Structure-Activity Relationship of Dietary Flavonoids for Inhibitory Activity of Mouse Brain Monoamine Oxidase (MAO) in vitro

Abstract

Thirty-seven naturally occurring flavonoids were investigated for their inhibitory activities on the mouse brain monoamine oxidase (MAO). Three flavonoids, apigenin (IC50; 1 µmol), liquiritigenin (IC50; 4 µmol) and genistein (5 µmol), were judged to be very active, four flavonoids, quercetin, leuteolin and eupatilin were moderately active with IC50 values of 10-20 µmol. 6- Methoxy flavonoids showed considerable activities with IC50 values of 20 µmol-40 µmol, while most isoflavones except genistein and daidzein showed weak activities. According to the results of structure-activity relationship of flavonoids as inhibitors of MAO, three hydroxyl groups at C-5 and C-7 on A ring and C-4' on B ring of flavone/flavanone seems to be essential for the inhibitory activity, and ortho catechol moiety at B ring showed somewhat weaker activity. Flavone/flavanone glycosides showed weaker inhibitory activity than their corresponding flavone/flavanone, and double bonds between C-2 and C- 3 were not critical for the inhibitory activity on MAO enzyme. As for polymethoxylated flavones, 6-methoxyl group was assumed to be important for the moderate inhibitory activity.