Biological activity of benzylating N-nitroso compounds. Models of activated N-nitrosomethylbenzylamine

Comparison of responses of base-specific Salmonella tester strains with the traditional strains for identifying mutagens: the results of a validation study

The ability of a TA7000 series of Salmonella his- mutant tester strains to detect mutagens as classified by the traditional tester strains (TA100, TA98, TA1535, TA1537, TA97, TA102 and TA104) was evaluated using 30 coded chemicals, 5 of which were duplicates with different code numbers. The TA7000 series of tester strains were TA7001, TA7002, TA7003, TA7004, TA7005 and TA7006, each of which reverts by a specific base substitution. In addition, each chemical was tested in a mixture of the base-specific strains (the Mix), plus the traditional strains, TA98 and TA1537. A liquid version of the Salmonella mutagenicity assay was performed in microtiter plates to allow partial automation for increased throughput. The results were compared to those in the National Toxicology Program (NTP) database, which were obtained from the traditional strains in the preincubation assay. In the two strains common to both protocols, TA98 and TA1537, the agreement was 80% and 85%, respectively. When compared to the NTP results for TA100, the Mix gave a 72% concordance, while the addition of the frameshift tester strain, TA98, increased the agreement to 76%. The overall agreement on positive or negative classifications of mutagenicity was 88% for the 25 chemicals tested. There were three notable exceptions to the overall agreement. Benzaldehyde was detected as a mutagen in TA7005 in contrast to its classification as a non-mutagen in the NTP database. This does not necessarily contradict the NTP results because the base-specific strains may respond to different mutagens. Two weak mutagens in the NTP database, 1-chloro-2-propanol and isobutyl nitrite, were not detected as mutagens in the base-specific new strains in the liquid protocol. While there are a number of major differences in the two assays, it was concluded that the results from each procedure are comparable.

Esophageal carcinogenesis in the rat: a model for aerodigestive tract cancer

A number of chemical carcinogens have been used to study the process of esophageal carcinogenesis. Among the most prominent of these models is the induction of cancer of the esophagus in the rat by the nitrosamine N-nitrosomethylbenzylamine (NMBA). In the rat, tumors can occur within 15 weeks of carcinogen administration. The rat model has been used to investigate the mechanism of action of several chemopreventive agents. Among these, the garlic-derived agent diallyl sulfide has been shown to be a specific inhibitor of NMBA metabolism. Other investigators have used the model to seek out the relationship of dietary factors and alcohol in esophageal tumorigenesis. With striking histologic parallels to human esophageal carcinoma, the NMBA model provides useful information to study this cancer.

N-nitrosamines: bacterial mutagenesis and in vitro metabolism

Many nitrosamines are potent mutagens. The rate-limiting step in their in vitro metabolism to mutagens is usually a single enzymatic reaction catalyzed by one or more of the many cytochrome P-450-dependent mixed-function oxidases present in the microsomal cell fraction. Current evidence indicates that this reaction activates nitrosamines to alpha-hydroxynitrosamines, which have half-lives on the order of seconds. This product decomposes to an aldehyde and a much shorter-lived ultimate metabolite which is probably an alkyl diazonium ion or an alkyl carbocation. This may react with DNA leading to premutagenic adducts. Such adducts represent a very small fraction of the ultimate mutagen, with the rest reacting with water to yield the corresponding alcohol. Evidence for this pathway includes (1) the observation of deuterium isotope effects in metabolism and mutagenesis, (2) products (aldehydes, alcohols, and N2) consistent with this pathway, (3) studies on metabolism of nitrosamines using purified cytochrome P-450, (4) formation of DNA adducts such as O6-alkylguanines which are consistent with those expected from the ultimate mutagen, (5) expected products and genotoxic effects of other sources of activated nitrosamines, e.g., alpha-acetoxynitrosamines, alkanediazotates and related compounds. Hydroxylation of nitrosamines at other positions also occurs in vitro (usually to a lesser extent), but these products are generally stable and must be further metabolized to exert mutagenic effects (with the exception of N-nitrosoalkyl(formylmethyl)amines, which are direct-acting mutagens). Because only low percentages of nitrosamines are metabolized in vitro, the contribution to mutagenesis by secondary metabolism is small. In this respect, in vitro metabolism can differ significantly from in vivo metabolism. Bacterial mutagenesis by nitrosamines has most often been studied in Salmonella typhimurium and to a lesser extent E. coli. Mutagenesis by nitrosamines generally requires a source of microsomes (a 9000 X g supernatant fraction is often used), and NADPH. Liver fractions from Aroclor-1254- or PB-induced rodents have been most frequently employed but liver fractions from untreated animals, and homogenates of other organs (lung, kidney, nasal mucosa, and pancreas) have also been utilized. Liver homogenates from humans are generally similar to those from untreated rats in metabolizing nitrosamines to mutagens but large interindividual variations are observed. Mutagenesis is often most effective using a liquid preincubation, a slightly acidic incubation mixture and hamster liver fractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Use of diethyldithiocarbamate as a probe to detect stable intermediates during the decomposition of several mutagenic and nonmutagenic N-nitroso compounds

By showing that methyldiethyldithiocarbamate is formed from the reaction of methylnitrosourea and disulfiram, we demonstrated in previous experiments that one of the anticarcinogenic/antimutagenic mechanisms of disulfiram is the scavenging of reactive species. We propose that this reaction may be employed additionally as a model for elucidating the following: (a) possible reactions between alkylating species and nucleophilic sites within the cell, and (b) the existence of stable intermediates during the metabolism of N-nitroso compounds. With structurally related pairs of nitrosoureas (n-propyl/isopropyl; cyclopropyl/allyl; 2-phenylethyl/l-phenylethyl), for which each alkylating group of the first compound can spontaneously rearrange to form the alkylating group of the second isomer, we investigated whether the alkylation proceeds via a monomolecular (sn1) or a bimolecular substitution (sn2). For this, we comparatively determined the relative mutagenic activities of each isomer in Salmonella typhimurium TA 1535, as well as their reactivities towards diethyldithiocarbamate (DDTC) by identifying the reaction products. These studies were aimed at revealing the possible formation of a free carbonium ion in the decomposition of several nitrosoureas in the rat liver supernatant fraction. Our system showed that DDTC reacts by two competing mechanisms: attack at the diazonium ion and at the free carbonium ion. Therefore the striking differences which were observed in the mutagenic potency of cyclopropylnitrosourea and N-nitrosoallylurea as well as of N-nitroso-2-phenylethylurea and N-nitroso-1-phenylethylurea cannot be explained only by the different electrophilic reactivities of the respective intermediates.