A new flavonol glycoside and bioactive flavonoids from the fruits of Citrus unshiu

Citrus unshiu Marcow (Rutaceae) is cultivated around the subtropical region of Southeast Asia. In Korea, it is distributed only in the southern area, especially in Jeju Island. It has been used as a traditional Korean medicine for improving skin elasticity, relieving fatigue and cough, and preventing bronchitis, flu, and various cancers [1, 2]. Earlier studies have showed that flavonoids and limonoids were present in C. unshiu [3, 4], and the extracts of C. unshiu displayed diverse bioactivities including anti-oxidant, anti-inflammatory, and anti-cancer activities [1, 5]. Chemical investigation of the EtOH extract of the fruits of C. unshiu resulted in the isolation and identification of a new flavonol glycoside (1) along with 7 (2 – 8) known flavonoids, and a limonoid (9). The new compound was identified as limocitrunshin and the structure of new compound was elucidated by spectroscopic analysis, including 1D and 2D NMR, HR-MS and chemical reaction. All compounds were tested for their effects on acetylcholine esterase (AChE) inhibition, protection against cisplatin-induced nephrotoxicity, and inhibition of nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells. Among the compounds studied in this work, compound methyl nomilinate (9) at a concentration of 30 µg/mL inhibited acetylcholinesterase activity by more than 50% presented significant acetylcholinesterase inhibitory activity, and compound (2S)-narirutin (8) inhibited potent NO production in LPS-stimulated macrophages (IC₅₀= 65µM) and was the strongest inhibitor as compared to the control. In addition, compounds kaempferol 3-(2^G-rhamnosylrutinoside) (5) and (2S)-narirutin (8) showed the most potent inhibition of renal cell damage by inhibiting the apoptosis pathway through the JNK-p53-caspase apoptotic cascade. Thus, the fruits of C. unshiu could serve as a valuable natural source of bioactive compounds with health benefits for potential application in functional foods.

Acknowledgements: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1C1A1A02037383).

Keywords: Citrus unshiu, Rutaceae, Flavonoid, Acetylcholine esterase, Nephrotoxicity, Nitric oxide.

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