Phytochemical investigations of four medicinal plants of Malawi: What next?

JEROME D. MSONTHI

Chemistry Department
Chancellor College
University of Malawi
P.O. Box 280, Zomba, Malawi

ABSTRACT

Results of the phytochemical investigations of four plants of Malawi used in traditional medicine are given. The biological activity of the isolated compounds indicate that information from traditional healers is vital, as it gives useful leads in the selection of medicinal plants to be studied. The question on how the results obtained in phytochemical investigations, such as this, are usefully utilised and developed for the benefit of the people, has not yet been fully addressed at. In this paper suggestions on this issue are given.

Introduction

Research on plants of Malawi used in traditional medicine has gathered momentum. The selection of plants with acclaimed biological properties is made possible from information obtained from traditional healers. The traditional healers in Malawi have formed a professional association called the Herbalists Association of Malawi, chaired by Chief C.W. Mbatata. This Association collaborates with the Government, medical personnel, and scientific researchers, in their endeavour to promote good health to the people of the country, under a politically stable environment prevailing in Malawi.

The information obtained from traditional healers gives useful leads to plants that may have biological activity, and, in most cases, the plants so investigated do show remarkable biological properties.

The plants

In Eastern, Southern and Central Africa, the tuberous roots of Mondia whytei Skeels (Asclepiadaceae, Milkweed family) are ground to a powder, and taken orally in porridge, beer, soup or tea, as an aphrodisiac, and also to treat anorexia, schistosomiasis, constipation, and gonorrhoea (Gelfand et al. 1985). A phenolic glycoside was isolated from the methanol extract of the tubers, using combined chromatographic techniques. The structure of 1 was determined by spectroscopic methods (proton and carbon-¹³ NMR, ultraviolet and infrared spectroscopy) and by synthesis of the aglycone.

The powdered tuber was extracted successively with dichloromethane, methanol and water at room temperature. The methanol extract was separated by droplet counter current chromatography (DCCC) (chloroform:methanol:isopropanol:water 5:6:1:4, descending mode), followed by column chromatography on Sephadex LH- 20 (Methanol (MeOH)). Final purification was achieved by medium pressure liquid chromatography (MPLC), RP-8 (MeOH-H₂O, step-wise gradient).

Acid hydrolysis of the glycoside with 5% ethanolic sulphuric acid afforded the aglycone, xylose and glucose (thin layer chromatography (TLC)). The mass spectral (MS) data indicated that xylose was the terminal sugar. The interglycosidic linkage was deduced from carbon-13 nuclear magnetic resonance (¹³-NMR) data.

Synthesis of the aglycone from 2,4-dihydroxybenzoic acid was achieved in three steps. Methylation, to give 2-hydroxy-4-methoxymethyl benzoate, followed by reduction to yield 2-hydroxy-4-methoxybenzyl alcohol, and then partial oxidation of the primary alcohol with pyridinium chlorochromate (PCC) to give 2-hydroxy-4-methoxybenzaldehyde, the NMR data of which were identical to those of the aglycone, which was obtained after hydrolysis of the glycoside.

The pharmacological interest in the genus Hypoxis (Hypoxidaceae) arises from its use in traditional medicine by people in Eastern, Central and Southern Africa. Infusions of the tuber are used as a remedy for prostate hypertrophy and uterine cancer (Gelfand et al., 1985).

Compounds so far isolated from various Hypoxis plants are zeatin and zeatin glycoside (Van Staden, 1981), hypoxoside from H. obtusa (Marini-Bettolo et al., 1982), acuminoside from H. acuminata, nyasicoside (Marini-Bettolo et al., 1985), nyasicoside from H. nyasica (Galefi et al., 1987) and 1-(3",4"-dihydroxyphenyl)-5', 4'-dihydroxyphenyl)pent-l-en-4-yne from H. rooperi. These compounds show strong anticancer activity (Drewes et al., 1989).

Phytochemical investigations of Hypoxis obtusa have led to the isolation of a new phenolic glycoside named obtusaside, together with known compounds such as, accuminoside, hypoxososide and nyasoside, from the methanol extract of the whole plant, using chromatographic separation techniques. The structure of the glycoside was established by spectroscopic methods and chemical transformations.

The whole plant was cut into small pieces and extracted with methanol. The methanol extract was washed with dichloromethane and n- butanol, following which, the n-butanol extract was fractioned chromatographically.

Enzymatic hydrolysis of the glycoside with b-D-glucosidase, gave 2,5- dihydroxybenzyl alcohol from the ethyl acetate extract, identified as its triacetate, whereas acid hydrolysis with 5% ethanolic sulphuric acid gave 3- hydroxy-2, 6-dimethoxyethyl benzoate and glucose, as the sole monosaccharide in the aqueous solution (TLC). The presence of glucose was confirmed by the formation of pentaacetyl glucitol, and by comparison with an authentic sample (gas chromatography (GC)).

The glycoside was converted to the hexaacetyl derivative, while permethylation only gave the tetramethyl ether, due to steric hindrance of one phenolic hydroxyl group by the sugar moiety. The glycoside, an off-white armophous powder, gave a dark blue colour with iron (III) chloride, a positive test phenolic hydroxyl groups.

The spectroscopic data was consistent with the structure of the glycoside.

From the methanol extract of the tubers of Hypoxis nyasica, three glycosides: hypoxoside (previously isolated from H. obtusa), nyasoside and nyasicoside were isolated, together with two new monoglucosides named mononyasine A and mononyasine B.

These glycosides have the same aglycones, nyasoside (1-(4'-hydroxyphenyl)-3- (4"-hydroxyphenyl))-1,4-pentadiene. The structures were assigned by comparison of their spectroscopic data (and of the corresponding methyl and tretrahydromethyl derivatives) with those of nyasoside (and tetrahydronyasoside) (Messana et al., 1987).

In our continued studies on plants used in traditional medicine, we undertook the phytochemical investigations of Sesamum angolense Wel. (Pedaliaceae). This plant is used in traditional medicine to treat leprosy and related skin diseases. It is also used as a substitute of soap to wash women's hair. It is also endowed in particular with haemostatic properties and is used in Malawi to prevent bleeding after tooth extraction. Sesangolin and fatty acids have been previously isolated from the steam distillation of the leaves (Msonthi, 1984). The methanol extract of the root bark has resulted in the isolation of two new naphthoxirene derivatives (Potterat et al., 1987), and a new iridoid glucoside methyl antirrinoside-4- carboxylate, sesamoside, together with known compounds; phlomiol, pulchelloside-1, b-hydroxyipolamiide and a phenylpropanoid glycoside called verbasicoside (Potterat et al., 1988).

The methanolic extract from the root bark of S. angolense was submitted to DCCC (chloroform-methanol-isopropanol-water (5:6:1:4) as solvent system in the ascending mode). Further purification by medium pressure liquid chromatography on RP-8 afforded these compounds, which were characterised by spectroscopic methods and by comparison with authentic samples (TLC and HPLC). Tests are underway to determine if these compounds could be responsible for the haemostatic properties of the plant.

Having got these results, there is a need for the government to take action on how best we can utilize these findings, through participation of local pharmaceutical industries and other relevant institutions in developing these compounds for their ultimate use, if any, by the general public.

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