Free radical reactions of methionine in peptides: mechanisms relevant to beta-amyloid oxidation and Alzheimer's disease.
The pathogenesis of Alzheimer's disease is strongly associated with the formation and deposition of beta-amyloid peptide (beta AP) in the brain. This peptide contains a methionine (Met) residue in the C-terminal domain, which is important for its neurotoxicity and its propensity to reduce transition metals and to form reactive oxygen species. Theoretical studies have proposed the formation of beta AP Met radical cations as intermediates, but no experimental evidence with regard to formation and reactivity of these species in beta AP is available, largely due to the insolubility of the peptide. To define the potential reactions of Met radical cations in beta AP, we have performed time-resolved UV spectroscopic and conductivity studies with small model peptides, which show for the first time that (i) Met radical cations in peptides can be stabilized through bond formation with either the oxygen or the nitrogen atoms of adjacent peptide bonds; (ii) the formation of sulfur-oxygen bonds is kinetically preferred, but on longer time scales, sulfur-oxygen bonds convert into sulfur-nitrogen bonds in a pH-dependent manner; and (iii) ultimately, sulfur-nitrogen bonded radicals may transform intramolecularly into carbon-centered radicals located on the (alpha)C moiety of the peptide backbone.
Schöneich
Christian
C
Department of Pharmaceutical Chemistry, University of Kansas, 2095 Constant Avenue, Lawrence, Kansas 66047, USA. schoneic@ukans.edu
Pogocki
Dariusz
D
Hug
Gordon L
GL
Bobrowski
Krzysztof
K
D000544:Alzheimer Disease / Q000378:metabolism*; D000577:Amides / Q000737:chemistry; D016229:Amyloid beta-Peptides / Q000737:chemistry* / Q000378:metabolism; D004563:Electrochemistry; D005609:Free Radicals / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008715:Methionine / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
D000577:Amides; D016229:Amyloid beta-Peptides; D005609:Free Radicals; D010446:Peptide Fragments; C006379:N-acetylmethionine; D008715:Methionine