title: Contribution of CYP1A2 in the hepatic metabolism of melatonin: studies with isolated microsomal preparations and liver slices.
abstract: The objective of the present studies was to define the enzyme systems catalysing the 6-hydroxylation of melatonin, by monitoring the levels of 6-sulphatoxymelatonin in rat hepatic postmitochondrial preparations and in precision-cut liver slices. Melatonin 6-hydroxylase activity was localized in microsomes and was supported by NADPH, but not NADH. Treatment of rats with beta-naphthoflavone more than tripled 6-sulphatoxymelatonin formation from melatonin, but gave rise only to a moderate increase (25%) in the sulphate conjugation of 6-hydroxymelatonin. Treatment of rats with phenobarbitone, acetone, dexamethasone and clofibrate did not increase 6-sulphatoxymelatonin generation when either melatonin or 6-hydroxymelatonin served as substrates. Of a number of cytochrome P450 inhibitors investigated, only furafylline inhibited markedly the conversion of melatonin to 6-sulphatoxymelatonin without any concomitant effect on the sulphoconjugation of 6-hydroxymelatonin. When liver slices were incubated with melatonin, treatment of rats with beta-naphthoflavone, and to a lesser extent phenobarbitone, elevated the levels of 6-sulphatoxymelatonin in the culture medium. No such increase was seen when slices from beta-naphthoflavone-treated rats were incubated with 6-hydroxymelatonin, whereas a modest increase was seen with slices from phenobarbitone-treated rats. Treatment of rats with acetone, dexamethasone or clofibrate failed to modulate the levels of 6-sulphatoxymelatonin generated from either melatonin or 6-hydroxymelatonin. Molecular modelling analysis revealed that melatonin had a high area/depth(2) ratio, displayed characteristics of CYP1A2 substrates and could be readily accommodated into the human CYP1A2 active site in a position favouring 6-hydroxylation. Collectively, all the above data provide strong experimental evidence that CYP1A2 is an important catalyst of the 6-hydroxylation of melatonin.
authors.affiliation: School of Biological Sciences, University of Surrey, Guildford, Surrey, GU2 5XH, UK.
authors.forename: D J
authors.initials: DJ
authors.lastname: Skene
authors.forename: E
authors.initials: E
authors.lastname: Papagiannidou
authors.forename: E
authors.initials: E
authors.lastname: Hashemi
authors.forename: J
authors.initials: J
authors.lastname: Snelling
authors.forename: D F
authors.initials: DF
authors.lastname: Lewis
authors.forename: M
authors.initials: M
authors.lastname: Fernandez
authors.forename: C
authors.initials: C
authors.lastname: Ioannides
mesh_terms: D000818:Animals; D001571:Benzoflavones / Q000494:pharmacology; D002753:Chlorzoxazone / Q000494:pharmacology; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism*; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066298:In Vitro Techniques; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008550:Melatonin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism*; D008862:Microsomes, Liver / Q000378:metabolism; D008958:Models, Molecular; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D051381:Rats; D017208:Rats, Wistar; D013806:Theophylline / Q000031:analogs & derivatives* / Q000494:pharmacology
chemical_list: D001571:Benzoflavones; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D004791:Enzyme Inhibitors; C054513:6-sulfatoxymelatonin; D009249:NADP; C011512:alpha-naphthoflavone; D013806:Theophylline; C050131:furafylline; D019388:Cytochrome P-450 CYP1A2; D002753:Chlorzoxazone; D008550:Melatonin