Metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific carcinogen, by rabbit nasal microsomes and cytochrome P450s NMa and NMb.

Rabbit nasal olfactory and respiratory microsomes were found to catalyze the alpha-hydroxylation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) with specific activities of 262 and 136 pmol/min/mg protein in the formation of keto aldehyde, and of 318 and 190 pmol/min/mg protein in the formation of keto alcohol respectively. The formation of NNK-N-oxide was observed in experiments with rabbit olfactory and respiratory microsomes, but not with rat nasal microsomes. However, the rat nasal microsomes had higher activity in catalyzing the alpha-hydroxylation of NNK. In a reconstituted system, rabbit P450NMa, a major constitutive P450 isozyme in nasal microsomes, displayed high activities in the formation of the keto aldehyde and the keto alcohol with apparent Km values of 15 and 9 microM respectively. In comparison, rabbit olfactory specific P450NMb had a low activity in catalyzing the formation of keto aldehyde (Km = 186 microM) and no activity in the formation of keto alcohol. The P450NMa-catalyzed oxidation of NNK was inhibited by nicotine and diallyl sulfide. Kinetic studies indicated that nicotine is a competitive inhibitor. These results demonstrate that enzymes in rabbit nasal microsomes, especially P450NMa, efficiently catalyze the bioactivation of NNK.