Abstract
The objective of these studies was to investigate interactions between melatonin and the cytochrome P450 system. CYP1A2 was identified as the cytochrome P450 enzyme responsible for the 6-hydroxylation of melatonin, its principal route of metabolism, in rat liver. Interaction studies were undertaken, in both rat and human liver, to establish if melatonin metabolism, through 6-hydroxylation and suphation, is influenced by frequently co-administered drugs. Fluvoxamine and 5-methoxypsoralen strongly inhibited the 6-hydroxylation of melatonin in rat liver, with the effect of 5-methoxypsoralen being even more pronounced in human liver. Diazepam, tamoxifen, paracetamol and 17α-ethinyloestradiol failed to modulate the 6-hydroxylation of melatonin in both rat and human liver. 17α-Ethinyloestradiol, however, significantly decreased the sulphate conjugation of 6-hydroxymelatonin in human liver. Pre-treatment of rats with caffeine significantly increased the nocturnal peak of endogenous 6-sulphatoxymelatonin in the urine, but not the total 24-hour production. Caffeine pre-treated animals excreted significantly more 6-sulphatoxymelatonin in the urine between 8 to 12 hours following melatonin administration. A significant correlation (R=0.8108) was observed between the nocturnal peak of endogenous 6-sulphatoxymelatonin in the urine and CYP1A2 activity, in both control and caffeine-treated animals. A similar correlation was found between the amount of 6-sulphatoxymelatonin excreted in the urine following melatonin administration and CYP1A2 activity. Thus, both nocturnal endogenous and exogenous 6-sulphatoxymelatonin could be used for assessing CYP1A2 activity in humans. Daily 24h rhythms were observed in CYP1A1, CYP2B1 and CYP2E1 activity. In contrast, no rhythms were evident in CYP1A2, CYP2A1/2, CYP2C11, CYP3A and testosterone 16α- and 16β-hydroxylase activities, in CYP2D1 and CYP4A1 expression, as well as in the glutathione S-transferase, UDP-glucuronide transferase and epoxide hydrolase. Lesions of the suprachiasmatic nuclei led to the disappearance of the 24h rhythms in CYP1A1 and CYP2E1 activity indicating that the SCN circadian clock may play a role in the rhythmic expression of these enzymes.