Mutagenicity in lung of Big Blue (R) 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 re acting with acetoxy radicals generated from ambient aldehydes in the presen ce 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 exposu re). 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 molec ular 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 for mation. Insertion of AA opposite the dimerized GG would account for the tan dem 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 th e 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 b ound covalently to DNA. Our results indicate that PAN is weakly mutagenic i n the lungs of mice and in Salmonella and that PAN produces a unique signat ure 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 pos sible 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.