4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is activated to DNA alkylating species via two different alpha-hydroxylation pathways. Methylene hydroxylation leads to DNA methylation, whereas methyl hydroxylation yields DNA pyridyloxobutylation. We have developed a high-pressure liquid chromatography assay utilizing radiochemical detection that permits the determination of the extent of metabolism through each pathway in microsomal preparations. Levels of 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB) were used to measure the extent of methyl hydroxylation, whereas levels of the aldehyde, 4-oxo-1-(3-pyridyl)-1-butanone (OPB), were used to quantify the extent of methylene hydroxylation. Incubations of [5-3H]NNK with microsomes and cofactors were conducted in the presence of 5 mM sodium bisulfite to trap the reactive OPB. The inclusion of bisulfite did not affect the rate of NNK metabolism. Trapping the aldehyde also inhibited its further oxidation to the corresponding acid or reduction to HPB. Furthermore, the conversion of HPB to OPB made only a minor contribution to the OPB levels under our incubation conditions. Analysis of incubation mixtures containing [5-3H]NNK, cofactors, and either A/J mouse liver or lung microsomes demonstrated that OPB was a significant metabolite of NNK. The OPB:HPB ratio was greater in liver (1.5) than in lung (0.2-1) microsomal preparations. Apparent Km values for OPB and HPB formation in lung microsomes were 23.7 and 3.6 microM, respectively, whereas the corresponding values for liver microsomes were 19.1 and 73.8 microM, respectively. These data are consistent with the involvement of more than one cytochrome P-450 isozyme in the activation of NNK to DNA reactive species.