ?ex = 280 nm and 295 nm, respectively. The binding interactions between HSA and PpIX have been studied by means of molecular docking simulation using the CLC Drug Discovery Workbench (CLC DDWB) computer program. PpIX creates a strong 'sandwich-type' complex between its highly conjugated porphine system and aromatic side chains of tryptophan and tyrosine. In summary, fluorescent studies on binding interactions between HSA and PpIX have been confirmed by the results of computer simulation.
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Project description:Bilirubin can be coupled covalently to albumin by using water-soluble carbodi-imide as coupling reagent. The optimal specificity in the attachment of bilirubin to the high-affinity site on the albumin molecule was obtained by treating an albumin-bilirubin complex with carbodi-imide in low concentrations and for a short period. The product was reduced, carboxymethylated and digested with trypsin. By fractionation on Sephadex G-50 (superfine grade) a peptide fraction containing most of the bilirubin label was isolated. Further purification by paper chromatography gave one peptide, consisting of residues 240-258. The peptide containined a single lysine residue, 240, and had an intact disulphide bridge. The results indicate that bilirubin is bound to lysine residue 240 at its high-affinity site on human serum albumin.
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Project description:Human serum albumin (HSA) is the most abundant plasma protein, which functions to transport a large range of ligands within the circulation. These interactions have important implications for human health and disease. The primary binding site for CuII ions on HSA is known to be the so-called amino-terminal CuII and NiII binding (ATCUN) motif. However, the number and identity of secondary binding sites is currently not understood. In this study, we harnessed a suite of contemporary electron paramagnetic resonance (EPR) spectroscopy methods to investigate recombinantly produced constructs of HSA bearing single-histidine knockouts, with the aim to characterise its endogenous CuII ion binding sites.
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Project description:Binding of warfarin, digitoxin, diazepam, salicylate and Phenol Red, individually or in different pair combinations, to defatted human serum albumin at ligand/protein molar ratios less than 1:1 was studied at pH 7.0. The binding was determined by ultrafiltration. Some of the experiments were repeated with the use of equilibrium dialysis in order to strengthen the results. Irrespective of the method used, all ligands bind to one high-affinity binding site with an association constant in the range 10(4)-10(6) M-1. High-affinity binding of the following pair of ligands took place independently: warfarin-Phenol Red, warfarin-diazepam, warfarin-digitoxin and digitoxin-diazepam. Simultaneous binding of warfarin and salicylate led to a mutual decrease in binding of one another, as did simultaneous binding of digitoxin and Phenol Red. Both effects could be accounted for by a coupling constant. The coupling constant is the factor by which the primary association constants are affected; in these examples of anti-co-operativity the factor has a value between 0 and 1. In the first example it was calculated to be 0.8 and in the latter 0.5. Finally, digitoxin and salicylate were found to compete for a common high-affinity binding site. The present findings support the proposal of four separate primary binding sites for warfarin, digitoxin (and salicylate), diazepam and Phenol Red. An attempt to correlate this partial binding model for serum albumin with other models in the literature is made.
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Project description:Propofol and halothane are clinically used general anaesthetics, which are transported primarily by HSA (human serum albumin) in the blood. Binding characteristics are therefore of interest for both the pharmacokinetics and pharmacodynamics of these drugs. We characterized anaesthetic-HSA interactions in solution using elution chromatography, ITC (isothermal titration calorimetry), hydrogen-exchange experiments and geometric analyses of high-resolution structures. Binding affinity of propofol to HSA was determined to have a K(d) of 65 microM and a stoichiometry of approx. 2, whereas the binding of halothane to HSA showed a K(d) of 1.6 mM and a stoichiometry of approx. 7. Anaesthetic-HSA interactions are exothermic, with propofol having a larger negative enthalpy change relative to halothane. Hydrogen-exchange studies in isolated recombinant domains of HSA showed that propofol-binding sites are primarily found in domain III, whereas halothane sites are more widely distributed. Both location and stoichiometry from these solution studies agree with data derived from X-ray crystal-structure studies, and further analyses of the architecture of sites from these structures suggested that greater hydrophobic contacts, van der Waals interactions and hydrogen-bond formation account for the stronger binding of propofol as compared with the less potent anaesthetic, halothane.
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