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2-Nitropropane (2-NP) is a rat liver carcinogen, whilst the 1-isomer is noncarcinogenic in rodents. Although DNA repair tests in the rat liver discriminated clearly between the carcinogenic and the non-carcinogenic isomer, uniformly negative results have been published for the mouse bone marrow micronucleus test (BMMN test) with both isomers. Therefore, the latter assay did not discriminate between the carcinogenic and the noncarcinogenic isomer. To investigate whether this is due to endpoint specificity or organospecificity of 2-NP, studies were carried out in the rat in which micronucleus induction (bone marrow and liver) and unscheduled DNA synthesis (UDS) induction (liver) were measured after oral treatment with either nitropropane isomer. 2-NP induced UDS in the liver whilst the 1-isomer was negative, thus confirming the published studies. In the BMMN test, occasional small increases in the incidence of micronuclei were found for both compounds, but results were interpreted as negative after considering the control background data and the lack of reproducibility. By contrast, the liver micronucleus test revealed a clastogenic effect of 2-NP in the liver. This indicates that 2-NP induces chromosome aberrations as well as DNA repair , but it seems to act organospecifically. For 1-NP a slightly increased incidence of micronuclei was found in the liver, which was accompanied by a markedly increased mitotic index. It therefore remains questionable as to whether this increased micronucleus frequency for 1-NP is an indicator of a clastogenic effect, or whether it is caused by an increased cell proliferation induced by 1-NP. Consequently, it is too early to conclude whether the liver micronucleus assay is able to discriminate between the carcinogenic and non-carcinogenic isomer. However, the results provide further evidence that bone marrow assays are insufficient for the detection of all genotoxic carcinogens . This indicates the need for analysing a second tissue, pzrticularly when negative bone marrow results have been obtained with genotoxins. [ABSTRACT FROM PUBLISHER]

The most widely used assays for detecting chemically induced gene mutations are those employing bacteria. The plate incorporation assay using various Salmonella typhimurium LT2 and E. coli WP2 strains is a short-term bacterial reverse mutation assay specifically designed to detect a wide range of chemical substances capable of causing DNA damage leading to gene mutations. The test is used worldwide as an initial screen to determine the mutagenic potential of new chemicals and drugs.The test uses several strains of S. typhimurium which carry different mutations in various genes of the histidine operon, and E. coli which carry the same AT base pair at the critical mutation site within the trpE gene. These mutations act as hot spots for mutagens that cause DNA damage via different mechanisms. When these auxotrophic bacterial strains are grown on a minimal media agar plates containing a trace of the required amino-acid (histidine or tryptophan), only those bacteria that revert to amino-acid independence (His(+) or Tryp(+)) will grow to form visible colonies. The number of spontaneously induced revertant colonies per plate is relatively constant. However, when a mutagen is added to the plate, the number of revertant colonies per plate is increased, usually in a dose-related manner.This chapter provides detailed procedures for performing the test in the presence and absence of a metabolic activation system (S9-mix), including advice on specific assay variations and any technical problems.