Antimycobacterial activity and possible mode of action of newly isolated neodiospyrin and other naphthoquinones from Euclea natalensis

The roots of Euclea natalensis are used by the indigenous people of southern Africa against various bacterial infections. Six naphthoquinones; diospyrin, isodiospyrin, mamegakinone, 7-methyljuglone, neodiospyrin and shinanolone were isolated from root extracts of this plant. In the case of neodiospyrin, this was a first time isolate. The activity of the six isolated compounds were tested against Mycobacterium tuberculosis using the radiometric BACTEC assay, providing a first report on the antimycobacterial activity of isodiospyrin, neodiospyrin and mamegakinone. The MIC values of diospyrin (8.0 μg/ml), isodiospyrin (10.0 μg/ml), 7-methyljuglone (0.5 μg/ml) and neodiospyrin (10.0 μg/ml) compared well to those of the known antimycobacterial drugs, ethambutol, isoniazid and rifampicin. A hypothesis on the structure-activity relationship for the naphthoquinones and a possible mode of action is discussed.