title: What Are the Potential Sites of Protein Arylation by N-Acetyl-p-benzoquinone Imine (NAPQI)?
abstract: Acetaminophen (paracetamol, APAP) is a safe and widely used analgesic medication when taken at therapeutic doses. However, APAP can cause potentially fatal hepatotoxicity when taken in overdose or in patients with metabolic irregularities. The production of the electrophilic and putatively toxic compound N-acetyl-p-benzoquinone imine (NAPQI), which cannot be efficiently detoxicated at high doses, is implicated in APAP toxicity. Numerous studies have identified that excess NAPQI can form covalent linkages to the thiol side chains of cysteine residues in proteins; however, the reactivity of NAPQI toward other amino acid side chains is largely unexplored. Here, we report a survey of the reactivity of NAPQI toward 11 N-acetyl amino acid methyl esters and four peptides. (1)H NMR analysis reveals that NAPQI forms covalent bonds to the side-chain functional groups of cysteine, methionine, tyrosine, and tryptophan residues. Analogous reaction products were observed when NAPQI was reacted with synthetic model peptides GAIL-X-GAILR for X = Cys, Met, Tyr, and Trp. Tandem mass spectrometry peptide sequencing showed that the NAPQI modification sites are located on the "X" residue in each case. However, when APAP and the GAIL-X-GAILR peptide were incubated with rat liver microsomes that contain many metabolic enzymes, NAPQI formed by oxidative metabolism reacted with GAIL-C-GAILR exclusively. For the peptides where X = Met, Tyr, and Trp, competing reactions between NAPQI and alternative nucleophiles precluded arylation of the target peptide by NAPQI. Although Cys residues are favorably targeted under these conditions, these data suggest that NAPQI can, in principle, also damage proteins at Met, Tyr, and Trp residues.
authors.affiliation: School of Chemistry and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Bio21 Institute of Molecular Science and Biotechnology, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Leeming
authors.affiliation: School of Chemistry and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Bio21 Institute of Molecular Science and Biotechnology, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Luke F
authors.initials: LF
authors.lastname: Gamon
authors.affiliation: School of Chemistry and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Bio21 Institute of Molecular Science and Biotechnology, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Uta
authors.initials: U
authors.lastname: Wille
authors.affiliation: School of Chemistry, University of New South Wales , Sydney, NSW 2052, Australia.
authors.forename: William A
authors.initials: WA
authors.lastname: Donald
authors.affiliation: School of Chemistry and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Bio21 Institute of Molecular Science and Biotechnology, University of Melbourne , Parkville, Victoria 3010, Australia.
authors.forename: Richard A J
authors.initials: RA
authors.lastname: O'Hair
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D016227:Benzoquinones / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007097:Imines / Q000378:metabolism*; D008861:Microsomes / Q000378:metabolism; D010455:Peptides / Q000378:metabolism; D011485:Protein Binding
chemical_list: D000596:Amino Acids; D016227:Benzoquinones; D007097:Imines; D010455:Peptides; C028473:N-acetyl-4-benzoquinoneimine