Some factors involved in the N-oxidation of 3-substituted pyridines by microsomal preparations in vitro.

1. Pyridine-N-oxides have been identified as metabolites in vitro of several 3-substituted pyridines. 2. Factors affecting the metabolism of these pyridines in vitro have been studied, and conditions which give the most metabolism have been established. 3. 'Pyridine-N-oxidase' activity resides mainly in the hepatic and pulmonary microsomal fractions. 4. A species difference was evident with 'pyridine-N-oxidase' activity decreasing in the order hamster, rabbit, mouse, guinea-pig and rat. 5. A sex difference in 'pyridine-N-oxidase' activity was also established in rats and mice. 6. The appropriate kinetic factors, Km and Vmax for the N-oxidation of pyridine, 3-methylpyridine and 3-chloropyridine are reported.