Chemical constituents and biological activities of African medicinal tree Sterculia setigera Delile stem bark

Stem bark of Sterculia setigera Delile, family Sterculiaceae, is used in traditional medicine in Sudan and other African countries to treat various diseases. This study aimed to investigate the chemical constituents, antioxidant, enzyme inhibition and antiproliferative activities of stem bark extracts and an isolated compound. Compounds were extracted using hexane and dichloromethane: ethanol (DE) (1:1). Chromatographic techniques were performed to characterize the phenolic compounds. We isolated lupeol as a bioactive compound in S. setigera from DE extract. The antioxidant and enzyme inhibitory properties of each extract were evaluated. The antiproliferative activity of the DE extract was determined. Results showed that five known compounds namely; procyanidins as dimer B, trimer C1 and tetramer, 3,4-dimethoxyphenol ß-D-apiofuranosyl (1″→6″)- ß -D-glucopyranoside and (+)- catechin were identified or tentatively elucidated, in addition to the isolation and characterization of lupeol. DE extract showed significantly (p<0.05) higher antioxidant activity than that observed from the hexane extract. Both extracts displayed considerable enzyme inhibitory activity. DE extract revealed remarkable tyrosinase inhibitory activity (115.26±1.10 mg kojic acid equivalents (KAE)/g) and displayed high antiproliferative activity against human colon adenocarcinoma HT29 (IC50 4.4 ± 0.8 µg/ml) and HCT116 (IC50 7.3 ± 2.2 µg/ml) cell lines without any toxicity to Vero normal cells. Isolated lupeol exerted high tyrosinase inhibitory (37.68±5.65 mg KAE/g) and α-amylase (27.64±0.55 mg galanthamine equivalents/g) activities. In silico study showed that 3,4-Dimethoxyphenol ß -D-apiofuranosyl (1″→6″)-ß-D-glucopyranoside revealed the best docking score for tyrosinase. In conclusion, stem bark of S. setigera could be a rich natural sources of bioactive agents.