title: Characterization of the spherical intermediates and fibril formation of hCT in HEPES solution using solid-state 13C-NMR and transmission electron microscopy.
abstract: Human calcitonin (hCT) is a 32-amino acid peptide hormone that contains an intrachain disulfide bridge between Cys1 and Cys7 and a proline amide at the C-terminus. hCT tends to associate to form a fibril precipitate of the same type as amyloid fibrils, and hence has been studied as a model of amyloid fibril formation. The fibrillation process in N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (HEPES) solution was examined using transmission electron microscopy. The rate of hCT fibrillation in HEPES solution was much lower than in phosphate buffer and acetic acid solution. Spherical intermediate aggregates (nuclei) were observed during the early stage of fibril formation. Short proto-fibrils appeared on the surface of the spherical intermediates. Subsequently, the spherical intermediates transformed directly into long proto-fibrils, which then elongated into mature hCT fibrils. The fibrillation process was also examined using solid-state (13)C-NMR spectroscopy, which indicated that the fibril structure was a β-sheet in the central region and a mixture of random coils and β-sheets at the C-terminus. The kinetics of fibril formation was examined in terms of a two-step autocatalytic reaction mechanism. The first-step nucleation rate (k1) was lower in HEPES solution than in phosphate buffer and acetic acid solution because the half-life of the intermediates is significantly longer in HEPES solution. In contrast, the second-step fibril elongation rate (k2) was similar in HEPES solution and acidic solutions. Specific interaction of HEPES molecules with hCT may stabilize the spherical intermediates and consequently inhibit the fibril elongation process of hCT.
authors.affiliation: Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan. naito@ynu.ac.jp.
authors.forename: Hikari
authors.initials: H
authors.lastname: Itoh-Watanabe
authors.forename: Miya
authors.initials: M
authors.lastname: Kamihira-Ishijima
authors.forename: Izuru
authors.initials: I
authors.lastname: Kawamura
authors.forename: Masashi
authors.initials: M
authors.lastname: Kondoh
authors.forename: Masamichi
authors.initials: M
authors.lastname: Nakakoshi
authors.forename: Michio
authors.initials: M
authors.lastname: Sato
authors.forename: Akira
authors.initials: A
authors.lastname: Naito
mesh_terms: D000595:Amino Acid Sequence; D002116:Calcitonin / Q000737:chemistry*; D057846:Cell-Penetrating Peptides; D006531:HEPES; D006801:Humans; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D046529:Microscopy, Electron, Transmission; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010446:Peptide Fragments / Q000737:chemistry*; D017433:Protein Structure, Secondary; D012996:Solutions
chemical_list: D057846:Cell-Penetrating Peptides; D010446:Peptide Fragments; D012996:Solutions; C497136:calcitonin (9-32), human; D002116:Calcitonin; D006531:HEPES