Metabolism of Melatonin by Human Cytochromes P450

Abstract

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, Odemethylated MEL. The K m (M) 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 IC 50 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.

Melatonin Metabolism in the Central Nervous System

“…CYP1A1 and the extrahepatic CYP1B1 contribute to the formation of 6-hydroxymela- tonin, whereas CYP2C19 and, to a much smaller extent, CYP1A2 can also demethylate melatonin to its precursor, NAS [79,93]. Since the dealkylating activity of CYP1A1 is well-known for ethoxylated substrates [114,115,156], NAS formation from melatonin seems likely for this isoenzyme, too.…”

“…Since the dealkylating activity of CYP1A1 is well-known for ethoxylated substrates [114,115,156], NAS formation from melatonin seems likely for this isoenzyme, too. In addition to other CYPs not related to melatonin metabolism, CYP1A2 [60,88,126], CYP1A1 [60,88], CYP1B1 [79,88], and CYP2C19 [58,59,159] are also expressed in the brain. The presence of these CYP isoenzymes indicates that, at least, a certain fraction of melatonin should be either 6-hydroxylated or O-demethylated in the CNS (Fig.…”

Oxidation of Endogenous N-Arachidonoylserotonin by Human Cytochrome P450 2U1

“…This is a reaction analogous to that of P450 2E1 and some other P450s with indole (49) and can be envisioned as hydroxylation followed by tautomerization. Because of the nature of the reaction, we considered melatonin as a possible substrate (50). However, we did not see consistent oxidation, and when products were observed in some reactions, they were sensitive to the inclusion of catalase in the reaction, supporting the view that they were formed by the action of H 2 O 2 adventitiously produced in the P450 2U1 reaction.…”

“…N-Arachidonoylserotonin was first reported in 1998 as a synthetic inhibitor of fatty acid amide hydrolase, an enzyme that hydrolyzes anandamide agonists of cannabinoid receptors (21), being the most active of a series of compounds (IC 50 12 M). Subsequently N-arachidonoylserotonin has been characterized in terms of its analgesic actions, with antagonistic activity at vanilloid TRPV1 receptors, with an IC 50 of 37-40 nM (55).…”

Antioxidant potential of melatonin enhances the response to L-dopa in 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine-parkinsonian mice

“…In the grid walking test, statistical analysis using ANOVA indicated a difference in the percentage of foot slip errors after treatment with the selected regimens at the two time points, at week 4 [F (6,28 Fig. 3C).…”