CYSTEINE CONJUGATE BETA-LYASE-CATALYZED BIOACTIVATION OF BROMINE-CONTAINING CYSTEINE S-CONJUGATES - STOICHIOMETRY AND FORMATION OF 2,2-DIFLUORO-3-HALOTHIIRANES
Mb. Finkelstein et al., CYSTEINE CONJUGATE BETA-LYASE-CATALYZED BIOACTIVATION OF BROMINE-CONTAINING CYSTEINE S-CONJUGATES - STOICHIOMETRY AND FORMATION OF 2,2-DIFLUORO-3-HALOTHIIRANES, Chemical research in toxicology, 9(1), 1996, pp. 227-231
1,1-Dichloroalkene-derived S-(1-chloroalkenyl)-L-cysteine conjugates,
but not 1,1-difluoroalkene-derived S-(2,2-dihalo-1,1-difluoroethyl)-L-
cysteine conjugates, are mutagenic in the Ames test. Recent studies ha
ve showed, however, that bromine-containing, 1,1-difluoroalkene-derive
d S-(2-bromo-2-halo-1,1-difluoroethyl)-L-cysteine conjugates are mutag
enic [Finkelstein, M. B., et al. (1994) Chem. Res. Toxicol. 7, 157-163
] and that alpha-thiolactones are formed as reactive intermediates and
glyoxylate as a terminal product [Finkelstein, M. B., et al. (1995) J
. Am. Chem. Sec. 117, 9590-9591]. The present studies were undertaken
to examine the stoichiometry of cysteine conjugate beta-lyase-catalyze
d product formation from a panel of bromine-containing and bromine-lac
king cysteine S-conjugates and to search for additional metabolites. T
he cysteine S-conjugates were incubated with rat renal homogenates, an
d pyruvate:product (glyoxylate, bromide, fluoride, dihaloacetate, trih
aloethene) ratios were measured. Pyruvate: glyoxylate ratios for S-(2-
bromo-1,1,2-trifluoroethyl)-L-cysteine S-(2-bromo-2-chloro-1,1-difluor
oethyl)-L-cysteine, and S-(2,2-dibromo-1,1-difluoroethyl)-L-cysteine r
anged from 1:0.13 to 1:0.16. With S-(2-bromo-2-chloro-1,1-difluoroethy
l)-L-cysteine and S-(2-bromo-1, 1,2-trifluoroethyl)-L-cysteine, pyruva
te:bromide ratios were 1:1, but with the dibrominated conjugate S-(2,2
-dibromo-1,1-difluoroethyl)-L-cysteine, the pyruvate:bromide ratio was
1:1.2. All bromine-containing cysteine S-conjugates gave less than co
mplete conversion to fluoride. A search for additional metabolites led
to the consideration of 2,2-difluoro-3-halothiiranes as putative inte
rmediates. 2,2-Difluoro-3-halothiiranes may arise by internal displace
ment of bromide and cyclization of 2-bromo-2-halo-1,1-difluoroethaneth
iolates, which are beta-elimination products of cysteine S-conjugates.
Such halogenated thiiranes may eliminate sulfur to give 1,1-difluoro-
2-haloethenes. GC/MS analysis showed that trifluoroethene, 2-chloro-1,
1-difluoroethene, and 2-bromo-1,1-difluoroethene were terminal product
s of S-(2-bromo-1,1,2-trifluoroethyl)-L-cysteine, S-(2-bromo-2-chloro-
1,1-difluoroethyl)-L-cysteine, and S-(2,2-dibromo-1,1-difluoroethyl)-L
-cysteine, respectively. The bromine-lacking conjugate S-(2-chloro-1,1
,2-trifluoroethyl)-L-cysteine did not yield glyoxylate or trifluoroeth
ene as products, but the formation of chlorofluoroacetate was confirme
d. The pyruvate:chlorofluoroacetate ratio was 1:0.38, indicating that
other products are formed. This is the first report of the stoichiomet
ry of the beta-lyase-catalyzed biotransformation of haloalkene-derived
cysteine S-conjugates and of the formation of 2,2-difluoro-3-halothii
ranes as reactive intermediates in the biotransformation of bromine-co
ntaining cysteine S-conjugates.