Phytochemical and pharmacological studies of species of African Combretaceae

Katerere, David R.

Thesis

Phytochemical and pharmacological studies of species of African Combretaceae

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Creator

Katerere, David R.

Rights statement

Strathclyde Thesis Copyright

Awarding institution

University of Strathclyde

Date of award

2001

Thesis identifier

T10406

Qualification Level

Doctoral (Postgraduate)

Qualification Name

Doctor of Philosophy (PhD)

Department, School or Faculty

Abstract

The isolation of pharmacologically active stilbenoids like combretastatin and its analogues from the African Combretaceae in previous studies prompted the work described in the present study. It was postulated that secondary metabolites with chemotherapeutic potential first isolated in Combretum caffrum would also occur in related species from Southern Africa. Starting from this premise, a general search for these and other compounds was mounted on six of twelve plants gathered from Zimbabwe in May 1999. These were Combretum albopunctatum (a mixture of fruit and leaves was investigated), C. apiculatum (leaves), C. collinum (leaves), C. hereroense (fruit) and C. imberbe (leaves), as well as Terminalia stulhmanii (stem bark). Phytochemical fractionation was carried out on ground plant material in a search for both polar and non-polar compounds. Once the powdered plant material was defatted with n-hexane, the non-polar compounds were extracted by macerating with dichloromethane (DCM). These were subsequently fractionated by Sephadex LH-20 and preparative HPLC using acetonitrile-water as eluents in a gradient system. Thirty-two compounds were isolated by this method and are reported here. Of these compounds, the vast majority are known but have been isolated from Combretaceae for the first time in this study. Seven compounds were previously unknown, among them a bibenzyl from C. apiculatum, two cyclobutane chaicone dimers from C. apiculatum and C. albopunctatum, and three acetylated rhamnosides from C. imberbe. There was an attempt to isolate polar constituents by routine HPLC techniques as well as a general search for evidence to suggest the presence of polyhydroxylated alkaloids. The former yielded only one pure compound, quercetin galactoside and it was concluded that the methods used are not suited for this type of work and the development of better methods is required. GC-MS and High Voltage Electrophoresis (HVE) gave preliminary data suggesting that polyhydroxylated alkaloids do occur in the Combretaceae. No further work was done to isolate pure compounds employing these methods due to limited time. The biological work performed was to evaluate the antimicrobial potency of all the natural products isolated and characterized in the first phase of the project. Two microbiological assays were used - the well-diffusion assay (WDA) and the microdilution assay (MDA). The former showed that the bibenzylic compound B14-7, isolated here for the first time from C. apiculatum was active against two yeasts NCYC 2251 and NCYC 327, and two novel bidesmosides from C. imberbe and T. stulhmanii were both active against Listeria monocytogenes. The most active compounds in the MDA were those aglycone and monodesmosides from C. imberbe which showed very low minimum inhibitory concentrations (MICs) against Mycobacterium fortuitum and Staphylococcus aureus. All the other compounds only mustered moderate activity against Candida albicans and S. aureus and were inactive against Proteus vulgaris and E. coli. Further work will involve evaluation of these compounds for anti-tumour activity. They will be submitted for the screening programme of National Cancer Institute (NCI) Natural Products Branch (NPB). This is the single largest phytochemical and pharmacological survey of species from both Combretum and Terminalia of the African Combretaceae growing in southern Africa to date. Compounds similar to the combretastatins were isolated only in three of the six plant species investigated, and two species gave only terpenoid constituents. There is need for a broader evaluation of the biological activity of the compounds herein isolated with a view to tapping into the potential of the Combretaceae to yield compounds which may be of pharmaceutical and agrochemical interest.

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