Mutagenicity in lung of Big Blue® mice and induction of tandem-base substitutions in Salmonella by the air pollutant peroxyacetyl nitrate (PAN):: predicted formation of intrastrand cross-links

Peroxyacetyl nitrate (PAN) is a ubiquitous air pollutant formed from NO2 reacting with acetoxy radicals generated from ambient aldehydes in the presence of sunlight and ozone. It contributes to eye irritation associated with photochemical smog and is present in most urban air. PAN was generated in a chamber containing open petri dishes of Salmonella TA100 (gas-phase exposure). After subtraction of the background mutation spectrum, the spectrum of PAN-induced mutants selected at 3.1-fold above the background mutant yield was 59% GC --> TA, 29% GC --> AT, 2% GC --> CG, and 10% multiple mutations - primarily GG --> TT tandem-base substitutions. Using computational molecular modeling methods, a mechanism was developed for producing this unusual tandem-base substitution. The mechanism depends on the protonation of PAN near the polyanionic DNA to release NO2+ resulting in intrastrand dimer formation. Insertion of AA opposite the dimerized GG would account for the tandem GG --> TT transversions. Nose-only exposure of Big Blue(R) mice to PAN at 78 ppm (near the MTD) was mutagenic at the lacI gene in the lung (mutant frequency +/-S.E. of 6.16 +/- 0.58/10(5) for controls versus 8.24 +/- 0.30/10(5) for PAN, P = 0.016). No tandem-base mutations were detected among the 40 lacI mutants sequenced. Dosimetry with H-3-PAN showed that 24 h after exposure, 3.9% of the radiolabel was in the nasal tissue, and only 0.3% was in the lung. However, based on the molecular modeling considerations, the labeled portion of the molecule would not have been expected to have been bound covalently to DNA. Our results indicate that PAN is weakly mutagenic in the lungs of mice and in Salmonella and that PAN produces a unique signature mutation (a tandem GG --> TT transversion) in Salmonella that is likely due to a GG intrastrand cross-link. Thus, PAN may pose a mutagenic and possible carcinogenic risk to humans, especially at the high concentrations at which it is present in some urban environments. (C) 2000 Elsevier Science B.V. All rights reserved.