Evidence that 6-hydroxydopamine is an inhibitor of catechol-O-methyltransferase in intact tissue

The effect of in-vitro sympathetic denervation on the O-methylation of 2-hydroxyoestradiol was examined using the rabbit thoracic aorta. Aortic segments were exposed to 6-hydroxydopamine (400 mg litre-1, 10 min), incubated in 2-hydroxyoestradiol (5 microM, 60 min), and the 2-methoxyoestradiol formed was quantified using HPLC with electrochemical detection. Pre-exposure to 6-hydroxydopamine reduced O-methylation by 90% in intact aortic strips. However, O-methylation was also reduced (81%) in rabbit aorta that had been surgically denervated before exposure to 6-hydroxydopamine. Furthermore, the O-methylation of 2-hydroxyoestradiol (20 microM, 15 min) by partially purified soluble catechol-O-methyltransferase was inhibited by 6-hydroxydopamine (400 mg litre-1) by 95% and 51% in the absence and presence of antioxidant, respectively. These results suggest that the inhibition of catechol-O-methyltransferase by 6-hydroxydopamine reported for the purified enzyme, applies to the intact tissue and that the inhibition is dependent on oxidation of the 6-hydroxydopamine. Subsequent experiments using dialysis techniques demonstrated that the inhibition of soluble catechol-O-methyltransferase by 6-hydroxydopamine was irreversible in part, but the degree of irreversibility was influenced by pH and by the extent of in-vitro oxidation of 6-hydroxydopamine.