title: Metabolism of melatonin by human cytochromes p450.
abstract: In humans, the pineal hormone melatonin (MEL) is principally metabolized to 6-hydroxymelatonin (6-HMEL), which is further conjugated with sulfate and excreted in urine. MEL O-demethylation represents a minor reaction. The exact role of individual human cytochromes P450 (P450s) in these pathways has not been established. We used a panel of 11 recombinant human P450 isozymes to investigate for the first time the 6-hydroxylation and O-demethylation of MEL. CYP1A1, CYP1A2, and CYP1B1 all 6-hydroxylated MEL, with CYP2C19 playing a minor role. These reactions were NADPH-dependent. CYP2C19 and, to some extent CYP1A2, O-demethylated MEL. The K(m) (microM) and V(max) (k(cat), pmol min(-1) pmol(-1) P450) for 6-hydroxylation were estimated as 19.2 +/- 2.01 and 6.46 +/- 0.22 (CYP1A1), 25.9 +/- 2.47 and 10.6 +/- 0.32 (CYP1A2), and 30.9 +/- 3.76 and 5.31 +/- 0.21 (CYP1B1). These findings confirm the suggestion of others that CYP1A2 is probably the foremost hepatic P450 in the 6-hydroxylation of MEL and a single report that CYP1A1 is also able to mediate this reaction. However, this is the first time that CYP1B1 has been shown to 6-hydroxylate MEL. The IC50 for the CYP1B1-selective inhibitor (E)-2,4,3',5'-tetramethoxystilbene was estimated to be 30 nM for MEL 6-hydroxylation by recombinant human CYP1B1. Comparison of brain homogenates from wild-type and cyp1b1-null mice revealed that MEL 6-hydroxylation was clearly mediated to a significant degree by CYP1B1. CYP1B1 is not expressed in the liver but has a ubiquitous extrahepatic distribution, and is found at high levels in tissues that also accumulate either MEL or 6-HMEL, such as intestine and cerebral cortex, where it may assist in regulating levels of MEL and 6-HMEL.
authors.affiliation: Laboratory of Metabolism, Center for Cancer Research, National Cancer Institute, Bethesda, MD 20892, USA.
authors.forename: Xiaochao
authors.initials: X
authors.lastname: Ma
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Idle
authors.forename: Kristopher W
authors.initials: KW
authors.lastname: Krausz
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Gonzalez
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics / Q000378:metabolism; D001921:Brain / Q000201:enzymology / Q000378:metabolism; D002853:Chromatography, Liquid; D065633:Cytochrome P-450 CYP1B1; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000235:genetics; D006801:Humans; D006900:Hydroxylation; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D008297:Male; D013058:Mass Spectrometry; D008550:Melatonin / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout
chemical_list: D018076:DNA, Complementary; D007527:Isoenzymes; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; C585496:CYP1B1 protein, human; C585498:Cyp1b1 protein, mouse; D065633:Cytochrome P-450 CYP1B1; D008550:Melatonin