Bacterial mutagenicity of the three isomeric dicyclopenta-fused pyrenes: the effects of dicyclopenta topology

Abstract

Cyclopenta[cd]pyrene (1) and its congeners dicyclopenta [cd,mn]-(2), dicyclopenta [cd,fg]-(3), dicyclopenta [cd,jk]pyrene (4), which were all identified as constituents of combustion exhausts, as well as their partially hydrogenated derivatives 3,4-dihydrocyclopenta[cd]-(5), 1,2,4,5-tetrahydrodicyclopenta[cd,mn]-(6), 5,6,7,8-tetrahydrodicyclopenta[cd,fg]- (7) and 1,2,6,7-tetrahydrodicyclopenta[cd,jk]pyrene (8), were assayed for mutagenicity in the Salmonella typhimurium strain TA98 using different concentrations of microsomal protein in the metabolic activation system (S9-mix, with S9-fraction from liver of Aroclor-1254-treated rats: 2, 4 and 10% (v/v), respectively). Whereas a positive mutagenic response is found for 1-4 in the presence of S9-mix, 5-8 exert no mutagenicity either with or without S9-mix. Since for 1-4 the highest response is observed with S9-mix 2% (v/v) instead of the standard 4% (v/v), a one-step activation pathway, i.e. epoxidation of the five-membered ring olefinic bonds, appears to be operational. Surprisingly, 3 and, to a lesser extent, 2 (11.7 versus 4.2 His revertants/nmol) also give a positive response in the absence of S9-mix. Hence, 2 and 3 are expected to contribute to the direct-acting mutagenicity of the non-polar fraction of combustion exhausts. Presumably for the direct-acting mutagenicity one-electron transfer processes play a role in bioactivation. The experimental observations are supported by semi-empirical AM1 calculations on the possible ultimate metabolites, i.e. mono-epoxides (2a-4a), cis-di-epoxides (2b-4b) and trans-di-epoxides (2c-4c) and the related mono-hydroxy carbocations (2d-4d and 2e-4e), and the radical anions 1 •− -4 •− .

Di-epoxides of the three isomeric dicyclopenta-fused pyrenes: ultimate mutagenic active agents

“…one-step epoxidation of the five-membered ring olefinic bond. This was also observed for compounds 2-4 [21]. The results suggested that the ultimate mutagenic active forms in the presence of S9-mix are the di-epoxides 6-8 (Fig.…”

“…2-5 the dose-response curves are shown for the direct-acting (−S9-mix) and metabolic activation-dependent mutagenicity (+S9-mix) for the (di-)epoxides 5-8. The results are compared with those previously found for CP-PAH 1-4 in the same dose range (0.0-2.0 g/plate) and under similar conditions [21]. The specific mutagenic activity of 1-8, viz.…”

“…the number of His revertants/nmol of test compound, is reported in Table 2. Note that the bacterial mutagenic response of the CP-PAH 1-4 [21] and their (di-)epoxides 5-8 was assayed using the same S. typhimurium TA98 frozen permanent and the same microsomal protein fraction (S9) for the exogenous metabolic activation mixture (see Section 2.2).…”

“…We recently reported the bacterial mutagenic response of 2-4 in the bacterial microsome/histidine reverse mutation assay (Salmonella typhimurium strain TA98) [21] according to the standard protocol by Ames and coworkers [22,23]. The number and the topology of the cyclopenta moieties along the pyrene perimeter were found to markedly affect the mutagenic response of 2-4.…”

“…The importance of the olefinic bonds in the five-membered rings for bio-activation, presumably via di-epoxide formation, was determined by assessing the mutagenic response of the corresponding dihydro derivatives of 2-4 in which the five-membered rings contain a saturated instead of an olefinic carbon-carbon bond. These partially saturated CP-PAH were found to be nonmutagenic both in the absence or presence of exogenous metabolic activation mixture (±S9-mix) [21]. A typical feature of CP-PAH is their low protein content requirement in the exogenous metabolic activation mixture (S9-mix) to exert maximal mutagenic response, which is indicative of a one-step metabolic activation pathway [7,8,24], viz.…”