The intestinal metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) was investigated in male and female Sprague-Dawley (SD) rats and male F344 rats, using isolated perfused intestinal segments. [1-C-14]-NNK at 1 mu M was metabolized by alpha-hydroxylation, pyridine N-oxidation and carbonyl reduction. Jejunal segments from control female rats metabolized 26.2% of the NNK during transepithelial transfer to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL, 12.2%), 4-(methylnitrosamino)-1-(3-pyridyl-N-oxide)-1-butanone (NNK-N-oxide, 7.7%), 4-oxo-4-(3-pyridyl)-butanol (KAlc, 2.7%), 4-(methylnitrosamino)-1-(3-pyridyi-N-oxide)-1-butanol (NNAL-N-oxide, 1.8%), 4-oxo-4-(3-pyridyl)butyric acid (KA, 1.1%) and 4-hydroxy-4-(3-pyridyl)butyric acid (HA, 0.7%). Ileal segments metabolized 20.8% of the NNK during absorption, with no difference in metabolite distribution as compared to jejunal segments. In control male SD and F344 rats, jejunal presystemic metabolism was 2.3-fold higher (56.4% and 60.8% respectively), mainly because of a 4-fold increase in NNAL formation (44.1% and 48.5%). Total NNK metabolism was also induced in female rats by starvation (84.4% metabolites), acetone (89.3%), phenobarbital (PB, 75.3%) and Clophen A50 (61%). PB and Clophen A50 induced N-oxidation to 38.9% (4 x) and 27.8% (3 x), and to a lesser extent NNAL formation and alpha-hydroxylation (2 x). Starvation mainly increased N-oxidation with a time-dependent increase from 1 day to 3 days of starvation (4 x and 8 x versus controls), whereas alpha-hydroxylation and NNAL formation was elevated only after 1 day starvation. Acetone pretreatment (3 days) stimulated all three pathways (NNAL 2 x, N-oxidation 4 x, alpha-hydroxylation 4 x). In male F344 rats, starvation and acetone induced N-oxidation (5 x and 7 x)and alpha-hydroxylation (3 x and 5 x), and decreased NNAL formation by 40%, probably due to substrate competition or further metabolism of NNAL. In acetone-induced female SD rats, NNK metabolism was inhibited by in vivo pretreatment with phenethylisothiocyanate (PEITC) or in vitro addition of 1% ethanol to the perfusate. Both inhibition experiments reduced total metabolism by 20%; N-oxidation and alpha-hydroxylation were reduced to values found in control rats, whereas NNAL formation increased from 31% to 51%. Inhibition of NNK metabolism by PEITC in male F344 rats was less pronounced compared to female SD rats; again a decrease in ol-hydroxylation (6.7% to 3.3%) and N-oxidation (73.6% to 35.3%) was accompanied by increased NNAL formation (9.8% to 41.0%). These experiments demonstrate the presence of NNK metabolism in the intestine of rats, the presence of sex differences and the importance of the induction state on both extent of NNK metabolism and metabolite composition.
