K-region aziridines of polycyclic aromatic hydrocarbons reverted Salmonella typhimurium his- (TA100, TA98) and Escherichia coli trp- strains (WP2 uvrA), without requiring activation by mammalian enzymes. The number of revertants induced per nmol in S. typhimurium TA100, the most responsive strain, varied from 6 to 10,000 for the seven monoaziridines and the two bisaziridines tested. Interestingly, the mutagenic potencies (y) of the monoaziridines were closely related (r = 0.984) with those of the corresponding epoxide analogues (x) by the equation y = 19.6 x0.97, i.e., the aziridines were about 20-fold stronger mutagens than were the epoxides.

One of the aziridines, benzo(a)pyrene (BP)-4,5-imine, was investigated in several additional mutagenicity test systems: toxicity in DNA repair-deficient (rec-) and -proficient (rec+) Bacillus subtilis strains; induction of 6-thioguanine resistance in V79 Chinese hamster cells; and induction of sister chromatid exchanges in cultured human fibroblasts. In all systems, BP-4,5-imine was much more active than the epoxide analogue, BP-4,5-oxide. The difference in activity was particularly large in the two test systems with mammalian target cells in which several hundredfold higher concentrations of the epoxide had to be used in order to elicit equipotent effects. Even r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydro-BP, which is one of the most potent mutagens known for V79 cells, was less active in the mammalian cells than was BP-4,5-imine.

One reason that arene imines are such potent mutagens may be that they are poorly detoxified. Addition of highly purified microsomal epoxide hydrolase, which strongly reduced the mutagenicity of BP-4,5-oxide and benz(a)anthracene-5,6-oxide in S. typhimurium, had no effect on the mutagenicity of the corresponding aziridines. Furthermore, while benz(a)anthracene-5,6-oxide was inactivated by highly purified cytosolic epoxide hydrolase, benz(a)anthracene-5,6-imine was not inactivated.

It is noteworthy that the arene imines are isomeric with and structurally closely related to aromatic amines. Some aziridines derived from nonaromatic structures (ethylene imines) have been reported as metabolites of xenobiotics; others are used as chemotherapeutics. At present, however, the results are mainly of theoretical interest in that a new type of arene derivatives with exceptionally potent, probably ultimate, mutagenicity was discovered and may be exploited for the study of mechanisms of chemical carcinogenesis.

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We thank the Deutsche Forschungsgemeinschaft (SFB 302), The Israel Fund for Basic Research, and the Israel Academy for Science and Humanities for financial support.

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