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

Abstract

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)

The Use of Deuterium Isotope Effects to Probe the Active Site Properties, Mechanism of Cytochrome P450-Catalyzed Reactions, and Mechanisms of Metabolically Dependent Toxicity

“…Deuterium substitution for hydrogen at the terminal bromomethyl carbon atom of tris(2,3-dibromopropyl)phosphate (Tris-BP) and 1,2-dibromo-3-chloropropane (DBCP) resulted in 5-to 6-fold decreased rates of mutagenicity and formation of the highly mutagenic metabolite, 2-bromoacrolein (Nelson et al, 1984;Omichinski et al, 1988). However, evidence from investigations with deuterated and methylated analogs suggested that tissue organ damage in rats caused by Tris-BP and DBCP resulted from GSH conjugation as the major rate-limiting reaction (Dybing et al, 1989;Pearson et al, 1990). Both pathways may be involved in organ toxicity since sequential metabolism of Tris-BP and DBCP by oxidation and glutathione conjugation forms reactive metabolites of these compounds as well (Fig.…”

Cyclodextrin-catalyzed oxidation of glutathione in solution and in an ion trap

“…[11][12][13] In this paper, we present preliminary evidence on the enhancement effect of a-and b-CD (CDs) on the rate of oxidation of GSH. [14][15][16] A reaction mechanism is suggested based on data resulting from collision-induced dissociation of (CD/GSH) complexes in an ion trap.…”

Deguanylation of Guanine Based-Nucleosides and Calf Thymus DNA Induced by Halogenated Alkanes at the Physiological Condition

“…Time response reaction was performed with 128 µL of 2-B-2-MP, 2,3-dBPe, 2-BP, BE and 2-IP at a time interval of 8h for 0, 8,16,24,32,40 and 48 h at the physiological condition, respectively. At the end of the reaction 300 µL of 1 M HCl was added and centrifuged for 10 min at 13,000 rpm.…”

“…Short-chain halogenated alkanes, which we utilized for the study of deguanylation of guanine based-nucleosides, have been widely used industrially as chemical intermediates, extraction solvents, degreasing compounds, copolymer cross-linking agents, or have been reported to be mutagenic and carcinogenic. [8][9][10][11][12][13][14][15][16][17] Among the sixteen different halogenated alkanes, we observed massive deguanylation of nucleosides by 2-bromo-2-methylpropane (2-B-2-MP), 2,3-dibromopropene (2,3-dBPe), 2-bromopropane (2-BP), bromoethane (BE) and 2-iodopropane (2-IP). …”