High mutagenic activity of N-nitrosobis(2-oxopropyl)amine and N-nitrosobis(2-hydroxypropyl)amine in the host-mediated assay in hamsters: evidence for premutagenic methyl and hydroxylpropyl adducts.

The carcinogenic nitrosamines N-nitrosobis(2-oxopropyl)-amine (BOP) and N-nitrosobis(2-hydroxypropyl)amine (BHP) were tested in excision-repair-deficient strains of hisG46 Salmonella mutants in the intrasanguinous host-mediated mutagenesis assay (HMA) in male Syrian hamsters. The major adducts produced by BOP in the hamster are methylguanines, while BHP leads to hydroxypropylguanines as well as methylguanines. Both nitrosamines were potent mutagens in bacteria recovered from the liver. On a comparison of administered dose, BOP was more potent, but when compared at doses producing similar levels of O6-methylguanine (O6-MeG) in host liver DNA, or at equitoxic doses in the hamster, BHP was more potent. BHP was approximately 10 times less mutagenic in an excision-repair-proficient strain of Salmonella, but the mutagenicity of BOP was not reduced. The effects of excision repair on in vitro mutagenesis induced by the direct-acting analogs N-(2-oxopropyl)-N-nitrosourea (OPNU), a methylating agent, and N-(2-hydroxypropyl)-N-nitrosourea (HPNU), a hydroxypropylating agent, were also examined. Mutagenesis by HPNU, but not OPNU was very sensitive to excision repair. Thus BOP appears to lead to mutagenesis via methylation, while mutagenesis by BHP apparently proceeds via hydroxypropylation. BOP, BHP, OPNU and HPNU were several times less mutagenic in hisG428 than hisG46 strains. In contrast to hisG46 strains, which are reverted mainly by base-pair substitutions at G:C base pairs, hisG428 strains are generally more sensitive to mutagenesis at A:T base pairs. Taken together the above results and observations that > 90% of the adducts from BOP and BHP were alkylguanines, suggest that the major premutagenic adducts produced from BOP and BHP are alkylguanines as opposed to other alkylated bases. BOP and BHP were weak mutagens in the Salmonella/S-9 mutagenesis assay using hamster liver S-9 fraction. When compared with results in the HMA, BOP and BHP were orders of magnitude less mutagenic in vitro. This observation suggests: (i) the pathways or enzymes involved in the activation of these carcinogens (although uncertain) may be different in vivo and in vitro; or (ii) the pathways for the in vitro and in vivo metabolism may be similar, but the conditions used for the in vitro activation of these nitrosamines are inadequate to generate significant levels of nitrosamine metabolites.