Variation in huperzine A and B in Australasian Huperzia species

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Abstract

Huperzine A (HupA) and huperzine B (HupB) are alkaloids with acetylcholine esterase inhibitory activity and potential applications in the treatment of Alzheimer's disease. Both alkaloids were isolated in the 1980s from the Chinese lycopod Huperzia serrata (Thunb. ex Murray) Trevis., which has been used since the Tang dynasty as a traditional Chinese medicine. Most of the HupA currently used in herbal supplements and medicines worldwide is sourced from H. serrata which on average contains only 0.18 mg g−1 dry wt HupA and is experiencing a rapid decline in China due to over-harvesting. Eleven Australasian Huperzia species were surveyed and nine species were found to contain both alkaloids, with a significant positive relationship observed between HupA and the lower abundance HupB. An Australian Huperzia carinata plant had one of the highest HupA concentrations recorded for a plant (1.03 mg g−1 dry wt) and an Huperzia phlegmaria plant had the highest HupB value of all species surveyed (0.23 mg g−1 dry wt). Intra-specific variation in huperzine concentration was examined for H. phlegmaria and Huperzia phlegmarioides and both HupA and HupB varied substantially within each species, but this variation was unrelated to foliar nitrogen levels.

Introduction

The lycopod Huperzia serrata (Thunb. ex Murray) Trevis. has a long history of use in Chinese folk medicine where it is known as Qian Ceng Ta, and has been used for the treatment of numerous ailments including contusions, strains, swellings, schizophrenia and myasthenia gravis since as early as the Tang Dynasty (College, 1985, Ma et al., 2007). In the 1980s the alkaloids huperzine A (HupA) and huperzine B (HupB; Fig. 1) were isolated from Qian Ceng Ta and since then the plant has become well known worldwide as a traditional Chinese medicinal plant (Liu et al., 1986a, Liu et al., 1986b). HupA and HupB are reversible, potent and selective acetylcholine esterase (AChE) inhibitors (Bai et al., 2000, Jiang et al., 2003, Liang and Tang, 2004, Liu et al., 1986b, Wang et al., 1986, Xu and Tang, 1987). This property makes both huperzines candidates for the treatment of diseases that result in hypofunction of cholinergic neurons, such as Alzheimer's disease, vascular dementia and myasthenia gravis (Bai et al., 2000, Chiu and Zhang, 2000, Geib et al., 1991, Jiang et al., 2003, Tang and Han, 1999, Zangara, 2003, Zhu et al., 2004).

The potency and duration of HupA-mediated AChE inhibition is similar or superior to the pharmaceutical drugs tacrine, donepezil, rivastigmine and galanthamine, with fewer toxic effects (Cheng et al., 1996, Liang and Tang, 2004, Wang et al., 1986, Zhao and Tang, 2002). HupA has recently been approved for use in China as a palliative therapy for Alzheimer's disease, and is currently used in USA as a supplement for preventing further memory degeneration (Zangara, 2003). HupB demonstrates lower anti-AChE activity than HupA, but has a longer duration of action and exhibits a higher therapeutic index, and like HupA, shows a relative lack of toxicity (Xu and Tang, 1987, Yan et al., 1987). Therefore HupA, and to a lesser extent HupB, have attracted recent pharmaceutical interest.

The major source of both huperzines remains extraction from plant tissues, despite reports on the synthesis of HupA (Kozikowski and Tückmantel, 1999, Lucey et al., 2007, Qian and Ji, 1989) and HupB (Lee et al., 2004, Wu and Bai, 1997). Published syntheses produce racemic mixtures of (±)-huperzine which exhibit less potent inhibition of AChE than does the plant extracted (−)-HupA isomer (Tang et al., 1994, Zhang et al., 2002). Plants such as H. serrata that produce HupA and/or HupB are lycopods of the genus Huperzia within the family Lycopodiaceae (Ma et al., 2005, Ma et al., 2006, Ma et al., 2007, Zhu et al., 2004). Lycopods are small and slow growing, and the huperzine concentrations in the tissues of surveyed species are relatively low, although there is some indication that concentrations may vary within a species (Ma et al., 2005, Ma et al., 2006).

Most of the HupA currently used in herbal supplements and medicines is sourced from Chinese H. serrata which on average contains only 0.18 mg g−1 dry wt HupA and is experiencing a rapid decline in China due to over-harvesting (Ma et al., 2005, Ma et al., 2006, Ma et al., 2007). This source remains in use despite there being over 200 species in the genus Huperzia, many of which are already utilized in traditional medicine (Bai et al., 2000). Relatively low concentrations of HupA have been determined in 40 of these Huperzia species, most of which occur in China (see Ma et al., 2005, Ma et al., 2006), and the HupB concentration has been rarely quantified. Within Australasia, only Huperzia varia (R.Br.) Trevis. from New Zealand has been shown to contain HupA (Ainge et al., 2002). This paper describes a survey of HupA and HupB in 11 Huperzia species from Australasia, and examines intra-specific variation within two of these species for the first time.

Section snippets

Plant material

Foliage from 11 Huperzia species was collected from natural populations where possible, or from nursery stock originally sourced from natural populations in Queensland Australia, Papua New Guinea, Fiji or Borneo (see Table 1). Plants were identified with reference to published descriptions and confirmed by A.R. Field. Voucher specimens of each species were collected by A.R. Field and deposited in the Queensland Herbarium (BRI) Australia (see Table 1 for voucher specimen numbers). Following

Survey results

The identification of HupA and HupB in extracts from Australasian Huperzia was achieved firstly by following an established RP-HPLC protocol (Wu and Gu, 2006) and comparing retention times and UV spectra with published values. Fig. 2a shows a representative HPLC chromatograph of an extract from Huperzia phlegmaria with characteristic peaks at ca. 8.2 min (compound 1) and 14.5 min (compound 2), each with dual UV maxima at 230 and 313 nm, indicative of HupB and HupA, respectively. Secondly,

Conclusion

Commercial HupA production uses traditional cultivation practices based upon the clonal propagation of lycopods. Assuming a genetic basis to huperzine variation, if the majority of individuals comprising the founding population for cultivation are sourced from genetically high HupA producing plants, the result will likely be an increase in HupA yield. Given the slow growth rates, low biomass accumulation and relatively low huperzine yield of lycopods, the observations presented here highlight

Acknowledgements

We thank Harry & Rita Kupke, Owen & Coral Rawlins, Keith & Yvonne Nunn and Holly Field for access to Huperzia plants and Wei Han Lim for laboratory assistance.

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