[Competitive inhibition of catechol-O-methyltransferase by the tetrahydroisoquinoline alkaloids salsolidine and 1-carboxysalsoline].

The tetrahydroisoquinoline alkaloids salsolidine (1-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline) and 1-carboxysalsoline (1-carboxy-1-methyl-6-hydroxy-7-methoxy-1,2,3,4-tetrahydroisoquinoline), which are themselves not substrates for the enzyme catechol-O-methyltransferase, are - contrary to a hitherto existing view - able to inhibit competitively the methylation of the catecholamine metabolite 3,4-dihydroxybenzoic acid by the enzyme enriched from rat liver. The inhibitor constants were determined to be Ki = 0.19 mM for salsolidine and Ki = 0.44 mM for 1-carboxysalsoline.