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

Abstract

Objective: The present study was carried out to identify the cytochrome P ₄₅₀ enzyme(s) involved in the 6-hydroxylation and O-demethylation of melatonin. Methods: The formation kinetics of 6-hydroxymelatonin and N-acetylserotonin were determined using human liver microsomes and cDNA yeast-expressed human enzymes (CYP1A2, 2C9 and 2C19) over the substrate concentration range 1–1000 µM. Selective inhibitors and substrates of various cytochrome P ₄₅₀ enzymes were also employed. Results: Fluvoxamine was a potent inhibitor of 6-hydroxymelatonin formation, giving 50±5% and 69±9% inhibition at concentrations of 1 µM and 10 µM, respectively, after incubation with 50 µM melatonin. Furafylline, sulphaphenazole and omeprazole used at low and high concentrations substantially inhibited both metabolic pathways. cDNA yeast-expressed CYP1A2, CYP2C9 and CYP2C19 catalysed the formation of the two metabolites, confirming the data obtained with specific inhibitors and substrates. Conclusions: Our results strongly suggest that 6-hydroxylation, the main metabolic pathway of melatonin, is mediated mainly, but not exclusively, by CYP1A2, the high-affinity enzyme involved in melatonin metabolism, confirming the observation that a single oral dose of fluvoxamine increases nocturnal serum melatonin levels in healthy subjects. Furthermore, the results indicate that there is a potential for interaction with drugs metabolised by CYP1A2 both at physiological levels and after oral administration of melatonin, while CYP2C19 and CYP2C9 are assumed to be less important.