Metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in isolated rat lung and liver

Abstract

The tobacco specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a strong lung carcinogen in all species tested. To elicit its tumorigenic effects NNK requires metabolic activation which is supposed to take place via α-hydroxylation, whereas N-oxidation is suggested to be a detoxification pathway. The differences in the organ specific metabolism of NNK may be crucial for the organotropy in NNK-induced carcinogenesis. Therefore, metabolism of NNK was investigated in the target organ lung and in liver of Fischer 344 (F344) rats using the model of isolated perfused organs. High activity to metabolize 35 nM [5-³H]NNK was observed in both perfused organs. NNK was eliminated by liver substantially faster (clearance 6.9 ± 1.6 ml/min, half-life 14.6 ± 1.2 min) than by lung (clearance 2.1 ± 0.5 ml/min, half-life 47.9 ± 7.4 min). When the clearance is calculated for a gram of organ or for metabolically active cell forms, the risk with respect to carcinogenic mechanisms was higher in lung than in liver. The metabolism of NNK in liver yielded the two products of NNK α-hydroxylation, the 4-oxo-4-(3-pyridyl)-butyric acid (keto acid) and 4-hydroxy-4-(3-pyridyl)-butyric acid (hydroxy acid). In lung, the major metabolite of NNK was 4-(methylnitrosamino)-1-(3-pyridyl-N-oxide)-1-butanone (NNK-N-oxide). Substantial amounts of metabolites formed from methyl hydroxylation of NNK, which is one of the two possible pathways of α-hydroxylation, were detected in lung but not in liver perfusion. Formation of these metabolites (4-oxo-4-(3-pyridyl)-butanol (keto alcohol), and 4-hydroxy-4-(3-pyridyl)-butanol (diol) can give rise to pyridyloxobutylating of DNA.

When isolated rat livers were perfused with 150 μM NNK, equal to a dosage which is sufficient to induce liver tumors in rat, glucuronidation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) was increased when compared to the concentration of 35 nM NNK. Nevertheless, the main part of NNK was also transformed via α-hydroxylation for this high concentration of NNK.