Lc. King et al., An evaluation of the mutagenicity, metabolism, and DNA adduct formation of5-nitrobenzo[b]naphtho[2,1-d]thiophene, CHEM RES T, 14(6), 2001, pp. 661-671
Thioarenes, sulfur-containing polycyclic aromatic compounds, are environmen
tal contaminants suspected of posing human health risks. In this study, 5-n
itrobenzo [b]naphtho[2,1-d]thiophene (5-nitro-BNT), a nitrated-thioarene, w
as examined for its mutagenicity, metabolism and subsequent formation of DN
A adducts. 5-Nitro-BNT was weakly mutagenic in Salmonella typhimurium strai
ns TA98 and TA100 without Aroclor-1254-induced rat liver S9 (S9), and its a
ctivity was increased in the presence of S9. Anaerobic metabolism of 5-nitr
o-BNT by S9 or xanthine oxidase (XO) produced one major metabolite, identif
ied as 5-amino-BNT by NMR, MS, and W spectroscopy and by comparison with an
authentic standard. Aerobic S9 metabolism of 5-nitro-BNT produced a major
metabolite, identified as trans-9,10-dihydroxy-9, 10-dihydro-5-nitro-BNT (5
-nitro-BNT-9,10-diol). Also present was a minor amount of 5-amino-BNT and t
rans-9,10-dihydroxy 9,10-dihydro-5-amino-BNT (5-amino-BNT-9,10-diol). DNA a
dduct analyses were performed using the P-32-postlabeling assay and reverse
d-phase HPLC. Three major XO-derived calf thymus DNA adducts were detected.
On the basis of their chromatographic mobilities, two adducts were identif
ied as reaction products of 5-nitro-BNT with 2 ' -deoxyguanosine and one ad
duct with 2 ' -deoxyadenosine. Incorporation of allopurinol (a specific XO
inhibitor) in the incubation mixture resulted in loss of all three adducts,
confirming enzymatic mediation by XO. Aerobic S9 activation of 5-nitro-BNT
with calf thymus DNA produced three adducts. On the basis of their chromat
ographic mobilities, two were identified as reaction products of 5-nitro-BN
T with 2 ' -deoxyguanosine and one with 2 ' -deoxyadenosine. Incorporation
of 1-aminobenzotriazole (a P450 inhibitor) in the incubation mixture result
ed in a loss of these adducts, confirming enzymatic mediation by P450. Aero
bic S9-catalyzed metabolism of 5-nitro-BNT-9, 10-diol produced the same DNA
adducts as observed with 5-nitro-BNT. Aerobic S9-catalyzed metabolism of 5
-amino-BNT-9,10-diol produced the same deoxyadenosine-derived DNA adducts a
s observed with 5-nitro-BNT and 5-nitro-BNT-9,10-diol. These results provid
e additional information that both ring oxidation and nitroreduction are in
volved in the metabolism, DNA adduct formation and mutagenicity of 5-nitro-
BNT.