Phytochemical Constituents from Cassia alata with Inhibition against Methicillin-Resistant Staphylococcus aureus (MRSA)

The methanolic extract of the leaves of Cassia alata was sequentially partitioned in increasing polarity to afford the hexane, chloroform, butanol and residual extract. Crude extracts were evaluated against MRSA using the agar well diffusion assay. The butanol and chloroform extracts both exhibited inhibition against MRSA with inhibition indexes of 1.03 +/- 0.16 and 0.78 +/- 0.07 at the concentration of 50 mg/mL. The butanol extracts were further purified using silica gel and reverse phase chromatography to afford kaempferol (11), kaempferol 3-O-beta-glucopyranoside (2), kaempferol 3-O-gentiobioside (3) and aloe emodin (4). The four constituents showed varying degrees of inhibition against MRSA. Both 1 and 4 exhibited MIC50 values of 13.0 +/- 1.5 mu g/mL and 12.0 +/- 1.5 mu g/mL, respectively. The kaempferol glycosides 2 and 3 were less active with MIC50 values of 83.0 +/- 0.9 mu g/mL and 560.0 +/- 1.2 mu g/mL, respectively. A free hydroxyl group at C-3 of the flavonol structure is a structural requirement for the inhibition of MRSA.