Phytochemistry and biological activity of the root extract of millettia oblata

Abstract

The genus Millettia belongs to Leguminoseae family, Tephroseae tribe and is known to elaborate prenylated flavonoids and isoflavonoids. In the search for bioactive principles Millettia oblata root was analysed. The dried and ground whole root of Millettia oblata was exhaustively extracted using dichloromethane: methanol (1:1) (CH2Cl2:MeOH (1:1)) followed by methanol by cold percolation. The CH2Cl2:MeOH (1:1) extract was then subjected to chromatographic isolation on normal silica gel and re-crystallisation leading to the isolation of five compounds. The structures of the isolated compounds were determined using spectroscopic methods including 1H and 13C NMR, comparison with literature and comparison with authentic samples. The isolated compounds included three isoflavones [isoerythrin A, 4’-(3-methylbut-2-enyl) ether (1), calopogoniumisoflavone B (2), 7,2’-dimethoxy-4’,5’- methylene dioxyisoflavone (4)], a chalcone 4-hydroxyonchocarpin (3) and the commonly occurring triterpene lupeol (5). This is the first report of these compounds from Millettia oblata.

In vitro anti-plasmodial activity of the crude extracts and isolated flavonoids was carried out against chloroquine sensitive D6 (CDC/Sierra Leone) and chloroquine resistant W2 (CDC/Rosewell Indochina III) strains of Plasmodium falciparum. The CH2Cl2:MeOH (1:1) crude extract showed anti-plasmodial activity against D6 and W2 P. falciparum strains with IC50 values of 8.26  1.7 and 11.49 µg/ml, respectively. The methanol extract showed anti-plasmodial activity against the D6 strain only with IC50 value of 14.84 µg/ml. All the isolated and identified flavonoids showed anti-plasmodial activity against D6 and W2 P. falciparum strains with the isoflavone isoerythrin A, 4’-(3-methylbut-2-enyl) ether (1) showing the highest potency with IC50 values of 6.61 2.8 and 15.10  4.8 µM against D6 and W2, respectively. Anti-bacterial activity of the crude extracts and isolated flavonoids was also carried out against gentamycin sensitive Staphylococcus aureus (NC 07447), Bacillus pumilus (NC 08241), and Escherichia coli (ATCC 25922). Anti-fungal activity of the crude extracts and isolated flavonoids was also carried out against nystatin sensitive Candida albicans. The crude extracts showed activity against the three bacteria but only the methanol extract showed anti-fungal activity against Candida albicans. Amongst the isolated compounds only the chalcone 4-hydroxyonchocarpin (3) showed anti-bacterial and anti-fungal activity. The critical inhibitory concentration (CIC) of the CH2Cl2):MeOH (1:1) crude extract and compound 3 were found to be below 6.45 and 1.53 mg/ml, respectively. The MICs (Minimum inhibitory concentration) of CH2Cl2:MeOH (1:1) crude extract and 4-hydroxyonchocarpin (3) were found to be 613 and 2.92 μg/ml, respectively against Staphylococcus aureas (NC07447), Bacillus pumilus (NC08241) and Escherichia coli (ATCC25922). The study has provided some flavonoids of Millettia oblata root as possible leads for the discovery, innovation and development of new anti-malarials and anti-bacterial agents. However, further bioassays including acute and chronic toxicity, pharmacokinetic and pharmacodynamic profiles should be carried out to fully establish the potential of Millettia oblata crude root extract and phytochemicals as safe and effective therapeutic agents.