Fourier-Transform Ion Cyclotron Resonance Mass Spectrometric Evidence for the Formation of α-Chloroenethiolates and Thioketenes from Chloroalkene-Derived, Cytotoxic 4-Thiaalkanoates

Article abstract of DOI:10.1021/tx00049a002

The cytotoxicity of chloroalkene-derived cysteine S-conjugates is thought to be associated with the formation of α-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 α-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-5-hexenoyl-CoA and subsequent processing by the fatty acid β-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 α-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 α-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 intemediates.