Mutagenicity of phenanthrene and phenanthrene K-region derivatives

Phenanthrene and 9 K-region derivatives, most of them potential metabolites of phenanthrene, were tested for mutagenicity by the reversion of histidine-dependent Salmonella typhimurium TA1535, TA1537, TA1538, TA98 and TA100 and the rec assay with Bacillus subtilis H17 and M45. The strongest mutagenic effects in the reversion assay were observed with phenanthrene 9,10-oxide, 9-hydroxyphenanthrene and N-benzyl-phenanthrene-9,10-imine. Interestingly, the mutagenic potency of the arene imine was similar to that of the corresponding arene oxide. This is the first report on the mutagenicity of an arene imine. The mutagenic effects of all these phenanthrene derivatives were much weaker than that of the positive control benzo[a]pyrene 4,5-oxide. Even weaker mutagenicity was found with cis-9,10-dihydroxy-9,10-dihydrophenanthrene and with trans-9,10-dihydroxy-9,10-dihydrophenanthrene. The other derivatives were inactive in this test. However, 9,10-dihydroxyphenanthrene and 9,10-phenanthrenequinone were more toxic to the rec^- B. subtilis M45 strain than to the rec⁺ H17 strain. This was also true for phenanthrene 9,10-oxide and 9-hydroxyphenanthrene, but not with the other test compounds that reverted (9,10-dihydroxy-9,10-dihydrophenanthrenes; N-benzyl-phenanthrene 9,10-imine; benzo[a]pyrene 4,5-oxide) or did not revert (phenanthrene, 9,10-bis-(p-chlorophenyl)-phenanthrene 9,10-oxide, 9,10-diacetoxyphenanthrene) the Salmonella tester strains. Although the K region is a main site of metabolism and although all potential K-region metabolites were mutagenic, phenanthrene did not show a mutagenic effect in the presence of mouse-liver microsomes and an NADPH-generating system under standard conditions. However, when epoxide hydratase was inhibited, phenanthrene was activated to a mutagen that reverted his^- S. typhimurium. This shows that demonstration of the mutagenic activity of metabolites together with the knowlegde that a major metabolic route proceeds via these metabolites does not automatically imply a mutagenic hazard of the mother compound, because the metabolites in question may not accumulate in sufficient quantities and therefore the presence and relative activities of enzymes that control the mutagenically active metabolites are crucial. N-Benzyl-phenanthrene 9,10-imine was mutagenic for the episome-containing S. typhimurium TA98 and TA100 but not for the precursor strains TA1538 and TA1535. This arene imine would therefore be useful as a positive control during routine testing to monitor in the former strains the presence of the episome which is rather easily lost.