INACTIVATION OF O-6-METHYLGUANINE-DNA METHYLTRANSFERASE IN-VIVO BY SN2 ALKYLATING-AGENTS

The cellular level of O-6-methylguanine-DNA methyltransferase (MGMT) i s important in mutagenic, carcinogenic and therapeutic effects of alky lating agents. I have investigated how SN2 alkylating agents affect th e activity of MGMT in vivo. As a model, adapted cultures of E. coli K1 2 strain AB2497 containing 2400 +/- 430 molecules of MGMT per cell wer e used. MGMT activity was assayed in the cell extracts of adapted cult ures challenged with various doses of MMS, DMS and for comparison the SN1 alkylating agents, MNNG and MNU. In control non-adapted cultures, with low constitutive levels of MGMT, the mutagenic potential of vario us doses of different alkylating agents was estimated to correlate wit h the O-6-methylguanine content produced in DNA by various treatments. Inactivation of MGMT by MNNG or MNU occurs only in doses able to prod uce a highly mutagenic level of O-6-methylguanine in DNA, which is con sistent with the consumption of MGMT activity in the DNA repair proces s. In contrast, non-mutagenic doses of MMS or DMS are sufficient to in activate MGMT in adapted E. coli cells. It may be concluded that SN2 a lkylating agents can block the main pathway of O-6-methylguanine-DNA r epair in vivo.