Chlorothioketene, the Ultimate Reactive Intermediate Formed by Cysteine Conjugate β-Lyase-Mediated Cleavage of the Trichloroethene Metabolite <i>S</i>-(1,2-Dichlorovinyl)-<scp>l</scp>-cysteine, Forms Cytosine Adducts in Organic Solvents, but Not in Aqueous Solution
作者:Wolfgang Völkel、Wolfgang Dekant
DOI:10.1021/tx980084d
日期:1998.9.1
Chlorothioketene has been suggested as a reactive intermediate formed by the cysteine conjugate beta-lyase-mediated cleavage of S-(1,2-dichlorovinyl)-L-cysteine, a minor metabolite of trichloroethene. Halothioketenes are highly reactive, and their intermediate formation may be confirmed by reactions such as cycloadditions and thioacylations of nucleophiles. A precursor of chlorothioketene, S-( 1,2-dichlorovinyl)thioacetate, is readly accessible by the reaction of dichloroethyne with thioacetic acid. In presence of base, S-(1,2-dichlorovinyl)thioacetate is cleaved to chlorothioketene. Chlorothioketene is not stable at room temperature and was characterized after transformation to stable products by reaction with compounds such as cyclopentadiene, N,N-diethylamine, and ethanol. In organic solvents, the cleavage of S-(1,2-dichlorovinyl)thioacetate in the presence of cytosine results in N-4-acetylcytosine, N-4-(chlorothioacetyl)cytosine, and small amounts of 3-(N-4-thioacetyl)cytosine. No reaction products were seen with guanosine, adenosine, and thymidine under identical conditions. When cytosine was reacted with S-(1,2-dichlorovinyl)thioacetate in aqueous solutions, only N-4-acetylcytosine was formed. N-4-(Chlorothioacetyl)cytosine and 3-(N-4-thioacetyl)cytosine were not detected even when using a very sensitive method, derivatization with pentafluorobenzyl bromide and electron capture mass spectrometry with a detection limit of 50 fmol/mu L of injection volume. Aqueous solutions of DNA cleave S-(1,2-dichlorovinyl)thioacetate to give N-4-acetyldeoxycyticline in DNA, but chlorothioketene adducts of deoxynucleosides were also not detected in these experiments. These results confirm the electrophilic reactivity of chlorothioketene toward nucleophilic groups of DNA constituents in inert solvents but also demonstrate that the formation of DNA adducts under physiological conditions likely is not efficient. Therefore, DNA adducts may not represent useful biomarkers of exposure and biochemical effects for trichloroethene.