The extractives from Sophora velutina and Calpurnia aurea and their biological activity.
This work is an account of the phytochemistry and biological activity of two related plant species within the plant family the Fabaceae, Sophora velutina from the subtribe Sophoreae and Calpurnia aurea of the subtribe Podalyrieae. Members of this family are known to contain quinolizidine alkaloids and flavonoids, which are chemotaxonomic markers in the Fabaceae. The phytochemical investigation of Sophora velutina resulted in the isolation of ten compounds, including five novel quinolizidine alkaloids, N-methylenehydroxycytisine (A-1), 7- hydroxylupanine (A-2), 6,7-dihydroxylupanine (A-3) and 17-oxo-thermopsine (A-4) from the fruits and velutinine (A-5) from the bark along with the known quinolizidine alkaloids Nmethylcytisine (A-6) and cytisine (A-7), and a cinnamate ester, methyl-3-(3',4'- dimethoxyphenyl)-2-propenoate (A-8) and triterpenoids lup-20(29)-ene-3β-ol (A-9) and 12- oleanen-3-one (A-10). The isolated compounds were tested for their antibacterial activity against Enterococcus faecalis and Pseudomonas aeruginosa. P. aeruginosa showed resistance against eight of the ten samples tested with only the cinnamate ester and the steroid 12-oleanen- 3-one (A-10) being slightly active at 200 and 175 μg mL-1, respectively. However, the quinolizidine alkaloid, N-methylcytisine (A-6) and 12-oleanen-3-one (A-10) showed good antibacterial activity against E. faecalis, with MIC values of 20.8 and 10.9 μg mL-1, respectively, with 17-oxo-thermopsine (A-4), another quinolizidine alkaloid, and the cinnamate ester showing moderate antibacterial activity against E. faecalis at concentrations of 125 μg mL-1 and 100 μg mL-1, respectively. Calpurnia aurea yielded five isoflavones, 4′,5,7-trihydroxyisoflavone (B-1), 7,3′-dihydroxy-5′- methoxyisoflavone (B-2), 7-hydroxy-4′,8-dimethoxyisoflavone (B-3), 7-acetoxy-4′,8- dimethoxyisoflavone (B-4) and 3',7-dihydroxy-4′,8-dimethoxyisoflavone (B-5), a pterocarpan (3- acetoxy-9-methoxypterocarpan) (B-6) and a quinolizidine alkaloid (calpurnine) (B-7) all of which were isolated from the stem and bark. These isoflavones were screened for in vitro anticancer activity against breast (MCF7), renal (TK10) and melanoma (UACC62) human cell lines, where 3',7-dihydroxy-4′,8-dimethoxyisoflavone (B-5) was found to be the most active amongst all the compounds tested, followed by 3',7-dihydroxy-5'-methoxyisoflavone (B-2), also with a hydroxyl and methoxy group on the phenyl ring but in the 3' and 5' positions, respectively. Elucidation of the compounds was done mainly by 1D and 2D NMR spectroscopy together with mass spectrometry, infrared, and ultraviolet spectroscopy. Antibacterial and anticancer assays were carried out using standard assays at the Centre for Scientific and Industrial Research (CSIR), Pretoria, South Africa.