Identification of butyrylcholinesterase adducts after inhibition with isomalathion using mass spectrometry: Difference in mechanism between (1R)- and(1S)-stereoisomers

Citation
Ja. Doorn et al., Identification of butyrylcholinesterase adducts after inhibition with isomalathion using mass spectrometry: Difference in mechanism between (1R)- and(1S)-stereoisomers, TOX APPL PH, 176(2), 2001, pp. 73-80
Citations number
29
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Pharmacology & Toxicology
Journal title
TOXICOLOGY AND APPLIED PHARMACOLOGY
ISSN journal
0041-008X → ACNP
Volume
176
Issue
2
Year of publication
2001
Pages
73 - 80
Database
ISI
SICI code
0041-008X(20011015)176:2<73:IOBAAI>2.0.ZU;2-I
Abstract
Previous kinetic studies found that butyrylcholinesterase (BChE) inhibited by (IR)-isomalathions readily reactivated, while enzyme inactivated by (IS) -isomers did not. This study tested the hypothesis that (1R)- and (1S)-isom ers inhibit BChE by different mechanisms, yielding distinct adducts identif iable by peptide mass mapping with matrix-assisted laser desorption/ionizat ion time-of-flight mass spectrometry (MALDI-TOF-MS). Equine BChE (EBChE) wa s inhibited to <10% of control activity with each isomer of isomalathion an d the reference compound isoparathion methyl. Control and treated enzyme wa s digested with trypsin, and peptides were fractionated with HPLC. Separate d and unseparated peptides were analyzed with MALDI-TOF-MS. Identity of an organophosphorus peptide adduct was confirmed by fragmentation using postso urce decay analysis. EBChE inhibited by (IR)-isomalathions or (S)-isoparath ion methyl readily reactivated after oxime treatment with 30-40% activity r ecovered. Enzyme inactivated by (IS)-isomalathions or (R)-isoparathion meth yl recovered <2% and <5% activity, respectively, after oxime treatment. MAL DI-TOF-MS analysis revealed that inhibition of EBChE by (IR)-isomalathions and (R)- or (S)-isoparathion methyl yielded O,S-dimethyl phosphate adducts. Enzyme inactivated by (IS)-isomalathions produced only O-methyl phosphate adduct. EBChE modified by (IR)-isomalathions or either enantiomer of isopar athion methyl yielded an O-methyl phosphate adduct as well. The results ind icate that EBChE inhibition by (IR)-isomalathions proceeds with loss of die thyl thiosuccinate, but inactivation by (IS)-isomers occurs with loss of th iomethyl as the primary leaving group followed by rapid expulsion of diethy l thiosuccinate to yield an aged enzyme. Furthermore, the data suggest that aging of the O,S-dimethyl phosphate adduct occurs via an S(N)2 process wit h loss of thiomethyl. (C) 2001 Academic Press.