Cytochrome P-450 isoforms involved in melatonin metabolism in human liver microsomes

Objective: The present study was carried out to identify the cytochrome P-4 50 enzyme(s) involved in the 6-hydroxylation and O-demethylation of melaton in. Methods: The formation kinetics of 6-hydroxymelatonin and N-acetylserotonin were determined using human liver microsomes and cDNA yeast-expressed huma n enzymes (CYP1A2, 2C9 and 2C19) over the substrate concentration range 1-1 000 muM. Selective inhibitors and substrates of Various cytochrome P-450 en zymes were also employed. Results: Fluvoxamine was a potent inhibitor of 6-hydroxymelatonin formation , giving 50 +/- 5% and 69 +/- 9% inhibition at concentrations of 1 muM and 10 muM, respectively, after incubation with 50 muM melatonin. Furafylline, sulphaphenazole and omeprazole used at low and high concentrations substant ially inhibited both metabolic pathways. cDNA yeast-expressed CYP1A2, CYP2C 9 and CYP2C19 catalysed the formation of the two metabolites, confirming th e data obtained with specific inhibitors and substrates. Conclusions: Our results strongly suggest that 6-hydroxylation, the main me tabolic pathway of melatonin, is mediated mainly, but not exclusively, by C YP1A2, the high-affinity enzyme involved in melatonin metabolism, confirmin g the observation that a single oral dose of fluvoxamine increases nocturna l serum melatonin levels in healthy subjects. Furthermore, the results indi cate that there is a potential for interaction with drugs metabolised by CY P1A2 both at physiological levels and after oral administration of melatoni n, while CYP2C19 and CYP2C9 are assumed to be less important.