Labelled 3-O-Methyl-L-DOPA. 3-O-Methyl-L-DOPA is the major metabolite of L-DOPA via catechol O-methyltransferase. 3-Methoxy-L-tyrosine is neither a substrate for nor an inhibitor of L-amino acid decarboxylase activity. Inhibition of COMT enhances the anti-Parkinson activity of L-DOPA.