Metabolism of a nitrate ester, dihydropyridine derivative in rabbit hepatic microsomes and cytosol

1. The metabolism of a nitrate ester-substituted dihydropyridine derivative (NND) in vitro was characterized with rabbit hepatic microsomes and cytosol. 2. Denitration activity was located in both the microsomal and cytosolic fractions, whereas oxidation to the pyridine analogue was solely located in the microsomal fraction. 3. Oxidation to the pyridine analogue required NADPH and was inhibited by carbon monoxide, miconazole and SKF-525A, suggesting that oxidation was catalysed by P450. 4. Denitration activity in the microsomes required either NADPH or GSH. Together with these results, responses to various inhibitors indicate participation of both P450 and glutathione S-transferase (GST). 5. Denitration activity in cytosol was activated by glutathione (GSH), and by dithiothreitol (DTT) to a greater extent. GSH-dependent denitration was inhibited by S-hexyl GSH, an inhibitor of GST, but DTT-dependent denitration was not. Moreover, the formation patterns of the mono-denitrated metabolites, M1 and M2, were shown to be different in each incubation condition. 6. These results suggest that the denitration of NND in cytosol could be catalysed by a GSH-independent enzyme as well as the GSH-dependent enzyme, GST.