Fourier-transform ion cyclotron resonance mass spectrometric evidence for the formation of alpha-chloroenethiolates and thioketenes from chloroalkene-derived, cytotoxic 4-thiaalkanoates.
The cytotoxicity of chloroalkene-derived cysteine S-conjugates is thought to be associated with the formation of alpha-chloroenethiolates and thioketenes as reactive intermediates. Recent studies indicate that the formation of 1,2-dichloroethenethiolate, which may give rise to chlorothioketene, is a key step in the bioactivation of 5,6-dichloro-4-thia-5-hexenoic acid (Fitzsimmons et al. (1995) Biochemistry 34, 4276-4286). We report here the use of Fourier-transform ion cyclotron resonance mass spectrometry to provide the first direct evidence for the formation of alpha-chloroenethiolate and thioketene species from a cytotoxic 4-thiaalkanoate. The bioactivation of 5,6-dichloro-4-thia-5-hexenoic acid involves conversion to the corresponding CoA thioester 5,6-dichloro-4-thia-5-hexenoyl-CoA and subsequent processing by the fatty acid beta-oxidation pathway. It has been proposed that the bioactivation of 5,6-dichloro-4-thia-5-hexenoyl-CoA involves loss of 1,2-dichloroethenethiolate, followed by loss of chloride to form chlorothioketene. 1,2-Dichloroethenethiolate and related alpha-chloroalkenethiolates have not been observed directly in aqueous solution. Fourier-transform ion cyclotron resonance mass spectrometric experiments show that S-propyl 5,6-dichloro-4-thia-5-hexenethioate reacts in the gas phase with base (hydroxide ion) to release 1,2-dichloroethenethiolate, which is observed directly in the mass spectrum of the products of the gas-phase reaction. Furthermore, the elimination of chloride from 1,2-dichloroethenethiolate on collision-induced decomposition is facile and provides evidence for chlorothioketene formation. Preliminary evidence for the formation of 1,2-dichloroethenethiolate and chlorothioketene from S-(1,2-dichlorovinyl)-N-acetyl-L-cysteine methyl ester was also obtained. These observations support the intermediacy of alpha-chloroenethiolates and chlorothioketenes in the bioactivation of cytotoxic, chloroalkene-derived 4-thiaalkanoates and cysteine S-conjugates and demonstrate the utility of Fourier-transform ion cyclotron mass spectrometry in studying the formation of reactive intermediates.
Zhang
T L
TL
Department of Chemistry and Biochemistry, University of Delaware, Newark 19716, USA.
Wang
L
L
Hashmi
M
M
Anders
M W
MW
Thorpe
C
C
Ridge
D P
DP
D000111:Acetylcysteine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000633:toxicity; D001711:Biotransformation; D002264:Carboxylic Acids / Q000737:chemistry*; D017357:Cyclotrons; D005583:Fourier Analysis; D006843:Hydrocarbons, Chlorinated / Q000737:chemistry*; D007477:Ions; D013058:Mass Spectrometry / Q000379:methods*; D013440:Sulfides / Q000737:chemistry*; D015262:Xenobiotics / Q000633:toxicity
D002264:Carboxylic Acids; D006843:Hydrocarbons, Chlorinated; D007477:Ions; D013440:Sulfides; D015262:Xenobiotics; C049705:S-1,2-dichlorovinyl-N-acetylcysteine; D000111:Acetylcysteine