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Contribution of CYP1A2 in the hepatic metabolism of melatonin: studies with isolated microsomal preparations and liver slices
Author(s) -
Skene Debra J.,
Papagiannidou Eleni,
Hashemi Elham,
Snelling Jacquie,
Lewis David F.V.,
Fernandez Michelle,
Ioannides Costas
Publication year - 2001
Publication title -
journal of pineal research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.881
H-Index - 131
eISSN - 1600-079X
pISSN - 0742-3098
DOI - 10.1034/j.1600-079x.2001.310408.x
Subject(s) - melatonin , cyp1a2 , clofibrate , microsome , medicine , endocrinology , hydroxylation , cytochrome p450 , chemistry , metabolism , pharmacology , biology , enzyme , biochemistry
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 β‐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 β‐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 β‐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.