Regeneration of serotonin from 5-methoxytryptamine by polymorphic human CYP2D6

Polymorphic cytochrome P450 2D6 (CYP2D6) is expressed in several types of central neurons but its function in human brain is currently unknown. Using recombinant enzymes and CYP2D6-transgenic mice, we established that 5-methoxytryptamine (5-MT), a metabolite and precursor of melatonin, is a specific and high-turnover endogenous substrate of CYP2D6. This potent serotonergic neuromodulator in numerous physiological systems binds tightly to recombinant CYP2D6 enzyme with an equilibrium dissociation constant (K_s) of 23.4 μmol/l, and is O-demethylated to serotonin (5-hydroxytryptamine, 5-HT) with a high turnover of 51.7 min^-1 and low Michaelis-Menten constant of 19.5 μmol/l. The production of 5-HT from 5-MT catalyzed by CYP2D6 was inhibited by selective serotonin reuptake inhibitors, and their inhibition potency (K_i, μmol/l) decreased in the order of fluoxetine (0.411 ) > norfluoxetine (1.38) > fluvoxamine (10.1) > citalopram (10.9). Liver microsomes prepared from CYP2D6-transgenic mice showed about 16-fold higher 5-MT O-demethylase activity than that from wild-type mice. After the intravenous co-administration of 5-MT (10 mg/kg) and pargyline (20 mg/kg), serum 5-HT level was about 3-fold higher in CYP2D6-transgenic mice than wild-type mice. When dosed with α,α,β,β-d₄-5-MT, α,α,β,β-d₄-5-HT was detected in CYP2D6 transgenic mouse serum, and its content was much higher than wild-type mouse, α,α,β,β-d₄-5-HT was not produced in CYP2D6-transgenic mice pretreated with quinidine. The regeneration of 5-HT from 5-MT provides the missing link in the serotonin-melatonin cycle. Up to 10% of the population lacks this enzyme. It is proposed that this common inborn error in 5-MT O-demethylation to serotonin influences a range of neurophysiologic and pathophysiologic events.