Phytochemistry and bioactivities of kava (Piper methysticum, Piperaceae) alkaloids and lactones.

摘要

Kava (Piper methysticum) rhizome extracts had become popular anxiolytic-sedative agents in the West that were considered safe and effective until recently. During 1998-2003, serious cases of irreversible hepatotoxicity were reported. The causal agent is unclear. The first reporting coincided with an acute shortage of kava rhizome material due to a surge in demand. This project explores the possibility that contamination from aerial parts, in particular leaves and stem peelings, were the cause of the problem. We discovered a co-elution of pipermethystine (PM) with particular kavalactones in HPLC. It means that PM may not have been detected in the industry where HPLC was used for quality control. We have quantitatively determined PM in different kava cultivars from Melanesia, Micronesia and Polynesia. PM, in form of its S-(-)-PM enantiomer, was concentrated at 1.5% in leaves and at 0.2% in stem peelings (cortex with epidermis tissues), but nearly missing in underground portions and peeled stems (0.02%). Three new alkaloids were also identified: awaine, (-)-deacetyl-PM, and (-)-epoxy-PM of which awaine and (-)-epoxy-PM were quantitatively determined in the aerial parts. Deacetyl-PM is proposed to be the evolutionary and biosynthetic precursor of PM. In vitro structure-activity studies with PM in human hepatoma, HepG2, cells suggest that the N-phenylpropanoid moiety and the 3,4-double bond, but not the O-acetyl rest, are required features for its toxicity. Both PM and deacetyl-PM are potential safety concerns because of the previous industrial use of aerial parts, which have been avoided in preparing the traditional kava drink. For the preparation of authentic standards of kavalactones and PM, an improved isolation method was developed. A rapid gas chromatography method for their simultaneous quantitation was also established and applied to experimental kava powders from different rhizome-near tissues. Because of the particular distribution of PM and its analogs, the use of aerial kava parts for human ingestion should be discouraged. Initial studies conducted herein regarding inhibition of Arabidopsis thaliana seed germination and root growth, and Phytophthora colocasiae zoospore germination suggest a wide range of biological activities of PM in addition to in vitro toxicities against HepG2 cells.