Structural requirements in the reaction of morphine uridine diphosphate glucuronyltransferase with opioid substances

The influence of the 3-hydroxyl and N-alkyl groups in the reactivity of narcotic compounds with morphine UDP-glucuronyltransferase was studied. Opioids possessing both, one or none of these groups were tested for inhibition of morphine glucuronidation in rabbit liver microsomal preparations. Compounds with only a 3-hydroxyl group (normorphine) or an N-methyl group (codeine, ethylmorphine) were less potent competitive inhibitors than those containing both groups (dextrorphan). Norcodeine, with neither of these groups, had no inhibitory effect. The synthetic narcotics (+)- and (-)-methadone, (-)-alpha-acetylmethadol and meperidine, with only an N-alkyl group, were effective competitive inhibitors. No stereoselectivity of the morphine glucuronyltransferase for opioid isomers was observed, and [methionine]enkephalin does not react with morphine glucuronyltransferase. Differences of pKa values and water/lipid solubility of narcotics could not explain the effects. Results indicate that the N-alkyl group plays a critical role in the interaction of narcotics with the morphine UDP-glucuronyltransferase.