title: N-terminal α-ketoamide peptides: formation and transamination.
abstract: We have previously reported that N-terminal α-ketoamide peptides can be formed through 4-oxo-2(E)-nonenal (ONE)-derived oxidative decarboxylation of aspartic acid (Asp), which converts angiotensin (Ang) II (DRVYIHPF) to pyruvamide-Ang II (Ang P, CH3COCONH-RVYIHPF). The pyruvamide group significantly inhibits Ang P binding to the Ang II type 1 receptor, which mediates the major biological effects of Ang II. In the present study, we found that ONE can also introduce an α-ketoamide moiety at the N-terminus of peptides containing N-terminal residues other than Asp. Subsequent investigation of alternative biosynthetic pathways for N-terminal α-ketoamide peptides revealed that hydroxyl radical-mediated formation is a much more efficient route. The proposed mechanism involves initial abstraction of the N-terminal α-hydrogen and hydrolysis of the ketimine intermediate. The resulting N-terminal α-ketoamide is then converted to the D- and L-amino acids by nonenzymatic transamination in the presence of pyridoxamine (PM). The formation of the epimeric N-terminus depended on the incubation time and the concentration of PM, and increased further upon the addition of Cu(II) ions. A conversion of approximately 60% after three days of incubation was observed for Ang P. We propose that the reaction intermediate contains a prochiral α-carbon and is stabilized by the chelate effect of Cu(II) ions. The ONE- and hydroxyl radical-derived formation of N-terminal α-ketoamide and its transamination in the presence of PM were also observed in amyloid β 1-11 (DAEFRHDSGYE), where the N-terminal Asp was converted to epimeric alanine. This suggests that these N-terminal modifications could occur in vivo and modulate the biological functions of peptides and proteins.
authors.affiliation: Department of Bio-analytical Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University , Aobayama, Aoba-ku, Sendai 980-8578, Japan.
authors.forename: Seon Hwa
authors.initials: SH
authors.lastname: Lee
authors.forename: Hyunsook
authors.initials: H
authors.lastname: Kyung
authors.forename: Ryo
authors.initials: R
authors.lastname: Yokota
authors.forename: Takaaki
authors.initials: T
authors.lastname: Goto
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Oe
mesh_terms: D000577:Amides / Q000737:chemistry*; D000586:Amination; D000595:Amino Acid Sequence; D002853:Chromatography, Liquid; D010455:Peptides / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D053719:Tandem Mass Spectrometry
chemical_list: D000577:Amides; D010455:Peptides