EFFICIENT REPAIR OF O6-ETHYLGUANINE, BUT NOT O4-ETHYLTHYMINE OR O2-ETHYLTHYMINE, IS DEPENDENT UPON O6-ALKYLGUANINE-DNA ALKYLTRANSFERASE ANDNUCLEOTIDE EXCISION REPAIR ACTIVITIES IN HUMAN-CELLS

The formation and persistence of O6-ethylguanine, O4-ethylthymine, and O2-ethylthymine were quantitated in the genomic DNA of human lymphobl asts exposed to 1.0 mM N-ethyl-N-nitrosourea using immunoslot-blot. Th e three cell lines used included one which lacks O6-alkylguanine-DNA a lkyltransferase, one deficient in nucleotide excision repair, and a th ird which is competent in both of these repair pathways. The activity of O6-alkylguanine-DNA alkyltransferase was further modulated with O6- benzylguanine, a specific inhibitor of this protein. Repair of the O-e thylated thymines was slow and not related to either DNA repair phenot ype. O6-Ethylguanine was repaired with a half-life of about 8 h in cel ls which expressed both O6-alkylguanine-DNA alkyltransferase and nucle otide excision repair functions. Cells expressing O6-alkylguanine-DNA alkyltransferase activity but lacking nucleotide excision repair showe d only slow repair of O6-ethylguanine (half-life of O6-ethylguanine, 4 3 h), while cells lacking the alkyltransferase showed little or no rep air of O6-ethylguanine regardless of nucleotide excision repair activi ty (half-lives of O6-ethylguanine, 53 to > 100 h). We conclude that O6 -alkylguanine-DNA alkyltransferase and nucleotide excision repair coop erate in the repair of O6-ethylguanine in human cells.