N⁶-Formyllysine as a Biomarker of Formaldehyde Exposure: Formation and Loss of N⁶-Formyllysine in Nasal Epithelium in Long-Term, Low-Dose Inhalation Studies in Rats

Exposure to both endogenous and exogenous formaldehyde has been established to be carcinogenic, likely by virtue of forming nucleic acid and proteins adducts such as N⁶-formyllysine. To better assess N⁶-formyllysine as a biomarker of formaldehyde exposure, we studied accumulation of N⁶-formyllysine adducts in tissues of rats exposed by inhalation to 2 ppm [¹³C²H₂]-formaldehyde for 7, 14, 21, and 28 days (6 h/day) and investigated adduct loss over a 7-day postexposure period using liquid chromatography-coupled tandem mass spectrometry. Our results showed formation of exogenous adducts in nasal epithelium and to some extent in trachea but not in distant tissues of lung, bone marrow, or white blood cells, with a 2-fold increase over endogenous N⁶-formyllysine over a 3-week exposure period. Postexposure analyses indicated a biexponential decay of N⁶-formyllysine in proteins extracted from different cellular compartments, with half-lives of ∼25 and ∼182 h for the fast and slow phases, respectively, in cytoplasmic proteins. These results parallel the behavior of DNA adducts and DNA-protein cross-links, with protein adducts cleared faster than DNA-protein cross-links, and point to the potential utility of N⁶-formyllysine protein adducts as biomarkers of formaldehyde.