Identification of Antibacterial Compounds in Lo Han Kuo fruit extract

Lo Han Kuo fruit (Momordica grosvenori) native to China has been used for various medicinal purposes in Chinese herbal medicine for centuries. Lo Han Kuo (LHK) extract is also used as zero-calorie high intensive natural sweetener, derived from the LHK fruit, which is 300X sweeter than sucrose and has recently been produced on an industrial scale. Critical research has not been conducted to examine LHK biological activities and any potential health effects. Objective: This study used a combination of purification and bioassay to isolate and describe bioactive compounds from LHK extract. Methods: LHK extracts were purified and bioactive compounds isolated following standard purification procedures. A resazurin-based microassay was used to screen for antimicrobial activity of purified LHK fractions and bacterial culturing was used to further confirm these bioactivities. Results: The fruit extract had a strong antimicrobial activity against the oral bacterium, S. mutans. Through bioactivity guided separation using column chromatographic, preparative thin layer chromatographic, and high performance liquid chromatographic methods, mogrosides including mogroside V, and other mixture compounds were obtained. The pure mogroside V showed no antimicrobial activity, while the mixture compounds which had absorptions in 254 and 365 nm, and were mostly composed of the polyphenols from 1H-, 13C-NMR spectra analysis, significantly inhibited the growth of S. mutans. A phenylpropanoid with an aldehyde group contributed to this activity. Conclusions: Our study demonstrated that mogroside V has no antibacterial activity, which is contrary to others' report that mogroside V had potent antibacterial activity. The likely reason in the bioactivity discrepancy is probably due to the impurity of mogroside V in others' study as those studies only used the LHK extract. The bioactive compounds of LHK fruit extract appear to be polyphenols and further studies will be carried out to confirm their chemical identities and structures. Supported by NIDCR grant R41DE17265-01.