Inhibitory activities of Cassia tora and its anthraquinone constituents on angiotensin-converting enzyme

As a component of our program that pertains to the isolation of antihypertensive agents derived from natural products, we screened the bioactivity of seeds from raw and roasted Cassia tora via angiotensin converting enzyme (ACE) inhibitory assays. We found that both of the MeOH extracts from the raw and roasted C. tora exhibited significant inhibitory properties against ACE, demonstrating more than 50% inhibition at a concentration of 163.93 microg/mL. Emodin (3), alaternin (4), gluco-obtusifolin (5), cassiaside (6), gluco-aurantioobtusin (7), cassitoroside (8), toralactone gentiobioside (9), and chrysophanol triglucoside (10) had been previously isolated; however, questin (1) and 2-hydroxyemodin 1-methylether (2) were isolated from C. tora for the first time in this study. Among them, only anthraquinone glycoside (7) demonstrated marked inhibitory activity against ACE, with an IC(50) value of 30.24 +/- 0.20 microM. Conversely, aurantioobtusin (7a), obtained from the acid hydrolysis of 7, showed no activity. Further inhibitory kinetics analyzed from Lineweaver-Burk plots showed 7 to be a competitive inhibitor with a Ki value of 8.3 x 10(-5) M. Moreover, compound 7 showed marked inhibitory and scavenging activities with an IC(50) value of 49.64 +/- 0.37 microM (positive control; trolox: 26.07 +/- 1.05 microM) for total reactive oxygen species generation, and 4.60 +/- 1.12 microM (positive control; penicillamine: 0.24 +/- 0.04 microM) for ONOO(-).