Catechol-O-methyltransferase (COMT, EC 2.1.1.6) plays a central role in the metabolic inactivation of
neurotransmitters and neuroactive
xenobiotics possessing a
catechol motif. 1-(3,4-Dihydroxy-5-nitrophenyl)-2-phenyl-ethanone (BIA 3-202) is a novel
nitrocatechol-type inhibitor of COMT, the potential clinical benefit of which is currently being evaluated in the treatment of
Parkinson's disease. In the present work we characterize the molecular interactions of
BIA 3-202 within the active site of COMT and discuss their implication on the regioselectivity of metabolic O-methylation. Unrestrained flexible-docking simulations suggest that the
solution structure of this complex is better described as an ensemble of alternative binding modes, in contrast to the well defined bound configuration revealed by the X-ray structures of related
nitrocatechol inhibitors, co-crystallized with COMT. The docking results wherein presented are well supported by experimental evidence, where the pattern of in vitro enzymatic O-methylation and O-demethylation reactions are analyzed. We propose a plausible explanation for the paradoxical in vivo regioselectivity of O-methylation of
BIA 3-202, as well as of its related COMT inhibitor
tolcapone. Both compounds undergo in vivo O-methylation by COMT at either meta or para
catechol hydroxyl groups. However, results herein presented suggest that, in a subsequent step, the p-O-methyl derivatives are selectively demethylated by a microsomal
enzyme system. The overall balance is the accumulation of the m-O-methylated metabolites over the para-regioisomers. The implications for the general recognition of
nitrocatechol-type inhibitors by COMT and the regioselectivity of their metabolic O-methylation are discussed.