Evidence that epichlorohydrin is not a toxic metabolite of 1,2-dibromo-3-chloropropane

Gingell, Ralph; Beatty, Patrick W.; Mitschke, H; Mueller, R.L.; Sawin, Virginia L.; Page, A. C.

doi:10.3109/00498258709043933

Cited by 16 publications

(5 citation statements)

References 7 publications

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“…The major urinary NAC conjugate of DBCP was identified by mass spectrometry as 7V-acetyl-S-(3-chloro-2hydroxypropyl)cysteine (III) and is in agreement with a previous report (28). It has been proposed that this metabolite may be formed by either direct GSH conjugation of DBCP or conjugation of l-bromo-3-chloroacetone (27), a cytochrome P450 metabolite of DBCP (14).…”

Section: Discussionsupporting

confidence: 90%

Identification of N-Acetylcysteine Conjugates of 1,2-Dibromo-3-chloropropane: Evidence for Cytochrome P450 and Glutathione Mediated Bioactivation Pathways

Weber¹,

Steenwyk²,

Nelson³

et al. 1995

Chem. Res. Toxicol.

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The haloalkane 1,2-dibromo-3-chloropropane (DBCP) is a carcinogen, mutagen, nephrotoxin, and testicular toxin. The identification of N-acetylcysteine conjugates of DBCP provides information on GSH mediated and cytochrome P450 mediated bioactivation pathways in the expression of DBCP-induced toxicities. N-Acetylcysteine conjugates excreted in the urine of male Sprague-Dawley rats administered DBCP, C1D2-DBCP, C2D1-DBCP, C3D2-DBCP, or D5-DBCP (80 mg/kg) were purified by reverse-phase HPLC as their methyl ester derivatives and characterized by fast atom bombardment tandem mass spectrometry. These metabolites were also converted to tert-butyldimethylsilyl ether derivatives and analyzed by gas chromatography-mass spectrometry (GC-MS) to facilitate the identification of N-acetyl-S-(2,3-dihydroxypropyl)cysteine (Ia), an apparent regioisomer of Ia, 2-(S-(N-acetylcysteinyl))-1,3-propanediol (Ib), N-acetyl-S-(3-hydroxypropyl)cysteine (IIa), and N-acetyl-S-(3-chloro-2-hydroxypropyl)-cysteine (III). Metabolites Ia, Ib, and III displayed quantitative retention of deuterium, an observation consistent with the formation of episulfonium ion intermediate(s) in their biogenesis. Mercapturate IIa retained three atoms of deuterium from D5-DBCP, and two atoms of deuterium from the dideuterio analogs (C1D2-DBCP and C3D2-DBCP), thus invoking P450 mediated formation of 2-bromoacrolein (2-BA) as an intermediate in the biogenesis of IIa. A mechanism is proposed in which conjugate addition of GSH to 2-BA, subsequent episulfonium ion formation, and addition of GSH afford 1,2-(diglutathion-S-yl)propanal. Glutathione mediated reduction is invoked to afford S-(3-hydroxypropyl)GSH which would be excreted in the urine as IIa. The quantitative retention of deuterium from C1D2-DBCP or C3D2-DBCP was indicative of isotopically sensitive branching of P450 metabolism at either C1 or C3 to afford 2-BA. C2D1-DBCP showed a 30% retention of 1 deuterium atom in IIa; the loss of the deuterium is consistent with 2-BA formation, whereas the retention of one deuterium atom is indicative of the formation of metabolite IIa through GSH conjugation of either 2,3-dibromopropanal or 2-bromo-3-chloropropanal. These data indicate that IIa is a marker metabolite for the potent direct-acting mutagen, 2-BA, or its metabolic precursors 2,3-dibromopropanal or 2-bromo-3-chloropropanal. Therefore, evidence has been presented for bioactivation of DBCP by glutathione and cytochrome P450 mediated mechanisms.

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Section: Discussionsupporting

confidence: 90%

Identification of N-Acetylcysteine Conjugates of 1,2-Dibromo-3-chloropropane: Evidence for Cytochrome P450 and Glutathione Mediated Bioactivation Pathways

Weber¹,

Steenwyk²,

Nelson³

et al. 1995

Chem. Res. Toxicol.

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“…In principle, conjugate addition of GSH to 2-bromoacrolein hydroxypropyl)glutathione, has been proposed to originate either from l-bromo-3-chloroacetone (Dohn et al, 1988) or via the intermediacy of an episulfonium ion (Dohn et al, 1988;Gingell et al, 1987). The former pathway involves oxidation at C2 and involves metabolic elimination of a hydrogen (or deuterium) atom at the C2 positions of DBCP.…”

Section: Resultsmentioning

confidence: 99%

Metabolic activation of 1,2-dibromo-3-chloropropane: evidence for the formation of reactive episulfonium ion intermediates

Pearson¹,

Søderlund²,

Dybing³

et al. 1990

Biochemistry

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The nematocide and soil fumigant 1,2-dibromo-3-chloropropane (DBCP) is a carcinogen and a mutagen and displays target-organ toxicity to the testes and the kidney. It has been proposed that both cytochrome P-450 mediated activation and glutathione (GSH) conjugation pathways are operative in DNA damage and organotropy induced by DBCP. To determine the chemical mechanisms involved in the bioactivation of DBCP and to assess a role for an episulfonium ion intermediate, the mechanism of formation of GSH conjugate metabolites of DBCP was investigated. Five biliary GSH conjugates of DBCP were isolated from rats and identified by fast atom bombardment tandem mass spectrometry: S-(2,3-dihydroxy-propyl)glutathione (I), S-(2-hydroxypropyl)glutathione (IIA), S-(3-chloro-2-hydroxypropyl)glutathione (III), 1,3-di(S-glutathionyl)propan-2-ol (IV), and 1-(glycyl-S-cysteinyl)-3- (S-glutathionyl)propan-2-ol (V). The mechanisms of conjugate formation were addressed by assessing deuterium retention in conjugates derived from [1,1,2,3,3-2H5] DBCP (D5-DBCP). GSH conjugates I, III, IV, and V displayed quantitative retention of deuterium, an observation consistent with the formation of an episulfonium ion intermediate. GSH conjugate IIA, however, retained three atoms of deuterium, thus invoking a P-450 mechanism in its genesis. The involvement of glutathione transferase (GST) and sequential episulfonium ion intermediates in the formation of metabolites I, III, and IV was demonstrated in vitro. Upon incubation of DBCP with GST, metabolites I, III, and IV were identified by tandem mass spectrometry and were found to arise with quantitative retention of deuterium when D5-DBCP was employed as a substrate. An additional GSH conjugate, 1,2,3-tri(S-glutathionyl)propane (VI), was observed as the major metabolite in incubations of GST with DBCP. When the incubations of DBCP with GST were performed in H2(18)O, metabolite I incorporated two atoms of 18O, and metabolites III and IV incorporated one atom of 18O. The ability of GST to catalyze the formation of the four GSH conjugates observed in vivo, with quantitative retention of deuterium and incorporation of 18O from H2(18)O, may be rationalized by a mechanism invoking the initial formation of S-(2-bromo-3-chloropropyl)glutathione. Rearrangement of this unstable conjugate via several reactive episulfonium ions, with either hydrolysis by water or alkylation of GSH at various stages, would account for the pattern of metabolites and their status of isotopic enrichment observed under various incubation conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

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“…Initial studies on the mechanism of DBCP bioactivation have proposed metabolic activation via the intermediacy of epichlorohydrin or epibromohydrin (11). However, a role for these electrophilic species in the metabolic activation of DBCP has since been refuted (12). In an alternative proposal it has been suggested that 2-bromoacrolein or l-bromo-3-chloroacetone may be involved in the testicular toxicity or nephrotoxicity of DBCP (13).…”

Section: Introductionmentioning

confidence: 99%

Metabolic activation of 1,2-dibromo-3-chloropropane to mutagenic metabolites: detection and mechanism of formation of (Z)- and (E)-2-chloro-3-(bromomethyl) oxirane

Pearson

Omichinski

Myers

et al. 1990

Chem. Res. Toxicol.

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1,2-Dibromo-3-chloropropane (DBCP), a haloalkane nematocide and soil fumigant, is metabolically activated to chemically reactive species that are direct-acting mutagens in a Salmonella typhimurium TA 100 test system. Studies in vitro with rat liver microsomes indicated that oxidation at carbon 3 resulted in the formation of an unstable gem-chlorohydrin that rearranged with elimination of hydrogen bromide to form (Z)-2-chloro-3-(bromomethyl)oxirane [(Z)-CBPO] and (E)-2-chloro-3-(bromomethyl)oxirane [(E)-CBPO]. Gas chromatography-mass spectrometry (GC-MS) with positive ion chemical ionization (CI) was employed to identify (Z)-CBPO and (E)-CBPO by comparison of characteristic fragment ions in their CI mass spectra with those observed for authentic standards. Quantitative GC-MS methodology was exploited to quantitate the rate of formation of (Z)-CBPO and (E)-CBPO from DBCP and analogues of DBCP specifically deuterated at carbon 1 and carbon 3. The rate of formation of Z- and E-isomers of CBPO was 31 and 33 pmol/(min.mg of protein), respectively, from DBCP; substitution with deuterium at carbon 1 increased the rate of epoxide formation by 50%, whereas CBPO formation could not be detected from a substrate labeled with deuterium at carbon 3. Both epoxides were directly acting mutagens to S. typhimurium TA 100. (Z)-CBPO caused approximately twice as many his+ revertants/nmol compared to (E)-CBPO. Oxidation at carbon 2 of DBCP resulted in the formation of a bifunctional alkylating agent, 1-bromo-3-chloroacetone, presumably via the intermediacy of an unstable gem-bromohydrin.(ABSTRACT TRUNCATED AT 250 WORDS)

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Evidence that epichlorohydrin is not a toxic metabolite of 1,2-dibromo-3-chloropropane

Cited by 16 publications

References 7 publications

Identification of N-Acetylcysteine Conjugates of 1,2-Dibromo-3-chloropropane: Evidence for Cytochrome P450 and Glutathione Mediated Bioactivation Pathways

Identification of N-Acetylcysteine Conjugates of 1,2-Dibromo-3-chloropropane: Evidence for Cytochrome P450 and Glutathione Mediated Bioactivation Pathways

Metabolic activation of 1,2-dibromo-3-chloropropane: evidence for the formation of reactive episulfonium ion intermediates

Metabolic activation of 1,2-dibromo-3-chloropropane to mutagenic metabolites: detection and mechanism of formation of (Z)- and (E)-2-chloro-3-(bromomethyl) oxirane

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