Metabolic activation studies of dibenzo[
a,
l]pyrene (DB[
a,
l]P) (dibenzo[
def,
p]chrysene), anextremely potent environmental carcinogen, have been focused on metabolism at the fjordregion, a region associated with high mutagenic and carcinogenic activities of the correspondingfjord-region DB[
a,
l]P-11,12-diol-13,14-epoxides. DB[
a,
l]P is metabolized by
![](/images/gifchars/beta2.gif)
-naphthoflavone(BNF)- and 3-methylcholanthrene-induced rat liver microsomes and a recombinant humanP450 1A1 system to two major dihydrodiols, the K-region dihydrodiol, DB[
a,
l]P-8,9-dihydrodiol(DB[
a,
l]P-8,9-diol), and the fjord-region dihydrodiol, DB[
a,
l]P-11,12-dihydrodiol. We haveinvestigated the further metabolic activation of DB[
a,
l]P-8,9-diol by BNF-induced rat livermicrosomes and a recombinant human P450 1A1 system with epoxide hydrolase to DB[
a,
l]P-bis-diols and to DNA adducts. (±)-
trans-DB[
a,
l]P-8,9-diol was synthesized and resolved intoits enantiomers. Racemic
trans-DB[
a,
l]P-8,9-diol was metabolized by BNF-induced rat livermicrosomes to six metabolites: two diastereomers of
trans,
trans-DB[
a,
l]P-8,9:11,12-bis-diol,two diastereomers of
trans,
cis-DB[
a,
l]P-8,9:11,12-bis-diol, and two diastereomers of
trans-DB[
a,
l]P-8,9:13,14-bis-diol as characterized by NMR, MS, and UV spectroscopy. Metabolic studiesusing both enantiomeric (-)- and (+)-
trans-DB[
a,
l]P-8,9-diol further demonstrated that eachdiastereomer of
trans,
trans-DB[
a,
l]P-8,9:11,12-bis-diol and
trans-DB[
a,
l]P-8,9:13,14-bis-diol wascomprised of two enantiomers. Similarly, incubations of enantiomeric or racemic
trans-DB[
a,
l]P-8,9-diol with a recombinant human P450 1A1 system and epoxide hydrolase also gavethe same two enantiomeric mixtures of diastereomers of
trans,
trans-DB[
a,
l]P-8,9:11,12-bis-diol and the same two enantiomeric mixtures of diastereomers of
trans-DB[
a,
l]P-8,9:13,14-bis-diol. This suggested that the microsomal oxidations of (-)- and (+)-
trans-DB[
a,
l]P-8,9-diolwere stereospecific. The stereospecific formation of enantiomers of
trans-DB[
a,
l]P-8,9-diol fromDB[
a,
l]P was examined using both BNF-induced rat liver microsomes and a recombinant humanP450 1A1 system with epoxide hydrolase. Stereospecificity was observed as both metabolicsystems favored the formation of (-)-
trans-DB[
a,
l]P-8,9-diol by 8-9-fold. DNA adduct studieswere undertaken using TLC/HPLC
32P-postlabeling techniques. In the presence of a recombinant human P450 1A1 system with epoxide hydrolase, DB[
a,
l]P gave two groups of calfthymus DNA adducts. The group of later-eluting adducts were identified as arising from
syn-and
anti-DB[
a,
l]P-11,12-diol-13,14-epoxides, while the more polar early-eluting adducts werederived, in part, from the further activation of
trans-DB[
a,
l]P-8,9-diol. Our data indicate that,in P450 1A1-mediated microsomal incubations, DB[
a,
l]P is metabolized to
trans-DB[
a,
l]P-8,9-diol which is further metabolized to DB[
a,
l]P-bis-diols.
trans-DB[
a,
l]P-8,9-diol is metabolicallyactivated to intermediates that can bind to DNA and give DNA adducts similar to thoseobserved with DB[
a,
l]P. These results indicate that DB[
a,
l]P can be metabolically activatedby both fjord-region and K-region pathways.