On the role of alkylating mechanisms, O-alkylation and DNA-repair in genotoxicity and mutagenicity of alkylating methanesulfonates of widely varying structures in bacterial systems

The Ames test and the SOS-chromotest are widely used bacterial mutagenicity /genotoxicity assays to test potential carcinogens. Though the molecular me chanisms leading to backmutations and to the induction of SOS-repair are in principle known the role of alkylation mechanisms, of different DNA-lesion s and of DNA-repair is in parts still unknown. In this study we investigate d 14 monofunctional methanesulfonates of widely varying structures for muta genicity in Salmonella typhimurium strain TA 1535 sensitive for O-6-guanine alkylation for comparison with strain TA 100 in order to obtain additional information on the role of alkylation mechanisms, formation of the procarc inogenic DNA-lesion O-6-alkylguanine and the role of DNA-repair in inductio n of backmutation. The substances were also tested in the SOS-chromotest wi th Escherichia coh strain PQ 37 and strain PQ 243 lacking alkyl base glycos ylases important for base excision repair in order to examine the role of a lkylation mechanisms, of base excision repair and the role of O-alkyl and N -alkyl DNA-lesions on the induction of SOS-repair. The secondary methanesul fonates with very high S(N)1-reactivity isopropyl methanesulfonate and 2-bu tyl methanesulfonate showed highest mutagenicities in both strains. The hig her substituted methanesulfonates with very high S(N)1-reactivity had lower mutagenic activities because of reduced half lives due to their high hydro lysis rates. A clear increase in mutagenicities in strain TA 100 was observ ed for the primary compounds methyl methanesulfonate and allyl methanesulfo nate with very high S(N)2-reactivity. The primary compound phenylethyl meth anesulfonate has a relatively high mutagenicity in both Salmonella strains which can be explained by an increased S(N)1-reactivity and by low repair o f the O-6-phenylethylguanine, Highest SOSIPs (SOS inducing potency) in stra ins PQ 37 and PQ 243 were found for methyl methanesulfonate and for the sec ondary compounds with high S(N)1-reactivity. The ratios in the SOSIPs betwe en strain PQ 243 and PQ 37, indirectly indicative for the role of O- and N- alkylation in tile induction of SOS-repair, was high for the primary methan esulfonates and lower for the secondary, indicating that the SOS-repair is, to a certain extent, also induced by other lesions than O-6-alkylation. Th e results indicate that O-6-alkylation is also a predominant lesion for bac kmutation in strain TA 100 and that in the case of monofunctional alkylatin g agents high S(N)2-reactivities are required to induce error prone repair mediated backmutations. The O-6-alkylguanine lesion is also important for i nduction of SOS-repair in the SOS-chromotest, however, other sites of alkyl ation which are repaired by the base pair excision repair system can also e fficiently contribute to the induction of SOS-repair. (C) 2001 Elsevier Sci ence Ireland Ltd. All rights reserved.