[6.7-Dihydroxycoumarin (Aesculetin) as a substrate for catechol-o-methyltransferase (author's transl)].

6,7-Dihydroxycoumarin (Aesculetin) was found to be a substrate of rat liver Catechol-O-methylfransferase (COMT) (EC 2.1.1.6). Incubation of this substrate with S-Adenosyl-L-[methyl-14C]methionine and/or S-Adenosylmethionin-hydrogensulfate in the presence of COMT yields the highly fluorescent compounds 7-hydroxy-6-methoxycoumarin (Scopoletin) and 6-hydroxy-7-methoxycoumarin (Isoscopoletin) in the ration of about 2:1. The O-methylated products obtained from Aesculetin were identified after separation by thin layer chromatography mainly by the reversed isotope dilution technique. The fluorescence of the isolated methylethers was proportional to concentration within the range from 10(-5)-10(-7) M. A reciprocal plot of activity versus substrate concentration gives a Km of 1 X 10(-6) M.