BISDIHYDRODIOLS, RATHER THAN DIHYDRODIOL OXIDES, ARE THE PRINCIPAL MICROSOMAL METABOLITES OF TUMORIGENIC ANS-3,4-DIHYDROXY-3,4-DIHYDRODIBENZ[A,H]ANTHRACENE

Several studies on metabolism and biological activity of tumorigenic d ibenz[a,h] anthracene (DBA) and its derivatives have led to the conclu sion that the M-region dihydrodiol, trans-3,4-dihydroxy-3,4-dihydro-DB A (DBA-3,4-dihydrodiol), is the precursor of the ultimate mutagenic an d tumorigenic metabolite of DBA with the presumed structure of a bay-r egion dihydrodiol oxide. Incubations of DBA-3,4-dihydrodiol (50 mu M) with the microsomal hepatic fraction of Sprague-Dawley rats pretreated with Aroclor 1254 yielded more than 13 metabolites upon separation by HPLC. nti-3,4-Dihydroxy-1,2-epoxy-1,2,3,4-tetrahydro-DBA [0.27 nmol/( nmol of P450.15 min)] could be identified for the first time by UV spe ctroscopy, by cochromatography with the synthetic reference compound, and by its nonenzymatic hydrolysis to 1,t-2,t-3,c-4-tetrahydroxy-1,2,3 ,4-tetrahydro-DBA, while firm evidence for the presence of the diaster eomeric syn-dihydrodiol oxide was not obtained. Major microsomal metab olites of the M-region dihydrodiol were however three bisdihydrodiols: trans,trans-3,4:8,9-tetrahydroxy-3,4,8,9 tetrahydro-DBA [0.32 nmol/(n mol of P450.15 min)l, trans,trans-3,4:10,11-tetrahydroxy-3,4, 10,ll-te trahydro-DBA [DBA-3,4:10,11-bisdihydrodiol; 1.44 nmol/(nmol of P450.15 min)l, and ns-3,4:12,13-tetrahydroxy-3,4,12,13-tetrahydro-DBA [0.70 n mol/(nmol of P450.15 min)], whose structures were verified by UV and m ass spectrometry as well as cochromatography with synthetic reference compounds and by the observation that they were not formed when epoxid e hydrolase was inhibited (1,1,1-trichloro-2-propene oxide, 1 mM). Det ermination of the bacterial mutagenicity in strain TA100 of Salmonella typhimurium in the presence of a metabolic activating system revealed a stronger mutagenic effect of DBA-3,4:10,11-bisdihydrodiol (52.3 his (+) revertants/ nmol) as compared to its metabolic precursor, DBA-3,4- dihydrodiol (34.5 his(+) revertants/nmol), while the two other bisdihy drodiols contribute only little to the genotoxic activity of the M-reg ion dihydrodiol. On the basis of these observations 1,2-epoxy-3,4:10,1 1-tetrahydroxy-1,2,3,4,10, 11-hexahydrodibenz[a,h] anthracene can be p redicted as an ultimate mutagenic metabolite of DBA-3,4-dihydrodiol pl aying a more important role in the genotoxicity of DBA than the simple bay-region dihydrodiol oxide. This assumption is further supported by the results of recent DNA binding studies with DBA and its derivative s.