5-Methoxypsoralen inhibits 6-hydroxylation of melatonin in the rat.

Pharmacokinetic studies of exogenous melatonin (100 or 1 micrograms) administered to 5-methoxypsoralen (MOP)-treated rats, showed that MOP administration induced an increase in the half-life of melatonin from 22 to 67 min. This delayed degradation of melatonin was the consequence of 5-MOP-induced inhibition of the 6-hydroxylation of melatonin. Under in vitro experimental conditions, 5-MOP did not affect melatonin synthesis and release. These results demonstrate that the elevated plasma melatonin concentrations observed in vivo following 5-MOP administration is due to inhibition of the hydroxylation of endogenous melatonin.