Anti-oxidant and Antimicrobial Flavonoid Glycosides from Alstonia boonei De Wild Leaves

Aims: Alstonia boonei De Wild (Apocynaceae) leaves are used traditionally in several parts of West Africa (including Nigeria) and Asia for the treatment of various ailments such as rheumatic and muscular pains as well as hypertension and malaria. The aim of this study is to isolate the phytoconstituents responsible for its anti-oxidant and anti-inflammatory activities.

Methodology: The leaves of Alstonia boonei were extracted in methanol and the methanol extracted subjected to series of chromatographic separation for the isolation of 8 flavonoid glycosides. The chemical structures of these compounds were elucidated by a combination of UV, HPLC-MS, 1 D and 2 D NMR spectroscopy. The antioxidant activity of the isolated flavonoid glycosides was determined by DPPH free radical scavenging model while the antimicrobial activity was determined by Agar well diffusion technique.

Results: The flavonoid glycosides were elucidated as Rutin (1), Quercetin robinobioside (2), Kaempferol-3-O-rutinoside (3), Kaempferol-3-O-robinobioside (4) and compounds 5, 7 and 8 which are glycosides of quercetin and compound 8 which is a flavonoid of kaempferol. Compounds 1, 2, 5, 7 and 8 which are all glycosides of quercetin showed good antioxidant activities (IC50 ˂66 μg/mL) on DPPH free radical scavenging model. Compounds 3, 4 and 6 which are derivatives of kaempferol, however, showed poor activity (IC50 >200 μg/mL). Of all the compounds, only 7 and 8 were active against Escherichia coli with IZD (inhibition zone diameter) values of 17 and 13 mm at 62.5 μg/mL respectively.

Conclusion: The profound antioxidant activity of the isolated quercetin derivatives may explain the ethnomedicinal use of the plant extracts in the management of inflammatory diseases and other disorders associated with oxidative stress.