INVESTIGATION OF THE BIOTRANSFORMATION OF 4-(METHYLNITROSAMINO)-1-(3-PYRIDYL)-1-BUTANONE BY PROSTAGLANDIN H SYNTHASE AND CYTOCHROME P450 2F
Fikree, Hana M.
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The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is believed to play a role in human lung cancer induced by tobacco smoking. NNK biotransformation may involve the enzymes prostaglandin H synthase (PHS)-1, PHS-2 and cytochrome 450 (CYP) 2F. PHS activity is thought to be important in extrahepatic tissues, where CYP activity is low. The CYP2F subfamily contains a single functional enzyme in humans (CYP2F1) and goats (CYP2F3); these enzymes are preferentially expressed in the lung, with little or no expression in other organs. The role of these enzymes in the pulmonary biotransformation of NNK was investigated. 4.2 µM [5-3H]NNK was incubated with human lung microsomes under NADPH-dependent and arachidonic acid-dependent conditions. Metabolites reflective of NNK α-carbon hydroxylation, N-oxidation and carbonyl reduction were detected in the presence of NADPH, and metabolite levels for all three biotransformation pathways were lower in the presence of arachidonic acid compared with NADPH (p<0.05, N=4). Incubation of microsomes with the PHS-1 selective inhibitor SC-560 and the PHS-2 selective inhibitor NS-398 did not change NNK biotransformation either in the presence of NADPH or in the presence of arachidonic acid (p>0.05, N=4). Incubation of [5-3H]NNK with ovine PHS-1 or PHS-2 did not result in formation of α-carbon hydroxylation or N-oxidation metabolites; 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) was measurable only in the presence of PHS-2. Incubation of goat recombinant CYP2F3 with [5-3H]NNK resulted in formation of keto acid, keto alcohol and NNK-N-oxide (65.0%, 17.5% and 30.0% (µmol enzyme)-1 minute-1, respectively). Metabolite formation was inhibited by 3-methylindole (3-MI), a mechanism-based inactivator of CYP2F3. Based on an N value of 3, incubation of human lung microsomes with 3-MI inhibited N-oxidation (p<0.05) but did not alter NNK bioactivation or carbonyl reduction (p>0.05). However, when metabolite formation was examined in lung microsomes from different individuals, decreases in NNK biotransformation (ranging from 19.6 to 68.5%) were observed and were more pronounced in some patients than others, suggesting inter-individual variability in CYP2F1 activity. These studies demonstrate the ability of CYP2F to biotransform NNK and suggest inter-individual variability in the importance of CYP2F1 for this activity in human lung. They also strongly argue against the involvement of PHS enzymes.