Mutagenicity testing with Salmonella microsome test

History of toxicology at the Technical University of Munich (TUM)

Toxicology at the TUM is mainly associated with the Faculty of Medicine at the Klinikum rechts der Isar (MRI). The Department of Clinical Toxicology has been founded in 1963. Max von Clarmann, the head, focused his activities on the treatment of intoxications and the development of analytical methods and established a poison information center. His successors, Thomas Zielker and Florian Eyer, further developed this department to an internationally renown institution.In 1967, the MRI became the TUM faculty of medicine with its Institute of Pharmacology and Toxicology. The director Melchior Reiter, formerly Institute of Pharmacology of the Ludwig Maximilians University (LMU), in 1970 initiated the foundation of the Department of Toxicology at the Gesellschaft für Strahlen- und Umweltforschung (GSF) with the director Gerhard Lange. The research focused on the neurotoxic effects of heavy metals and the metabolism and hepatoxicity of persistent chemicals. After Lange's unexpected death in 1973, he was succeeded in 1975 by Helmut Greim from the University of Tübingen. The now Institute of Toxicology rapidly expanded developing and standardizing in vitro test methods, investigating the mechanism of carcinogens and mutagens and heavy metal toxicity. Training courses in the 15 major areas of toxicology have been organized at the GSF and competent centers in Germany. In 1987, Greim became the director of the newly founded Institute of Toxicology and Environmental Hygiene of the TUM, with expanded research and teaching activities, especially in toxicology at the faculties of Chemistry of the TUM and LMU, which thereafter became mandatory for students of chemistry at German universities.

Liver subcellular fractions from rats treated by organosulfur compounds from Allium modulate mutagen activation

The effects of in vivo administration of naturally occurring organosulfur compounds (OSCs) from Allium species were studied on the activation of several mutagens. Male SPF Wistar rats were given p.o. one of either diallyl sulfide (DAS), diallyl disulfide (DADS), dipropyl sulfide (DPS) or dipropyl disulfide (DPDS) during 4 consecutive days and the ability of hepatic S9 and microsomes from treated rats to activate benzo[a]pyrene (BaP), cyclophosphamide (CP), dimethylnitrosamine (DMN), N-nitrosopiperidine (N-PiP) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was determined in the Ames test. Administration of DAS, DPS and DPDS resulted in a significant increase of the activation of BaP, CP, N-PiP and PhIP mediated by S9 and microsomes while DADS treatment only increased the mutagenicity of PhIP. In contrast, S9 from DADS-treated rats significantly inhibited the mutagenicity of N-PiP and BaP. DAS, DADS and DPS strongly inhibited DMN mutagenicity while DPDS enhanced it. To understand the mechanisms underlying these effects, the modifications of the activities of specific isozymes of CYP involved in the activation of these mutagens were studied. DAS, DPS and DPDS strongly enhanced pentoxyresorufin O-dealkylase (PROD) activity related to CYP2B and slightly increased ethoxyresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase (MROD) activities related to CYP1A family. DADS exerted the same effects than other OSCs but to a lesser extent. p-Nitrophenol hydroxylase (PNPH) activity related to CYP2E1 was inhibited by DAS and DADS, whereas DPDS significantly increased this activity. Hence, the effects of OSCs on the mutagenicity of several genotoxic compounds are mediated by modification (enhancement or inhibition) of specific CYP involved in their activation.

Hyperketonaemia markedly modulates the metabolic activation of chemical carcinogens

Male Wistar albino rats were rendered hyperketonaemic by oral administration of medium chain triacylglycerols or by a single intraperitoneal injection of the diabetogenic agent streptozotocin. Hepatic post-mitochondrial preparations from these animals were employed as activation systems in the Ames mutagenicity test. Activation systems from both groups of hyperketonaemic rats were more efficient than those of control rats in metabolically converting the precarcinogens Glu-P-1, Trp-P-1, Trp-P-2, N-nitrosopiperidine and N-nitrosopyrrolidine to mutagens. In contrast, when 2-aminofluorene was used as the precarcinogen, the preparations from the hyperketonaemic animals were less efficient than controls in activating this carcinogen. In all cases, the preparations from streptozotocin-treated rats displayed a more pronounced effect than those from triacylglycerol-treated rats, possibly reflecting the greater extent of hyperketonaemia in the former group. It is concluded that hyperketonaemia modulates the bioactivation of chemical carcinogens.

[Biotransformation of benzamidine and benzamidoxime by microsomal enzymes of the rabbit]

At pH 7.4 neither benzamidine (1) is ring-hydroxylated nor benzamidoxime (2) is N-hydroxylated, reduced or ring-hydroxylated by aerobic incubations with microsomal fractions (12000 g supernatant, microsomes) of rabbit liver homogenates and NADPH. Products of hydrolytic processes are also not detected. A very long incubation period and a pH 6.3 are necessary for the detection of a slight reduction of benzamidoxime (2) to benzamidine (1). Results are obtained by use of synthetic reference material and by newly developed HPLC methods. Thus, kinetic studies of the microsomal N-hydroxylation of benzamidine (1) to benzamidoxime (2) in the presence of N-methylbenzamidine (3) performed at pH 7.4 are not influenced by other transformations and provide evidence for the involvement of the same isoenzyme of cytochrome P-450 for both the N-hydroxylation of 1 and the N-dealkylation of 3.

Hepatic microsomal mixed-function oxidase activity in ethanol-treated hamsters and its consequences on the bioactivation of aromatic amines to mutagens

Male golden Syrian hamsters were maintained on ethanol-containing liquid diets for 4 weeks, corresponding to an average daily intake of 17 g/kg body wt. The p-hydroxylation of aniline was markedly enhanced by this treatment while minimal effects were seen in benzphetamine N-demethylase and ethoxyresorufin O-deethylase activities; there was no change in the microsomal levels of cytochromes P-450. Hepatic microsomal preparations from the ethanol-treated hamsters were more efficient than controls fed isocaloric diets in converting 2-aminofluorene, 4-aminobiphenyl, benzidine and 2-acetylaminofluorene into mutagens in the Salmonella mutagenicity test. The same treatment had no effect on the metabolic activation of 2-naphthylamine and even inhibited the mutagenicity of 2-aminoanthracene. No increase was seen in the activation of the two polycyclic aromatic hydrocarbons, benzo[a]pyrene and 3-methylcholanthrene to mutagens and an inhibitory effect was seen with the former. The ethanol-induced increase in the mutagenicity of 2-aminofluorene was inhibited by 2-butanol but not by the hydroxyl radical scavenger dimethylsulphoxide. It is concluded that chronic ethanol ingestion modulates the bioactivation of aromatic amines and amides to mutagens, the effect being substrate dependent. This effect of ethanol may be catalysed by unique form(s) of cytochrome P-450 whose synthesis is induced by such treatment.

Induction of rat hepatic mixed function oxidases by aromatic amines and its relationship to their bioactivation to mutagens

Hepatic microsomal mixed-function oxidase activities were determined in rats pretreated with the aromatic amines 2-aminoanthracene, 2-naphthylamine or 4-aminobiphenyl. All three amines stimulated the O-deethylation of ethoxyresorufin (cytochromes P-448) but none had any effect on the p-hydroxylation of aniline. 2-Aminoanthracene and 4-aminobiphenyl also stimulated the NADPH-dependent reduction of cytochrome c and 2-naphthylamine inhibited the N-demethylation of benzphetamine. Hepatic preparations from animals pretreated with 2-aminoanthracene were more efficient in converting this carcinogen to mutagens while in contrast pretreatment with Aroclor 1254 caused a marked decrease in mutagenicity. 4-Aminobiphenyl also enhanced its own activation but Aroclor-pretreated preparations were the most effective. The latter preparations were also more efficient than controls in activating 2-naphthylamine to mutagens. It is concluded that 4-aminobiphenyl and 2-aminoanthracene enhance their own activation at least partly, by inducing the synthesis of cytochromes P-448.

Metabolism of 2-aminofluorene by rat small-intestine fractions: differential effect of intragastric versus intraperitoneal administration of 3-methylcholanthrene

The influence of 3-methylcholanthrene (3-MC) induction on small intestinal enzymes was studied by measuring the enzymatic characteristics of 2-aminofluorene N-hydroxylase and the activation of this aromatic amine into mutagen(s) towards Salmonella typhimurium. When injected intraperitoneally to rats, 3-MC modified neither the N-hydroxylation nor the mutagenic properties of 2-aminofluorene (2-AF) in the presence of postmitochondrial and microsomal fractions from small intestine. On the other hand, the intragastric administration of the inducer slightly increased affinity and significantly induced maximum velocity of N-hydroxylase. Moreover, the ability of the small intestinal fractions to activate 2-AF into mutagenic intermediate(s) was enhanced after intragastric application. The mode of administration seems to be crucial in the induction of small intestinal enzymes required for metabolic and mutagenic activation of carcinogenic aromatic amines such as 2-AF.

Mutagenicity of some derivatives of dipyrido[1,2-a:3',2'-d]imidazoles in Salmonella typhimurium with metabolic activation by rat liver and small intestine subcellular fractions

The mutagenic effect of 2-amino-dipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2) was compared with that of the 3-amino, 3-nitro, or 3-N-hydroxylated derivatives of the same base ring with methyl groups at positions 4 and 6 of the molecule. The compounds were tested in Salmonella typhimurium strain TA98 without metabolic activation and in the presence of different concentrations of subcellular fractions from livers or small intestines of rats pretreated with different P448/P450 inducers. The 4,6-dimethyl compounds are always more mutagenic than Glu-P-2. Pretreatment with Aroclor 1254 (ARO) is the most effective inducer in the activation of the 2- and 3-amino compounds by liver S9, whereas the same fraction decreases the mutagenicity of the 3-nitro derivative. S9 from small intestine increased the mutagenic effect of the 3-nitro and 3-N-hydroxylated compounds, but it was unable to activate the amino compounds.

Activation of aromatic amines to mutagens by various animal species including man

2-Acetylaminofluorene, 2-aminofluorene, 4-aminobiphenyl, 2-naphthylamine, 2-aminoanthracene and benzidine were assayed for mutagenicity in the Ames test in the presence of hepatic microsomal preparations derived from mouse, hamster, rat, pig and man. Prior to each mutagenicity assay all activation systems were fully characterized with respect to mono-oxygenase and mixed-function amine oxidase activities. All compounds were metabolically activated to mutagens by all activation systems, but with markedly different efficiencies, hamster being the only species which readily activated all amines. The hamster also exhibited the highest ethoxyresorufin O-deethylase and dimethylaniline N-oxidase activities.

Evaluation of a methodology for the use of preparations from rat small intestine in the Salmonella/microsome assay

A methodology is evaluated for the use in the Ames assay of a microsomal metabolising system derived from villous tip cells of rat small intestine. The procedure involved high frequency vibration of everted gut segments followed by gentle lysis and homogenisation. This technique, which has previously been shown to result routinely in high levels of cytochrome P450 and linked enzymes, has now been investigated for its ability to yield preparations capable of activating several promutagens in the Salmonella/plate incorporation test. The data obtained have been compared with results observed with standard rat liver metabolising fractions. In the presence of intestinal microsomes, 2-aminoanthracene, 2-aminofluorene, 2-acetylaminofluorene, aflatoxin B1, benzo[a]pyrene and cyclophosphamide all caused dose-related increases in revertants, the maximum yields of which were lower than those detected with liver microsomes or S9 mix. These and other differences in dose-responses have been discussed in relation to the levels of microsomal protein and cytochrome P450 plated and with respect to the activities of relevant enzymes in the tissue extracts.

Comparison of liquid-liquid extraction and resin adsorption for concentrating mutagens in Ames Salmonella/microsome assays on water

The Ames Salmonella/microsome mutagenicity assay (AMES et al. 1975) is a relatively simple, cheap and rapid test for mutagenic activity (GREIM et al. 1980, SOBELS 1980). Although there are uncertainties about the health implications of mutagens, the Ames test detects 85 to 93% of known chemical carcinogens and is considered an essential part of the minimal battery of bioassays required in studies on environmental compounds which may damage chromosomal material (GREIM et al. 1980, SOBELS 1980). Since water may play an important role in the transmission of environmental carcinogens, which are responsible for an estimated 50 to 90% of human cancer cases, the Ames test is being used to determine the incidence of potential carcinogens in water supplies and their removal or formation by water treatment processes (HOOPER et al. 1978, NESTMANN et al. 1979, RAPPAPORT et al. 1979, SAXENA & SCHWARTZ 1979, SCHWARTZ et al. 1979, DENKHAUS et al. 1980, GRABOW et al. 1980, LOPER 1980). In many waters, particularly drinking-water supplies, the concentration of mutagens is generally too low for direct detection by the Ames test, and a wide variety of methods are being used to concentrate mutagens from large volumes of water (HOOPER et al. 1978, NESTMANN et al. 1979, RAPPAPORT et al. 1979, SCHWARTZ et al. 1979, GRABOW et al. 1980, LOPER 1980, GRIMM-KIBALO et al. 1981).