Intestinal metabolism of 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone in rats: sex-difference, inducibility and inhibition by phenethylisothiocyanate
The intestinal metabolism of 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone (NNK) was investigated in male and female Sprague-Dawley (SD) rats and male F344 rats, using isolated perfused intestinal segments. [1-14C]-NNK at 1 μM was metabolized by α-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)-l-(3-pyri-dyl)-l-butanol (NNAL, 12.2%), 4-(methylnitrosamlno)-l-(3-pyridyl-Af-oxide)-l-butanone (NNK-N-oxide, 7.7%), 4-oxo-4-(3-pyridyl)-butanol (KAlc, 2.7%), 4-(methylnitros-amino)- l-(3-pyrldyl-N-oxide)- 1-butanol (NN AL-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%). Deal 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 23-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 (893%), phenobarbital (PB, 75.3%) and Clophen A50 (61%). PB and Clophen A50 induced N-oxidation to 38.9% (4 ×) and 27.8% (3 ×), and to a lesser extent NNAL formation and α-hydroxylation (2 ×). Starvation mainly increased N-oxidation with a time-dependent increase from 1 day to 3 days of starvation (4 × and 8 × versus controls), whereas α-hydroxylation and NNAL formation was elevated only after 1 day starvation. Acetone pretreatment (3 days) stimulated all three pathways (NNAL 2 ×, N-oxidation 4 ×, α-hydroxylation 4 ×). In male F344 rats, starvation and acetone induced α-oxidation (5 × and 7 ×) and α-hydroxylation (3 × and 5 ×), 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 phenethylLso-thiocyanate (PEITC) or in vitro addition of 1% ethanol to the perfusate. Both inhibition experiments reduced total metabolism by 20%; N-oxidation and a-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