Bioassay-guided isolation of constituents of Piper sarmentosum using a mitochondrial transmembrane potential assay

Bioassay-guided fractionation was conducted on a chloroform-soluble extract of the aerial parts of Piper sarmentosum collected in Vietnam, monitored by a mitochondrial transmembrane potential assay using HT-29 human colon cancer cells. This led to the isolation of four new C-benzylated dihydroflavones, sarmentosumins A-D (1-4), as well as 14 known compounds. The structures of the new compounds were elucidated on the basis of spectroscopic data interpretation. Among these compounds, 1-4 as well as five known C-benzylated dihydroflavones (5-9) and a piperamide, pipercallosine (11), were found to induce apoptosis in HT-29 cells by moderately reducing the mitochondrial transmembrane potential (ΔΨm), with ED50 values ranging from 1.6 to 13.6 microM. Furthermore, 7-methoxydichamanetin (8) and pinocembrin (10) exhibited proteasome inhibitory activities in a human 20S proteasome bioassay with IC50 values of 3.45±0.18 and 2.87±0.26 microM, respectively. This is the first time that C-benzylated dihydroflavones have been reported to demonstrate an apoptotic effect associated with disruption of the mitochondrial transmembrane potential.