Fourier-transform ion cyclotron resonance mass spectrometric studies of elimination reactions of anionic bases with metabolites of a fluorinated anesthetic agent: towards modeling bioactivation in the gas phase

Sevoflurane (fluoromethyl 2,2,2-trifluoro-[2,2,2-trifluoromethyl]ethyl ethe r) is a volatile anesthetic agent that is widely used in the U.S. and abroa d. Sevoflurane undergoes degradation in the anesthetic circuit to form 2-(f luoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A). As it is metabo lized, Compound A alkylates the cysteine side chain in the tripeptide gluta thione, which acts as a sort of scavenger for xenobiotics such as Compound A. The S-alkylated glutathione or glutathione S conjugate loses its C-termi nal and N-terminal residues as it is further metabolized. This leaves a cys teine S conjugate of Compound A. The cysteine conjugate undergoes bioactiva tion by an enzyme known as beta-lyase to produce nephrotoxic metabolites. A lthough Compound A is nephrotoxic in rats, Compound A-associated nephrotoxi city has not been observed in the human clinical use of sevoflurane, appare ntly because beta-lyase activities are much lower in human kidney tissue th an in rat kidney tissue. Since beta-lyase reacts with carbonyl compounds by mechanisms involving deprotonation of the alpha-carbon, the reactions of C ompound A-derived cysteine conjugates with the basic anionic species hydrox ide, methoxide, and ethoxide were examined by Fourier-transform ion cyclotr on resonance mass spectrometry. The anionic bases examined react with the c ysteine conjugates by an initial deprotonation of the alpha-carbon to form an enolate intermediate followed by elimination of either a thiolate anion or of HF. Since the HF elimination leads to CF2 loss, it is suggested that the F atom eliminated as HF comes from a CF2 group. Collision-induced disso ciation (CID) of the product ions suggested structures consistent with this overall mechanistic picture. It is evident from these results that the sam e mechanism by which other cysteine conjugates are bioactivated could opera te in the case of Compound A. That is, deprotonation to form enolate interm ediates could lead to the release of very reactive species that might inact ivate enzymes by alkylating them or otherwise reacting irreversibly with th em. The thiolate product could alkylate an enzyme under appropriate conditi ons. Condensed-phase hydrolysis of the thiolate product could produce 2-(fl uoromethoxy)-3,3,3-trifluoropropionic acid, a known metabolite of Compound A. The HF loss channel produces not only HF, but also CF2, a very reactive species, Evidence is noted that a closely related enzyme substrate system r eacts to release fluoride in condensed phases suggesting that the activatio n of a CF3 observed here could be a more general process, (C) 2000 Elsevier Science B.V.