O6-Alkylguanine-DNA alkyltransferase was rapidly and irreversibly inactivated by exposure to O6-benzylguanine or the p-chlorobenzyl and p-methylbenzyl analogues. This inactivation was much more rapid than with O6-methylguanine: incubation with 2.5 microM O6-benzylguanine led to more than a 90% loss of activity within 10 min, whereas 0.2 mM O6-methylguanine for 60 min was required for the same reduction. O6-Benzylguanine was highly effective in depleting the alkyltransferase activity of cultured human colon tumor (HT29) cells. Complete loss of activity was produced within 15 min after addition of O6-benzylguanine to the culture medium and a maximal effect was obtained with 5 microM. In contrast, at least 100 microM O6-methylguanine for 4 hr was needed to get a maximal effect, and this reduced the alkyltransferase by only 80%. Pretreatment of HT29 cells with 10 microM O6-benzylguanine for 2 hr led to a dramatic increase in the cytotoxicity produced by the chemotherapeutic agents 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) or 2-chloroethyl(methysulfonyl)methanesulfonate (Clomesone). Administration of O6-benzylguanine to mice at a dose of 10 mg/kg reduced alkyltransferase levels by more than 95% in both liver and kidney. These results indicate that depletion of the alkyltransferase by O6-benzylguanine may be used to investigate the role of the DNA repair protein in carcinogenesis and mutagenesis and that this treatment may be valuable to increase the chemotherapeutic effectiveness of chloroethylating agents.

Human O6-alkylguanine-DNA alkyltransferase was rapidly inactivated by low concentrations of O6-benzylguanine, but the alkyltransferase from the Escherichia coli ogt gene was much less sensitive and alkyltransferases from the E. coli ada gene or from yeast were not affected. O6-Benzyl-2'-deoxyguanosine was less potent than the base, but was still an effective inactivator of the human alkyltransferase and had no effect on the microbial proteins. O6-Allylguanine was somewhat less active, but still gave complete inactivation of both the human and Ogt alkyltransferases at 200 microM in 30 min, slightly affected the Ada protein, and had no effect on the yeast alkyltransferase. O4-Benzylthymidine did not inactivate any of the alkyltransferase proteins tested. Inactivation of the human alkyltransferase by O6-benzylguanine led to the formation of S-benzylcysteine in the protein and to the stoichiometric production of guanine. The rate of guanine formation followed second-order kinetics (k = 600 M-1 s-1). Prior inactivation of the alkyltransferase by reaction with a methylated DNA substrate abolished its ability to convert O6-benzylguanine into guanine. These results indicate that O6-benzylguanine inactivates the protein by acting as a substrate for alkyl transfer and by forming S-benzylcysteine at the acceptor site of the protein. The inability of O6-benzylguanine to inactivate the microbial alkyltransferases may be explained by steric constraints at this site.(ABSTRACT TRUNCATED AT 250 WORDS)

This study provides the foundation for a phase II trial of O6-BG plus temozolomide in temozolomide-resistant MG.

Site-specific mutagenesis by O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine (O(6)-pobGua), a product of DNA pyridyloxobutylation by metabolites of the tobacco-specific nitrosamines N-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), was studied in Escherichia coli strain DH10B and human kidney cells (293) when the modified base was incorporated in either a double-stranded or a gapped shuttle vector. In the repair-competent E. coli strain, less than 3% of the colonies produced by double-stranded vectors harboring the modified base were mutant whereas 96% were mutant when DH10B cells were transformed with modified gapped vectors. By contrast, transformation of DH10B cells with plasmids derived from O(6)-pobGua-containing double-stranded and gapped vectors previously replicated in 293 cells produced 7 and 16% mutant colonies, respectively. These percentages increased to 42 and 82%, respectively, when the 293 cells were pretreated with O(6)-benzylguanine to inactivate the O(6)-alkylguanine-DNA alkyltransferase protein. These findings confirm that the adduct is readily repaired by the human O(6)-alkylguanine-DNA alkyltransferase in both double-stranded and gapped vectors and suggest that it is also highly mutagenic in both human cells and E. coli. In the E. coli strain, the adduct produced exclusively G --> A transition mutations although in human 293 cells it also produced G --> T transversions and more complex mutations in addition to G --> A transitions. These data suggest that O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine can contribute significantly to the mutagenic risk posed by exposure to both NNN and NNK in tobacco smoke.