title: Photo-isomerization and oxidation of bilirubin in mammals is dependent on albumin binding.
abstract: The bilirubin (BR) photo-conversion in the human body is a protein-dependent process; an effective photo-isomerization of the potentially neurotoxic Z,Z-BR as well as its oxidation to biliverdin in the antioxidant redox cycle is possible only when BR is bound on serum albumin. We present a novel analytical concept in the study of linear tetrapyrroles metabolic processes based on an in-depth mapping of binding sites in the structure of human serum albumin (HSA). A combination of fluorescence spectroscopy, circular dichroism (CD) spectroscopy, and molecular modeling methods was used for recognition of the binding site for BR, its derivatives (mesobilirubin and bilirubin ditaurate), and the products of the photo-isomerization and oxidation (lumirubin, biliverdin, and xanthobilirubic acid) on HSA. The CD spectra and fluorescent quenching of the Trp-HSA were used to calculate the binding constants. The results of the CD displacement experiments performed with hemin were interpreted together with the findings of molecular docking performed on the pigment-HSA complexes. We estimated that Z,Z-BR and its metabolic products bind on two independent binding sites. Our findings support the existence of a reversible antioxidant redox cycle for BR and explain an additional pathway of the photo-isomerization process (increase of HSA binding capacity; the excess free [unbound] BR can be converted and also bound to HSA).
authors: Department of Analytical Chemistry, University of Chemistry and Technology, Prague, Czech Republic. Electronic address: gonchari@vscht.cz.
authors: Iryna
authors: I
authors: Goncharova
authors: Institute of Medical Biochemistry and Laboratory Diagnostics, First Faculty of Medicine, Charles University in Prague, Czech Republic.
authors: Jana
authors: J
authors: Jašprová
authors: Institute of Medical Biochemistry and Laboratory Diagnostics, First Faculty of Medicine, Charles University in Prague, Czech Republic; Fourth Department of Internal Medicine, First Faculty of Medicine, Charles University in Prague, Czech Republic.
authors: Libor
authors: L
authors: Vítek
authors: Department of Physics and Measurements, University of Chemistry and Technology, Prague, Czech Republic.
authors: Marie
authors: M
authors: Urbanová
mesh_terms: D000818:Animals; D001663:Bilirubin / Q000031:analogs & derivatives / Q000737:chemistry* / Q000378:metabolism; D001664:Biliverdine / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D002942:Circular Dichroism; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D062105:Molecular Docking Simulation; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D012709:Serum Albumin / Q000737:chemistry* / Q000378:metabolism; D000075462:Serum Albumin, Human; D013050:Spectrometry, Fluorescence; D013237:Stereoisomerism; D013654:Taurine / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D014364:Tryptophan / Q000737:chemistry
chemical_list: C422455:ALB protein, human; D008024:Ligands; D012709:Serum Albumin; C013337:xanthobilirubic acid; D013654:Taurine; C045013:lumirubin; C054146:bilirubin ditaurine; D014364:Tryptophan; D001664:Biliverdine; D001663:Bilirubin; D000075462:Serum Albumin, Human
keywords: Antioxidant; Bilirubin conversion; Bilirubin–biliverdin reversible antioxidant redox cycle; Circular dichroism; Molecular docking; Photo-isomerization