This thesis reports on a series of investigations into the phytochemistry and pharmacology of plants belonging to the ginger family, Zingiberaceae (incl. Costaceae). The work falls into two main parts. The first part examines the pungent compounds and essential oil in 17 clones of ginger (Zingiber officinale) with a view to identify one or more with unique chemistry and consequent particular therapeutic (or flavouring) prospects. The second part comprises the screening of 41 taxa for inhibition of PGE2 and other biological activities, with the primary aim of identifying species with potential anti-inflammatory activity. This part tested the hypothesis that the combination of ethnobotanical and taxonomic information is a productive strategy to identify previously unrecognised plant species with therapeutic potential.
Chapter 1 provides a general introduction to plants as medicines and the field of ethnopharmacology. It also provides an overview of the process of inflammation, in particular arachidonic acid metabolism.
The main literature review is presented in Chapter 2. It reviews the literature relating to the chemistry and pharmacology of 15 genera in the Zingiberaceae, with the focus on species included in the experimental work. The Zingiberaceae is rich in species used as traditional medicines or spices, but extensive information about their chemistry and pharmacology is available only for a few species, most notably ginger and turmeric (Curcuma longa). These attributes make the family an ideal target for a screening project, since phylogenetically related plant species usually display a significant degree of similarity in the kinds of secondary metabolites they produce.
Chapter 3 describes the preliminary experimental work with ginger. This work aimed at determining a suitable extraction solvent and method, guided by the activity of the extracts in a cyclooxygenase-1 (COX-1) bioassay. It also established an HPLC method suitable for the quantification of gingerols and shogaols in the extracts.
Seventeen ginger clones, including commercial cultivars and 12 experimental clones, were analysed by HPLC for their content of pungent compounds. The result of this work is reported in Chapter 4. Because ginger is a sterile cultigen, there is an increased likelihood that chemically distinct and genetically stable clones may exist. This work identified one cultivar that when compared with other clones contained a significantly higher level of pungent gingerols. The analysis included 12 tetraploid clones, but these did not display elevated gingerol production compared with their diploid parent cultivar.
The essential oils obtained from the same 17 ginger clones by steam distillation were analysed by GC-MS. These results are presented in Chapter 5. The oil from one particular clone was distinctly different from the others; this was the same clone that had a very high gingerol content. The essential oil of this clone differed from the others by having a lower citral content and higher levels of sesquiterpene hydrocarbons. The unique chemistry of this clone in terms of aroma, pungency and flavour should make it of interest to the flavour, fragrance and pharmaceutical industries.
Chapter 6 presents the results of the screening of 41 taxa in an in vitro cell-based bioassay for inhibition of PGE2 production. A number of the samples were also tested for antioxidant activity in the oxygen radical absorbance capacity (ORAC) assay, for inhibition of nitric oxide production and for modulation of natural killer cell activity. Known medicinal plants, in particular ginger and turmeric, emerged as the most active in these assays. Included in the work were seven native Australian species not previously investigated for pharmacological activity. Two of these species showed good activity in the PGE2 assay and were selected for further investigations.
Chapter 7 reports on the bioactivity-guided fractionation of these two native Australian species, Curcuma australasica and Pleuranthodium racemigerum. Inhibition of PGE2 was used as the primary bioassay in this process, but fractions with high activity in that assay were also tested for cytotoxic properties. This work succeeded in isolating and structurally characterising a novel curcuminoid compound with potent PGE2 inhibitory activity from P. racemigerum as well as two known compounds from C. australasica.
The final chapter (Chapter 8) provides a short summary and concluding remarks, and identifies areas for future research arising from the present work.