Methoxytyrosine formation as an indicator of catechol-O-methyltransferase activity in rat liver in vivo

The O-methyl derivative methoxytyrosine accumulated rapidly in rat liver after an intraperitoneal injection of L-dopa (50 mg/kg) in combination with 3-hydroxybenzylhydrazine, an inhibitor of the aromatic amino acid decarboxylase. Methoxytyrosine levels reached a plateau 40--60 min after i.p. injection of L-dopa, of which the tissue concentration declined monoexponentially. Injection of various doses of L-dopa revealed that methoxytyrosine formation was saturable and followed enzyme kinetics in rat liver. The catechol-O-methyltransferase inhibitors pyrogallol, tropolone and alpha-propyldopacetamide as well as the inhibitor of the aromatic amino acid decarboxylase benserazide inhibited the formation of methoxytyrosine dose-dependently and concomitantly increased the tissue concentration of dopa. The accumulation of methoxytyrosine from exogeneously applied L-dopa appears to be a reliable indicator of the in vivo activity of catechol-O-methyltransferase.