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Nucleic Acids Research, 1982, Vol. 10, No. 15 4595-4604
© 1982

MOLECULAR BIOLOGY

Adaptive increase of O6-methlguanine-acceptor protein in HeLa cells following N-methyl-N'-nitro-N-nitrosoguanidine treatment

Evelyn A. Waldstein*, En-Hua Cao+ and Richard B. Setlow

Biology Department, Brookhaven National Laboratory Upton, NY 11973, USA

Received April 22, 1982. Revised June 24, 1982. Accepted June 24, 1982.

We have assayed in extracts of HeLa cells the amount of acceptor protein that removes O6-methylguanine adducts from alkylated DNA. Cells were treated with single or multiple nontoxic doses of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and the extracts were analyzed up to 32 h after the last exposure. The acceptor activity assayed immediately (1 h) after single exposures decreases linearly with dose indicating that the acceptor protein is used up by endogenous O6-methylguanine adducts in a stoichiometric reaction. Multiple exposures, assayed 8–24 h after the last exposure, increase the amount of acceptor protein in a dose dependent fashion followed by a decrease above a cumulative dose of 100 ng/ml. Under conditions of maximum induction, there are about 300,000 acceptor protein sites per cell, approximately 3 fold above the constitutive level. Both in adapted and unadapted cells the methyl group from O6-methylguanine adducts in the alkylated DNA is transferred to cysteine residues of the acceptor protein(s).

*Permanent address: Biochemistry Department, Tel Aviv University, Israel

+Permanent address: Institute of Biophysics, Academia Sinica, Beijing, China


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