with formaldehyde as the major source 
 of endogenous n6-formyllysine protein 
 adducts , we quantified endogenous 
 and exogenous n6-formyllysine in the nasal 
 epithelium of rats exposed by inhalation to 0.7 , 2 , 5.8 , and 9.1 ppm 
 [ 13c2h2]-formaldehyde using liquid 
 chromatography - coupled tandem mass spectrometry . 
 exogenous n6-formyllysine was detected in the nasal epithelium , 
 with concentration - dependent formation in total as well as fractionated 
 ( cytoplasmic , membrane , nuclear ) proteins , but was not detected in 
 the lung , liver , or bone marrow . 
 endogenous adducts dominated at all 
 exposure conditions , with a 6 h 9.1 ppm formaldehyde exposure resulting 
 in one - third of the total load of n6-formyllysine 
 being derived from exogenous sources . 
 the results parallel previous 
 studies of formaldehyde - induced dna adducts .