Mutagenicity of an antitumor protein, neocarzinostatin, in Escherichia coli

Neocarzinostatin as a probe for DNA protection activity--molecular interaction with caffeine

Neocarzinostatin (NCS), a potent mutagen and carcinogen, consists of an enediyne prodrug and a protein carrier. It has a unique double role in that it intercalates into DNA and imposes radical-mediated damage after thiol activation. Here we employed NCS as a probe to examine the DNA-protection capability of caffeine, one of common dietary phytochemicals with potential cancer-chemopreventive activity. NCS at the nanomolar concentration range could induce significant single- and double-strand lesions in DNA, but up to 75 ± 5% of such lesions were found to be efficiently inhibited by caffeine. The percentage of inhibition was caffeine-concentration dependent, but was not sensitive to the DNA-lesion types. The well-characterized activation reactions of NCS allowed us to explore the effect of caffeine on the enediyne-generated radicals. Postactivation analyses by chromatographic and mass spectroscopic methods identified a caffeine-quenched enediyne-radical adduct, but the yield was too small to fully account for the large inhibition effect on DNA lesions. The affinity between NCS chromophore and DNA was characterized by a fluorescence-based kinetic method. The drug-DNA intercalation was hampered by caffeine, and the caffeine-induced increases in DNA-drug dissociation constant was caffeine-concentration dependent, suggesting importance of binding affinity in the protection mechanism. Caffeine has been shown to be both an effective free radical scavenger and an intercalation inhibitor. Our results demonstrated that caffeine ingeniously protected DNA against the enediyne-induced damages mainly by inhibiting DNA intercalation beforehand. The direct scavenging of the DNA-bound NCS free radicals by caffeine played only a minor role.

An evaluation of tests using DNA repair-deficient bacteria for predicting genotoxicity and carcinogenicity. A report of the U.S. EPA's Gene-TOX Program

The detection of DNA-damaging agents by repair-deficient bacterial assays is based on the differential inhibition of growth of repair-proficient and repair-deficient bacterial pairs. The various methodologies used are described and recommendations are made for their improved use. In a survey of the literature through April 1979, 91 of 276 papers evaluated contained usable data, resulting in an analysis of 611 compounds that had been assayed in 1 or more of 55 pairs of repair-proficient and repair-deficient strains. The results indicate that (1) a liquid suspension assay is more sensitive than a spot (diffusion) test. In a review of the Escherichia coli polA assay, 45 compounds that gave "No Test" in the spot test were clearly positive or negative in the liquid suspension assay. (2) Of the 21 compounds analyzed by the E. coli polA assay and by other E. coli repair-deficient strains (e.g., rec, uvr, hcr, and exr derivatives of WP2 and AB1157), 10 were in complete agreement in all strains except uvrA strains. This indicates that strains other than polA+/polA- are useful for detecting DNA-damaging agents. However, in selecting strains for use in these assays, care should be taken to consider repair pathway specificity for particular compounds. (3) There was a 78% correspondence between results obtained with E. coli polA and Bacillus subtilis (H17/M45, 17A/45T) rec assay and between E. coli polA and Proteus mirabilis. (4) In a comparison of test results with carcinogenicity data, 44 of 71 (62%) carcinogenic compounds assayed by the polA system were positive, 10 (14%) were negative, and 17 (24%) gave No Test or doubtful results. 7 carcinogens were assayed by other E. coli strains and all were positive. 56 carcinogens were assayed in B. subtilis: 24 (43%) were positive, 9 (16%) were negative, and 23 (41%) gave No Test or doubtful results. Of the 7 carcinogens assayed in P. mirabilis, 6 (86%) were positive and 1 (14%) was negative. (5) The results were analyzed with respect to chemical classes. E. coli polA detected the highest percentage of hydroxylamines and alkyl epoxides. The B. subtilis rec assay detected the highest percentage of nitrosamines and sulfur and nitrogen oxides. It is concluded that some of these test systems are effective tools for the detection of DNA-damaging and potentially carcinogenic compounds, especially if the assay is done in liquid suspension and if more than 1 pair of tester strains is used. Advantages and disadvantages of the assay are discussed and suggestions are made for improvements in the system.

An evaluation of the Escherichia coli WP2 and WP2 uvrA reverse mutation assay

The methodology and status of the Escherichia coli WP2 reverse mutation system as it applies to chemical screening were reviewed using the available published literature. 163 documents were reviewed by the Working Group. These included abstracts, research articles, review articles and publicly available contract and grant final reports. From this group, 115 documents were rejected for critical evaluation by the Working Group. 48 documents were reviewed and the test results summarized. The general conclusion of the Working Group was the the E. coli WP2 reverse mutation system is a valuable tool for mutagenesis research, but that there is no evidence from a review of the literature that this assay will contribute significantly to the results obtainable from careful application of the Ames Salmonella assay. Another review of the role of this system in general screening may be warranted after more research and development with the plasmid-containing WP2 derivatives.

Short-term cytogenetic assays of nine cancer chemotherapeutic drugs with metabolic activation

Nine anticancer drugs were analyzed in cell cultures with respect to their cytogenetic effects with or without the addition of liver fraction S9 as an in vitro metabolic system. Among them, vincristine was the only drug which induced a significant accumulation of mitosis with or without S9. Adriamycin and neocarzinostatin were most potent in induction of chromosome breakage, but their clastogenic activities were reduced after metabolic conversion. On the other hand, cyclophosphamide had a dose-dependent activation of clastogenic effect by S9. The activity of bleomycin was enhanced by S9. S9 had no effect on the activities of actinomycin D, cytosine arabinoside, mitomycin C, and methotrexate. Vincristine showed no clastogenic property in our short-term assay system with or without S9. The activities of these anticancer drugs observed from our assay were compatible with those from other assay systems. The incorporation of S9 or other metabolic systems in routine clastogen assays should allow us to improve our understanding of the genetic toxicity of chemical agents.

Effect of DNA Repair systems on antibacterial and mutagenic activity of an antitumor protein, neocarzinostatin

Antibacterial and mutagenic effects of an antitumor protein, neocarzinostatin (NCS), on isogenic strains of Escherichia coli with normal or defective DNA repair systems were studied. Growth of the strains lacking recA gene was inhibited by NCS with much lower concentration than in the case of those possessing it, while the "differential inhibition for growth" (DIG) between the strains uvrA+ and uvrA- was not seen. NCS induces mutation in recA+ strains but not significantly in recA-, while no such difference of mutagenesis was noticed between the strains uvrA+ and uvrA-. These results suggest that NCS produces hardly excisable DNA damage which is repaired by an error-prone recombination process.