CHEMICAL STUDY OF THE ROOTS OF Burkea africana Hook (Fabaceae) WITH α-AMYLASE INHIBITORY POTENTIAL

ABSTRACT
The Fabaceae is a family plants comprising of about 18,000 species classified into over
650 genera. The genus Burkea contain two species, Burkea africana are used for tooth ache, stomach problems, antidote, cough and catarrh, gonorrhoea and syphillis. The extracts and compounds from the stem barks of Burkea. africana have been shown to possess anticancer, analgesic, antibacterial, antiproliferative, antidiarrhea, antibacterial, cytotoxic and antioxidant activities. From Burkea africana, mostly flavonoids and saponins have been isolated. Plants of the Fabaceae family in general and the genus Burkea in particular are known to be rich in diverse secondary metabolites principally flavonoids saponins and tannins which are classes of compounds known for biological activities including antidiabetic property. For this reason, this study involves the isolation and characterisation of secondary metabolites from the roots of Burkea. africana and evaluation of their inhibitory potential on a-amylase. The preliminary investigation of the methanol extract of the roots of this plant allowed us to the isolation of one new compound rubesanolidic acid alongside eleven known compounds including β-sitosterol, lupeol, betulinic acid, ursolic acid, diadezein, genistein, formononetin, 3-O-β-D-xylopyranosyl-(1→2)-β-D-glucopyranosyl-27-hydroxyalphitolic acid, 3-O-methylquercetin, β-sitosterol 3-O-β-D-glucopyranoside and stigmasterol 3-O-β-D-glucopyranoside. Their structures were elucidated from spectroscopic analyses (UV, IR, MS, 1D and 2D NMR) data and by comparison with data from previous studies. The extract and compounds were tested for their a-amylase inhibition. The extract was more active than the isolated compounds with a percentage inhibition of 51.0 ±2.5% at 400mg/mL and was the only sample showing above 50% inhibition at this dose. Amongst the isolated compounds and at the dose of 400mg/mL, the new diterpenoid Rubesanolidic acid exibited the highest percentage inhibition of a-amylase of 38.2 ±2.0% while β-sitosterol showed the lowest inhibition of 9.6±0.5%. The results indicate that Burkea africana is a potential source of antidiabetic compounds.

KEYWORDS
Burkea africana; chemical composition; rubesanolidic acid; antidiabetic potential; a-amylase inhibition