This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Pegg, A. E. Articles by Dolan, M. E. Search for Related Content PubMed PubMed Citation Articles by Pegg, A. E. Articles by Dolan, M. E.

Comparison of the Rates of Repair of O⁶-Alkylguanines in DNA by Rat Liver and Bacterial O⁶-Alkylguanine-DNA Alkyltransferase¹

Department of Physiology and Cancer Center, The Milton S. Hershey Medical Center, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033

The rates of loss of O⁶-methylguanine and O⁶-ethylguanine from rat liver DNA were determined over a time period of 15 min to 4 hr after various doses (5 µg/kg to 2 mg/kg) of dimethylnitrosamine and diethylnitrosamine which produced total amounts of these adducts in the range of 300 to 16,000 molecules/cell. This amount is considerably less than the content of O⁶-alkylguanine-DNA alkyltransferase protein (approximately 60,000 molecules/hepatocyte), and during the time period studied, the adducts were found to be lost with pseudo-first order kinetics. The half-life for O⁶-methylguanine was 47 min. O⁶-Ethylguanine was removed 3.6 times more slowly with a half-life of 172 min. The ability of partially purified rat liver O⁶-alkylguanine-DNA alkyltransferase to remove O⁶-methylguanine and O⁶-ethylguanine from [³H]alkyl-labeled DNA substrates in vitro was measured, and it was found that O⁶-methylguanine was removed 3.4 times more rapidly than was O⁶-ethylguanine. These results are consistent with the hypothesis that most, if not all, of the repair of these adducts which occurs within the first 4 hr after treatment is due to the alkyltransferase protein. Diethylnitrosamine, which is slightly more potent as a carcinogen to rat liver, produced a total amount of O⁶-ethylguanine of 3.7 µmol/mol guanine/mg compared to O⁶-methylguanine (28 µmol/mol guanine/mg) given by dimethylnitrosamine. The slower rate of loss of the ethyl adduct is not sufficient to account for this difference, and the results, therefore, support the concept that other DNA adducts (possibly O-alkylpyrimidines) contribute to the initiation of tumors by diethylnitrosamine.

Preliminary evidence that the rat liver alkyltransferase can also remove hydroxyethyl groups from DNA at a rate slower than removal of ethyl groups was also obtained. Bacterial O⁶-alkylguanine-DNA alkyltransferase was shown to remove methyl, ethyl, and hydroxyethyl groups from the O⁶ position of guanine in DNA using fluorescence detection to quantitate these adducts. The bacterial protein removed methyl groups very rapidly but was much slower than the rat liver protein on the larger adducts. These results suggest that the relative rates of repair of different alkyl groups may be species specific and must be determined experimentally in the cell of interest before conclusions concerning biological effects can be drawn.

¹ This work was supported by Grants CA-18137 and 1P30-18450 from the National Cancer Institute.

² To whom requests for reprints should be addressed.

Received 2/27/84. Accepted 6/ 5/84.

This article has been cited by other articles:

				G.J.S. Jenkins, S.H. Doak, G.E. Johnson, E. Quick, E.M. Waters, and J.M. Parry Do dose response thresholds exist for genotoxic alkylating agents? Mutagenesis, November 1, 2005; 20(6): 389 - 398. [Abstract] [Full Text] [PDF]

				M. Skorvaga, N. D. H. Raven, and G. P. Margison Thermostable archaeal O6-alkylguanine-DNA alkyltransferases PNAS, June 9, 1998; 95(12): 6711 - 6715. [Abstract] [Full Text] [PDF]