title: Cytochrome P450 isoforms involved in melatonin metabolism in human liver microsomes.
abstract: OBJECTIVE
  The present study was carried out to identify the cytochrome P450 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 microM. Selective inhibitors and substrates of various cytochrome P450 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 microM and 10 microM, respectively, after incubation with 50 microM 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.
authors.affiliation: Department of Medical Laboratory Sciences and Technology, Division of Clinical Pharmacology, Huddinge University Hospital, Karolinska Institutet, 14186 Stockholm, Sweden.
authors.forename: G
authors.initials: G
authors.lastname: Facciolá
authors.forename: M
authors.initials: M
authors.lastname: Hidestrand
authors.forename: C
authors.initials: C
authors.lastname: von Bahr
authors.forename: G
authors.initials: G
authors.lastname: Tybring
mesh_terms: D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000187:drug effects / Q000502:physiology*; D016666:Fluvoxamine / Q000494:pharmacology*; D006801:Humans; D006900:Hydroxylation / Q000187:drug effects; D007527:Isoenzymes / Q000187:drug effects / Q000502:physiology*; D008550:Melatonin / Q000031:analogs & derivatives* / Q000378:metabolism*; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology*; D013379:Substrate Specificity
chemical_list: D003577:Cytochrome P-450 Enzyme System; D016666:Fluvoxamine; D007527:Isoenzymes; D008550:Melatonin; D017367:Selective Serotonin Reuptake Inhibitors; C012365:6-hydroxymelatonin