Effects of Enantiomerically Pure β-Carboline Alkaloids from Picrasma quassioides on Human Hepatoma Cells

Four pairs of beta-carboline enantiomers (1a/1b-4a/4b), 2 beta-carboline derivatives (5-6) with a single enantiomeric configuration, together with 2 known achiral congeners (7-8) were isolated from the stems of Picrasma quassioides. Their structures were elucidated on the basis of extensive spectroscopic analyses and quantum mechanical calculations. Compound 5 possesses a 4,5-seco beta-carboline framework and represents the first example of this type of beta-carboline alkaloids from nature. A possible biosynthetic pathway is proposed to generate the racemate 4 and the enantiomerically pure compounds 5 and 6. All isolates were screened for their cytotoxicity against hepatocellular carcinoma Hep3B and HepG2 cells, which revealed that enantiomeric compounds 4a and 4b had distinctive effects in HepG2 cells. Further investigation showed that 4b could induce apoptosis in HepG2 cells.