Proximate Mutagen Research Articles

Identification of the mutagenic metabolites of fluoranthene, 2-methylfluoranthene, and 3-methylfluoranthene.

The metabolites of fluoranthene, 2-methylfluoranthene, and 3-methylfluoranthene obtained upon incubation with liver homogenate from Aroclor pretreated rats were assayed for mutagenicity in Salmonella typhimurium TA100. The mutagenic metabolites of fluoranthene and 2-methylfluoranthene were identified as 2,3-dihydro-2,3-dihydroxyfluoranthene and 4,5-dihydro-4,5-dihydroxy-2-methylfluoranthene, respectively. In contrast to these results, the major proximate mutagen detected among the in vitro metabolites of 3-methylfluoranthene was 3-hydroxymethylfluoranthene. Comparison of the mutagenic potential of 2-hydroxymethylfluoranthene demonstrated that the latter was a more powerful mutagen. Quantitative analyses of the metabolites of fluoranthene with that of 3-hydroxymethylfluoranthene demonstrated that the latter was a more powerful mutagen. Quantitative analyses of the metabolites of fluoranthene, 2-methylfluoranthene, and 3-methylfluoranthene indicated that similar amounts of dihydrodiols were formed. 4,5-Dihydro-4,5-dihydoxy-3-methylfluoranthene, however, was not found to be a potent mutagenic metabolite. These data suggest that the activation pathway to ultimate mutagens may differ for 2- and 3-methylfluoranthene.

Abstracts of papers presented at the Twenty-first Annual Meeting of the Japanese Teratology Society. Kumamoto, Japan, July 10-11, 1981.

Cyclophosphamide must be enzymatically activated to be either mutagenic or teratogenic. This activation is thought to be catalyzed by the cytochrome P-450 monooxygenase system. To study the relationship between the mutagenic and teratogenic metabolites of cyclophosphamide, the mutagenicity and teratogenicity of this drug were compared after activation by rats pretreated with chemicals (phenobarbital and beta-naphthoflavone) inducing different cytochromes P-450. Activation of cyclophosphamide to mutagenic metabolites by enzyme fractions from rats on day 13 of gestation was measured with the Ames test using Salmonella typhimurium TA1535. Teratogenicity was assessed in vivo by treatment of rats with cyclophosphamide on day 13 of gestation. Cyclophosphamide was activated to mutagenic metabolites to the same extent (on a tissue wet weight basis) by enzyme fractions from maternal liver, kidney and placenta, despite differences in cytochrome P-450 content. Fetal homogenates did not activate cyclophosphamide. Phenobarbital pretreatment increased the activation of cyclophosphamide to mutagenic metabolites by maternal liver microsomes 10-fold and liver cytochrome P-450 content 1.8 fold; however, this drug did not alter the activation of cyclophosphamide by maternal kidney, by placenta or by the fetus. Phenobarbital pretreatment increased the teratogenicity of cyclophosphamide in rats on day 13 of gestation (increased incidence of malformed embryos, decreased fetal weight). Pretreatment with beta-naphthoflavone did not induce liver cytochrome P-450 in the pregnant rat and did not change the activation of cyclophosphamide to mutagenic metabolites by liver, kidney, placenta or the fetus. Pretreatment with this polycyclic aromatic hydrocarbon had no effect or decreased the teratogenicity of cyclophosphamide. Thus, these experiments suggest that the mother, rather than the fetus, is the site of activation of cyclophosphamide; after phenobarbital pretreatment the predominant site of cyclophosphamide activation is the maternal liver. There appears to be a correlation between the teratogenicity and mutagenicity of cyclophosphamide after induction of the cytochromes P-450. We can speculate that the "proximate teratogen" of cyclophosphamide may also be the "proximate mutagen".

Isolation of mutants of Escherichia coli with increased resistance to alkylating agents: mutants deficient in thiols and mutants constitutive for the adaptive response.

A search has been made for mutants of E. coli that are constitutive for the adaptive response to alkylating agents. When selection was for resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) the resulting strains all proved to be low in intracellular thiols. As MNNG has to be converted by thiols to the proximal mutagen methylnitrosamine, these strains were resistant simply because their low thiol levels made them less readily methylated by MNNG. The selection was therefore undertaken for strains resistant to N-methylnitrosourea (MNU) because this alkylating agent does not require activation by thiols. Four MNU resistant mutants proved to be constitutive for the adaptive response. Following exposure to (3H) MNNG, they showed the same low ratio of O6-methylguanine (O6MeG) to 7-methylguanine (7MeG) as adapted wild type bacteria. The ratio of 3-methyladenine (3MeA) to 7MeG was the same in the wild type, the adapted wild type, and the constitutive strains. When exposed to very high MNNG doses, the constitutive mutants were even more resistant to mutation and killing than the adapted wild type. They have an even greater ability than the adapted wild type to remove O6MeG from methylated DNA (Lindahl personal communication).

Specific locus mutations induced in somatic cells of rats by orally and parenterally administered procarbazine.

A new test, the granuloma pouch assay, was used in detecting specific locus mutations in somatic cells of rats in vivo after the animals were treated orally and parenterally with procarbazine hydrochloride, an agent used in cancer chemotherapy. The results indicate that stable intermediates are formed in the body and distributed as proximate mutagens.

A study of chemical carcinogenesis. 19. Synthesis and mutagenicity of 5,11-dimethylchrysene and some methyl-oxidized derivatives of 5-methylchrysene

A series of compounds structurally related to the carcinogen and mutagen 5-methylchrysene (1) was synthesized and tested for mutagenicity toward S. typhimurium TA 100. The compounds prepared were 5,11-dimethylchrysene (2), 5-(hydroxymethyl)chrysene (3), 5-(acetoxymethyl)chrysene (4), 5-carbomethoxychrysene (5), 5-(hydroxymethyl)-1,2,3,4-tetrahydrochrysene (6), 5-carbomethoxy-1,2,3,4-tetrahydrochrysene (7), and 5H-chryseno[4,5-bcd]pyran-5-one (31). When tested in the presence of rat liver homogenate, 1 and 2 were active while 3--7 were less mutagenic than 1; 31 was highly mutagenic. The mutagenicity of 1 and 2 contrasts with the low activity of 5,12-dimethylchrysene, which supports the generalization that the structural requirements favoring activity are a bay-region methyl group and a free peri position, both adjacent to an unsubstituted angular ring. The low activity of 3--7 indicates that methyl oxidation is not an important activation process for 1. This agrees with previous studies in which the major proximate mutagen and carcinogen of 1 was identified as 1,2-dihydro-1,2-dihydroxy-5-methylchrysene.

Novel activation mechanism for the promutagenic herbicide diallate.

The potent bacterial mutagen 2-chloroacrolein is formed from the carcinogenic herbicide S-2,3-dichloroallyl diisopropylthiocarbamate (diallate) on incubation with hepatic microsomal monooxygenases or on reaction with m-chloroperbenzoic acid. A proposed activation mechanism for this promutagen involves sulfoxidation followed by [2,3] sigmatropic rearrangement and 1,2-elimination reactions. A portion of the highly reactive intermediate, diallate sulfoxide (proximate mutagens), is attacked by glutathione in a reaction which competes with its transformation to the ultimate mutagen, 2-chloroacrolein.