No abstract

The frequency of ethyl methanesulfonate (EMS)-induced mutations to 6-thioguanine resistance in a Chinese hamster ovary cells clone K1-BH4 was studied at many EMS doses including the minimally lethal range (0-100 microng/ml) as well as the exponential killing portion (100-800 microng/ml) of the survival curve. The mutation frequency increases approximately in proportion with increasing EMS concentration at a fixed treatment time. The pooled data for the observed mutation frequency, f(X), as a function of EMS dose X, is adequately described by a linear function f(X)=10(-6)(8.73+3.45 X), where 0 less than or equal to X less than or equal to 800 microng/ml. One interpretation of the linear dose-response is that, as a result of EMS treatment, ethylation of cellular constituents occurs, which is directly responsible for the mutation. Biochemical analyses demonstrate that most of the randomly isolated 6-thioguanine-resistant variants possess a highly reduced or undetectable level of HGPRT activity suggesting that the EMS-induced mutations to 6-thioguanine resistance affect primarily, if not exclusively, the HGPRT locus.

A study of meiotic and postmeiotic germ-cell-stage sensitivity of male mice to induction of unscheduled DNA synthesis (UDS) by acrylamide showed that DNA repair could be detected in early spermatocytes (after the last scheduled DNA synthesis) through about mid-spermatid stages. No DNA repair could be detected in later stages. The maximum UDS response was observed 6 hr after i.p. exposure and was about 5 times greater than the response measured immediately after treatment. This is the longest delay between chemical treatment and maximum UDS response yet observed in mouse germ cells. There was a linear relationship between the UDS response and acrylamide exposure from 7.8 to 125 mg/kg. By using 14C-labeled acrylamide it was determined that the temporal pattern of adduct formation in testes DNA paralleled that of the UDS response, with maximum binding occurring 4 to 6 hr after exposure. In contrast, the temporal pattern of adduct formation in liver DNA showed maximum binding within 1 to 2 hr after exposure and was an order of magnitude greater than that found for the testis DNA.

Exposure of Chinese hamster ovary (CHO) cells clone K1BH4 to ultraviolet (UV) light at doses up to 86 ergs/mm2 did not significantly reduce cell survival, but UV doses of 86-648 ergs/mm2 produced an exponential cell killing. Observed mutation frequency ro 6-thioguannine resistance induced by UV increases approximately in proportion to increasing doses up to 260 ergs/mm2 in a range of 5-648 ergs/mm2 examined. The pooled data of mutation frequency f(X) as a function of dose X from 0-260 ergs/mm2 is adequately described by f(X)=10(-6) (13.6 + 2.04 X). That the UV-induced mutations to 6-thioguanine resistance affects the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus is supported by the observation that all randomly isolated drug-resistant colonies contained highly reduced or undetectable HGPRT activity.

The genotoxic activities of 63, 2-nitronaphthofurans and related molecules were examined using two bacterial short-term tests, the Salmonella mammalian microsome assay test or Mutatest, a mutagenesis assay, and/or the SOS Chromotest, an assay for induction of an SOS function in Escherichia coli. Seven compounds were also investigated in the Chinese hamster ovary cells/hypoxanthine-guanine phosphoribosyl transferase (CHO/HGPRT) test, a mammalian gene mutation assay. Our main conclusions are the following: (a) Simple empirical rules relating structure to mutagenic activity in the Mutatest can be derived for some of the compounds. In particular, they account for the extremely high Mutagenic Potency of 7-methoxy-1-methyl-2-nitronaphtho[2,1-b]furan (R7372), approximately 2 X 10(6) mutants/nmol on strain TA100. (b) There is a good quantitative correlation between the Mutagenic Potency in the Salmonella/mammalian microsomes assay and the SOS-inducing potency in the SOS Chromotest. This, and previous evidence, suggests strongly that the 2-nitronaphthofurans derivatives are essentially recA and thus probably umuDC-dependent mutagens. (c) Four out of seven compounds tested in the CHO/HGPRT assay gave responses correlated with the bacterial responses. One of them, 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), is among, or is, the strongest mutagen described for mammalian cells. We briefly discuss the practical and theoretical implications of these results.