Most environmental mutagens and carcinogens require metabolic activation to electro- philic intermediates capable of reacting with cellular target structures, such as DNA. These electrophilic intermediates are in addition subject to metabolic detoxification. This metabolism is mainly controlled by enzymes whose expression is very variable. Among other things, various enzymes are inducible by environmental chemicals. Understanding the toxicology of chemicals (for example, species differences, idiosyncrasias, organotropisms) therefore requires knowledge of critical host factors. One approach towards this goal involves the use of purified enzymes in metabolism and toxicological studies (Glatt et al., 1983; Glatt and Oesch, 1986). This approach, however, is laborious, the purification may alter enzymatic properties, and it may be difficult to rule out the influence of impurities in the enzyme preparation, especially when large families of structurally closely related enzymes exist, as is common in xenobiotic metabolism.

The fjord-region diol-epoxides of benzo(c)phenanthrene combine high mutagenic and carcinogenic activity with low chemical reactivity. To study whether this is a unique property of these compounds or a more general characteristic of fjord-region diol-epoxides, we have synthesized the anti- and syn-diastereomers of r-9,t-10-dihydroxy-11,12-oxy-9,10,11,12-tetrahydrobenzo(c)chrysene and r-11-t-12-dihydroxy-13,14-oxy-11,12,13,14-tetrahydrobenzo(g)chrysene. These compounds as well as the anti- and syn-diastereomers of the fjord-region diol-epoxides of benzo(c)phenanthrene and of the bay-region diol-epoxides of phenanthrene, chrysene, and benzo(a)pyrene were investigated for their half-lives in a physiological buffer, for their mutagenicity in Salmonella typhimurium (reversion of the his- strains TA97, TA98, TA100, and TA104), for induction of SOS response in Escherichia coli (SOS chromotest in strain PQ37) and for their mutagenicity in V79 Chinese hamster cells (acquisition of resistance to 6-thioguanine). All six of the investigated fjord-region diol-epoxides were more stable in physiological buffer at 37-degrees-C (t1/2 > 2 h) than the six bay-region diol-epoxides (t1/2 = 0.011 to 1.2 h). The half-lives correlated negatively with the calculated DELTA-E(deloc) values for the formation of the benzylic carbocations, and were consistently shorter for the syn- than for the corresponding anti-diastereomer. All fjord-region diol-epoxides showed extraordinarily high activity in all six genotoxicity assays used. In mammalian cells, the anti-diol-epoxide of benzo(c)chrysene was 8.6 and 12 times more active than the anti-diol-epoxides of benzo(c)phenanthrene and benzo(a)pyrene, respectively, which were the most potent mutagens among the reference compounds. The other three newly available fjord-region diol-expoxides were also markedly more mutagenic in mammalian cells than the reference compounds. Whereas the syn-diastereomers of the simple bay-region diol-epoxides were clearly less mutagenic in mammalian cells than the corresponding anti-diastereomers, the differences in potency between diastereomers were small for the fjord-region diol-epoxides. In conclusion, the diol-epoxides of benzo(c)phenanthrene are not unique in their high biological activities. The two newly available diastereomeric pairs of fjord-region diol-epoxides of benzo(g)- and benzo(c)chrysene proved to be even more active. For one of them, the diol-epoxides of benzo(g)chrysene, the DELTA-E(deloc) value for the formation of the benzylic carbocation is lower than for the benzo(c)phenanthrene diol-epoxides, for the other it is higher.