ABSTRACT18 Carcinogens, including aflatoxin Bi, benzo(a)pyrene, acetylaminofluorene, benzidine, and dimethylamino-trans-stilbene, are shown to be activated by liver homogenates to form potent frameshift mutagens. We believe that these carcinogens have in common a ring system sufficiently planar for a stacking interaction with DNA base pairs and a part of the molecule capable of being metabolized to a reactive group: these structural features are discussed in terms of the theory of frameshift mutagenesis. We propose that these carcinogens, and many others that are mutagens, cause cancer by somatic mutation. A simple, inexpensive, and extremely sensitive test for detection of carcinogens as mutagens is described. It consists of the use of a rat or human liver homogenate for carcinogen activation (thus supplying mammalian metabolism) and a set of Salmonella histidine mutants for mutagen detection. The homogenate, bacteria, and a TPNHgenerating system are all incubated together on a petri plate. With the most active compounds, as little as a few nanograms can be detected.We have previously described the use of a set of mutants of Salmonella typhimurium for detecting and classifying chemical mutagens with great simplicity and sensitivity (1, 2). With this test we have also shown that the active forms of a large number of known carcinogens are mutagens (1-5). The active forms of carcinogens such as aflatoxin, polycyclic hydrocarbons, dimethylnitrosamine, and various aromatic amines are formed by mammalian metabolism, in particular by the TPNH-dependent microsomal enzymes of liver (6-11). The principal limitation of any bacterial system for detecting carcinogens as mutagens is that bacteria do not duplicate mammalian metabolism in activating carcinogens. Mammalian-liver homogenates have been used by Garner et al., (6) to activate aflatoxin B1 to a compound lethal to our bacterial tester strain lacking excision repair, by Malling (12) to activate dimethylnitrosamine to a compound that reverts one of our bacterial tester strains, and by Slater et al. (13) to activate dimethylnitrosamine to a compound lethal for bacteria lacking polymerase I. In this study we have extended this work and shown that carcinogens can be detected as mutagens simply and with great sensitivity by incubation of the carcinogen, a rat or human liver homogenate, and our bacterial tester strain together on a petri plate. MATERIALS AND METHODSCompounds. Glucose-6-phosphate, TPN, TPNH, and 2-naphthylamine were obtained from Sigma. Benzo(a)pyrene, 2-acetylaminofluorene, and benzidine were from Aldrich. DiAbbreviation: Me2SO, dimethylsulfoxide. methylsulfoxide (Me2SO), spectrophotometric grade, was obtained from Schwarz/Mann, sodium phenobarbital from Mallinckrodt, aflatoxin B1 from Calbiochem, and 3-methylcholanthrene from Eastman; 7,12-dimethylbenz(a)anthracene was a gift of P. L. Grover. Schuchardt (Munich) was the source for the other carcinogens.Bacterial Strains used are mutants of S. typhimurium LT-2 and have been discussed in detail (2).Sourc...

We previously described a set of four strains of Salmonella typhimurium designed for detecting the various types of mutagens, and showed their utility in detecting a wide variety of carcinogens as mutagens. The lipopolysaccharide that normally coats these bacteria is a barrier to penetration of mutagens to the cell membrane. The set of tester strains has been improved by adding a mutation ( rfa: deep rough) that results in a deficient lipopolysaccharide. The techniques for using these strains for detecting mutagens are presented and the tests are shown to be extremely sensitive and convenient. The specificity of frameshift mutagenesis is clarified. As adjuncts to the test with the four strains, we describe a test that compares mutagenic killing in deep rough strains with and without DNA excision repair, and a test using forward mutagenesis in a deep rough strain lacking excision repair.