The cytochrome P450-dependent liver monooxygenase system is influenced by many endocrine glands. The components of the system show circadian rhythmicity and therefore their activities change during the day. This may suggest that the pineal gland, whose activity is strongly related to the external environment, influences this metabolic system. A growing interest in melatonin used as a nonprescription drug that inhibits aging and cures various diseases prompted us to investigate the influence of pineal hormones on the liver's metabolic system. The aim of this study was to evaluate the effect of pinealectomy, melatonin, or serotonin on the cytochrome P450-dependent monooxygenase system. The experiment was performed on sexually mature male Wistar rats. The rats were divided as follows: a control group, pinealectomized rats, melatonin-treated rats, and seroto- nin-treated rats. Pinealectomy was done 14 days before decapitation. The isolated microsomal fraction was assayed for the activity of cytochrome P450-dependent monooxygenase system and for the activity of the enzymatic system, which non-obligatorily cooperates with the MFO system. Both pinealectomy and prolonged administration of melatonin or serotonin modified the activity of the mixed function oxidase system. In all experimental groups the components of microsome electron-transport chain I were inhibited. In microsome electron-transport chain II only melatonin and serotonin produced an inhibitory effect, while pinealectomy did not affect cytochrome b5 and NADH-cytochrome b5 reductase. 4-Aminopyrine N-deme- thylase and aniline hydroxylase did not alter either after pinealectomy or melatonin treatment (only a decreasing tendency was noted). In contrast, serotonin treatment decreased these activities significantly.
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