The E. coli Pol A-1 assay: a quantitative procedure for diffusible and non-diffusible chemicals

Simplified modeling of E. coli mortality after genome damage induced by UV-C light exposure

UV light is a group of high-energy waves from the electromagnetic spectrum. There are three types of UV radiations: UV-A, -B and -C. UV-C light are the highest in energy, but most are retained by the ozone layer. UV-A and -B reach the earth's surface and cause damage on living organisms, being considered as mutagenic physical agents. Numerous test models are used to study UV mutagenicity; some include special lamps, cell cultures and mathematical modeling. Mercury lamps are affordable and useful sources of UV-C light due to their emission at near the maximum absorption peak of nucleic acids. E. coli cultures are widely used because they have DNA-damage and -repairing mechanisms fairly similar to humans. In here we present two simple models that describe UV-C light incidence on a genome matrix, using fundamental quantum-mechanical concepts and considering light as a particle with a discontinuous distribution. To test the accuracy of our equations, stationary phase cultures of several E. coli strains were exposed to UV-C light in 30 s-intervals. Surviving CFUs were counted and survival/mortality curves were constructed. These graphs adjusted with high goodness of fit to the regression predictions. Results were also analyzed using three main parameters: quantum yield, specific speed and time of mortality.

Disposition of inhaled 1-chloro-2-propanol in F344/N rats

Propylene chlorohydrins, of which 1-chloro-2-propanol (1-CP) is a constituent, used as intermediates in the manufacture of propylene oxide and have been identified as potential air pollutants. The objective of these studies was to determine whether changes in the inhaled exposure concentration would affect the disposition of 1-CP in rats. In addition, experiments were conducted to identify the carbon atom of 1-CP that is metabolized to CO2. Rats were exposed nose-only to [14C]1-CP for 6 hr to 8.3 +/- 1.0 ppm (26.1 +/- 3.2 micrograms/liter air) or 77 +/- 4 ppm (245 +/- 13 micrograms/liter air) (mean +/- SE). There were two major routes of elimination of 14C, urinary and exhalation of CO2, which together accounted for about 80% of the total 14C in excreta and carcass. Half-times for elimination of 14C in urine as 14CO2 were between 3 and 7 hr with no effect of exposure concentration on the elimination half-times for either route. After the end of exposure, kidneys, livers, trachea, and nasal turbinates contained high concentrations of [14C]1-CP equivalents at both exposure concentrations (30-50 nmol 14C/g tissue for the 8 ppm exposure level and 200-350 nmol 14C/g tissue for the 80 ppm exposure level). Elimination of 14C from tissues was biphasic with about 50% of the material in a tissue being rapidly eliminated with a half-time of 1 to 3 hr and the remaining material slowly eliminated with a half-time of 40 to 80 hr. There was no effect of exposure concentration on elimination half-times in tissues. Major metabolites detected in urine and tissues (liver, kidney, and lung) were N-acetyl-S-(hydroxypropyl)cysteine and/or S-(2-hydroxypropyl)-cysteine. Little unmetabolized 1-CP (less than 1%) was detected in analyzed tissues or urine. We propose a metabolic scheme in which the major pathway for metabolism of 1-CP is to CO2 (which is exhaled) and to cysteine conjugates and mercapturic acids that are excreted in the urine. Both carbon-2 and carbon-3 are metabolized in part to CO2.

Genotoxicity of amebicide and anthelmintic drugs in Escherichia coli pol A+/pol A-

The amebicides dehydroemetine and chloroquine diphosphate and the anthelmintic pyrvinium pamoate, previously reported to be mutagenic in Salmonella typhimurium (Cortinas de Nava et al., 1983), were clearly shown to be genotoxic in the Escherichia coli pol A+/pol A- assay. Two other antiparasitic drugs, diiodohydroxyquin and 4-hexylresorcinol, were also found to be genotoxic in E. coli, while iodochlorhydroxyquin preferentially inhibited the pol A+ strain. From the 3 alternative testing methods employed, the liquid suspension succeeded in detecting 5 antiparasitic drugs as genotoxic; the microsuspension identified 2, and the disc diffusion method only 1. However, the metabolic activation system could only be coupled successfully and in a reproducible way to the microsuspension assay.

The Escherichia coli WP2/WP100 rec assay for detection of potential chemical carcinogens

46 chemicals of various classes and structures, including 30 known animal carcinogens, were evaluated for genotoxic effects using the Escherichia coli rec assay with strains WP2 (wild-type) and WP100 (uvrA- recA-) in qualitative and quantitative spot tests and in quantitative suspension tests. The rec assay detected 17 of 30 known carcinogens as genotoxic agents, including mitomycin C and diethylnitrosamine, both negative in the Salmonella/Ames test as utilized in these studies. The rec assay in conjunction with the Salmonella/Ames test detected 20 of 30 known carcinogens as genotoxic agents. Azo/aminoazo carcinogens showed little gentoxicity, and the aromatic amine 2-acetylaminofluorene was non-genotoxic in the rec assay. The rec assay was more effective than pol tests with E. coli strains W3110/p3478 and strains WP2/WP67. Effectiveness of the rec assay was related to the DNA repair-defective nature of the uvrA- recA- genotype of strain WP100.

Mutagenicity of chloroalkene epoxides in bacterial systems

6 alpha-chloroepoxides have been tested for in vitro activity in a variety of systems. The epoxides were cis- and trans-1-chloropropene oxide, cis- and trans-1,3-dichloropropene oxide, trichloroethylene oxide and tetrachloroethylene oxide. The epoxides were assayed for mutagenicity in the absence of metabolic activation in S. typhimurium TA1535 and E. coli WP2 uvrA and for preferential inhibition of growth of DNA-repair-deficient E. coli. All 6 epoxides possessed DNA-modifying activity as evidenced by their ability to preferentially inhibit DNA polymerase-deficient E. coli. All of the test chemicals except trichloroethylene oxide were mutagenic for S. typhimurium and all except trichloroethylene oxide and tetrachloroethylene oxide were mutagenic for E. coli Wp2 uvrA. Cis- and trans-1,3-dichloropropene oxide were the most potent mutagens and DNA modifiers. For all cases, the cis isomers were more active than the corresponding trans isomers. alpha-Chloroepoxides are considered likely to be the active intermediates of the carcinogenic parent halo-olefins. These mutagenicity studies are considered relevant in assessing the carcinogenicity of the parent hydrocarbons.

Comparative evaluation of different pairs of DNA repair-deficient and DNA repair-proficient bacterial tester strains for rapid detection of chemical mutagens and carcinogens

The aim of the present study was to evaluate the usefulness of different pairs of DNA repair-deficient and DNA repair-proficient bacterial tester strains in a mutagenicity/carcinogenicity screen, possibly as complements to the Ames test. 70 carcinogenic and non-carcinogenic compounds, representing a variety of chemical structures, were tested for their DNA-damaging effects, using 6 different DNA-repair-deficient bacterial strains. 2 Bacillus subtilis systems, H17/M45 and HLL3g/HJ-15, were used. The susceptibility of Escherichia coli AB1157 was compared with the susceptibility of 4 recombination-deficient mutants, JC5547, JC2921, JC2926 and JC5519. The test compounds were applied onto paper disks (spot test, ST), or incorporated into a top agar layer (agar-incorporation test, AT). The 2 B. subtilis systems were generally found to be more sensitive and reliable than the assays using E coli. The incorporation of the test compounds in the agar increased the sensitivity of the test for polycyclic aromatic hydrocarbons and other poorly water-soluble compounds. Hydrazines and several other highly polar chemicals could be tested more efficiently when applied onto paper disks. About 30% of the test compounds did not induce any growth inhibition and so could not be tested properly. In order to evaluate the ability of these DNA-repair tests to complement the Ames Salmonella mutagenicity test in a genetic toxicology screening program, results from this study were compared with published data both on mutagenicity in the Ames test and on carcinogenicity. 8 carcinogens generally found to be non-mutagenic for Salmonella were tested: 2 showed DNA-damaging properties (mitomycin C, 1,2-dimethylhydrazine), 5 failed to do so (actinomycin D, griseofulvin, thioacetamide, diethylstilbestrol, safrole), and one (thiourea) was not toxic, so that no classification was possible. 2 non-carcinogenic bacterial mutagens were examined; one, sodium azide, was equitoxic for repair-proficient and -deficient strains, while the other, nitrofurantoin, primarily inhibited repair-deficient strains. The DNA-repair tests failed to indicate the mutagenic and carcinogenic properties of acridine orange. Nalidixic acid, a non-mutagenic DNA synthesis inhibitor, damaged bacterial DNA. Apart from the differences summarized above, carcinogenicity was indicated correctly by the Salmonella S9 assay and most sets of DNA-repair-deficient and DNA-repair-proficient tester strains evaluated in this study. Thus, several more carcinogens could be detected by performing the Ames test and the bacterial DNA-repair tests in tandem than by using either test alone. Nevertheless, the use of both bacterial in vitro systems in a battery of short-term tests for mutagenicity/carcinogenicity evaluation is not considered to be ideal, since the Ames test and the pairs of DNA-repair-deficient and DNA-repair-proficient tester strains used had several shortcomings in common under the conditions of this study.

Carcinogen testing: current problems and new approaches

The classic procedures for testing potential carcinogens in animals have basically not changed in the past 50 years. Considerable knowledge of the mechanisms of carcinogenesis has accrued in the last 20 years, particularly concepts on the metabolic activation of chemicals to reactive electrophilic compounds that can interact with nucleophilic including DNA. These developments, in turn, have yielded a framework for integrating into carcinogen testing the determination of genetic effects of chemicals. A systematic decision point approach to carcinogen testing has been developed which entails a sequential decision-making process as specific tests are performed and evaluated prior to initiation of higher order, more complex tests. Compared to conventional bioassays in rodents, this approach provides knowledge based on mechanisms of carcinogenesis, yields a substantial amount of data at minimal cost, and forms a solid base for eventual heath risk assessment.

Promutagen activation with mammalian and avian S9 liver microsomes

Metabolic activation activity of Aroclor 1254 induced rat liver microsomes (S9) were compared with uninduced microsomes obtained from beef, pig, sheep and chicken liver. Using the Ames Salmonella typhimurium mutagenesis assay (strains TA98 and TA100), in conjunction with benzo(a)pyrene and 2-aminoanthracene, the highest promutagen activation levels were obtained with chicken liver S9. The S9 from the other domestic animals gave results which were close to those of the induced rat S9 values. Enhanced promutagen activation activity was observed with chicken S9 following freeze drying and reconstitution of the S9 preparation.

Non-mutagenic genotoxicants: novobiocin and nalidixic acid, 2 inhibitors of DNA gyrase

The antimicrobial agents nalidixic acid and novobiocin, both inhibitors of DNA gyrase, preferentially inhibit the growth of DNA repair-deficient Escherichia coli, Salmonella typhimurium and Bacillus subtilis. On the other hand, these agents exhibit no demonstrable mutagenicity for S. typhimurium and E. coli even when tested under conditions designed to take the metabolic properties of these agents into consideration. Novobiocin and nalidixic acid are widely used in animal and human medicine and further studies to assess their genetic and carcinogenic potentials appear to be in order.