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-oxopropyI)-amine(BOP) and N-nitrosobis(2-hydroxypropyl)amine (BHP) were testedin excision-repair-deficient strains of his G46 Salmonella mutantsin the intrasanguinous host-mediated mutagenesis assay (HMA)in male Syrian hamsters. The major adducts produced by BOP inthe hamster are methylguanines, while BHP leads to hydroxypropylguaninesas well as methylguanines. Both nitrosamines were potent mutagensin bacteria recovered from the liver. On a comparison of administereddose, BOP was more potent, but when compared at doses producingsimilar levels of O⁶-methylguanine (O⁶MeG) in host liver DNA,or at equitoxic doses in the hamster, BHP was more potent. BHPwas 10 times less mutagenic in an excision-repair-proficientstrain of Salmonella, but the mutagenicity of BOP was not reduced.The effects of excision repair on in vitro mutagenesis inducedby 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. Mutagenesisby HPNU, but not OPNU was very sensitive to excision repair.Thus BOP appears to lead to mutagenesis via methylation, whilemutagenesis by BHP apparently proceeds via hydroxypropylation.BOP, BHP, OPNU and HPNU were several times less mutagenic inhisG428 than hisG46 strains. In contrast to hisG46 strains,which are reverted mainly by base-pair substitutions at G: Cbase pairs, hisG428 strains are generally more sensitive tomutagenesis at A: T base pairs. Taken together the above resultsand observations that >90% of the adducts from BOP and BHPwere alkylguanines, suggest that the major premutagenic adductsproduced from BOP and BHP are alkylguanines as opposed to otheralkylated bases. BOP and BHP were weak mutagens in the Salmonella/S-9mutagenesis assay using hamster liver S-9 fraction. When comparedwith results in the HMA, BOP and BHP were orders of magnitudeless mutagenic in vitro. This observation suggests: (i) thepathways 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 metabolismmay be similar, but the conditions used for the in vitro activationof these nitrosamines are inadequate to generate significantlevels of nitrosamine metabolites.