Biological activity of five antibacterial flavonoids from Combretum erythrophyllum (Combretaceae)

Preliminary studies with Combretum erythrophyllum showed antimicrobial activity against Gram-positive and Gram-negative bacteria. Seven antibacterial flavonoids were subsequently isolated by bioassay-guided fractionation, i.e. apigenin; genkwanin; 5-hydroxy-7,4'-dimethoxyflavone, rhamnocitrin; kaempferol; quercetin-5,3'-dimethylether; rhamnazin. All compounds had good activity against Vibrio cholerae and Enterococcus faecalis, with MIC values in the range of 25-50 microg/ml. Rhamnocitrin and quercetin-5,3'-dimethylether also inhibited Micrococcus luteus and Shigella sonei at 25 microg/ml. With the exception of 5-hydroxy-7,4'-dimethoxy-flavone the flavonoids were not toxic towards human lymphocytes. This compound is potentially toxic to human cells and exhibited the poorest antioxidant activity whereas rhamnocitrin and rhamnazin exhibited strong antioxidant activity. Genkwanin; rhamnocitrin; quercetin-5,3'-dimethylether; rhamnazin had a higher anti-inflammatory activity than the positive control mefenamic acid. Although these flavonoids are known, this is the first report of biological activity with several of these compounds.