Dioscorea dumetorum Pax (family: Dioscoreaceae) is commonly known as bitter yam or cluster yam¹. It occurs wildly throughout Africa, predominantly in the tropics. Its use in treating schistosomiasis², as a topical anaesthetic³ and arrow poison⁴ has been reported.

An alkaloid present in the yam extract, dioscoretine, has been reported to possess hypoglycaemic effect⁵. Other alkaloids reported to be present in the yam tuber 4, 6_{, d}include dihydrodioscorine ioscorine⁷ and dumetorine⁴. Besides alkaloids, the tuber contains small quantities of a sapogenin, diosgenin⁴. Diosgenin is used as a precursor in the commercial synthesis of sex hormones, birth control pills and corticosteroids⁸.

In "folk medicine" , extracts of Dioscorea dumetorum have been used in the treatment of diabetes mellitus because of its hypoglycaemic effect. Diabetes mellitus is a syndrome characterized by chronic hyperglycaemia and disturbances of carbohydrate, fat and protein metabolism associated with absolute or relative deficiencies in insulin secretion and/or insulin action⁹. Impaired carbohydrate utilization in the diabetic leads to accelerated lipolysis, which results in elevated plasma triacylglycerol levels (hyperlipidaemia^10,11. The large amounts of fatty acids available to the liver in diabetic patients lead to excess acetyl CoA, which is converted to form ketone bodies. Individuals suffering from untreated diabetes mellitus produce large amounts of ketone bodies in the blood (hyperketonaemia)^{10, 11}. The increased availability of acetyl CoA from the β-oxidation of fatty acids are responsible for the hypercholesterolaemia^{10, 11}.

While the hypoglycaemic properties of this plant are well reported^{5, 12}, there are however no reports on the effect of the tuber on hyperlipidaemia, hypercholesterolaemia and hyperketonaemia associated with diabetes mellitus despite the relevance of such studies¹³. To reduce the risk of vascular complications in diabetes mellitus, control not only of blood glucose levels but also, lipid levels, blood pressure and weight are necessary¹⁴. This work was therefore aimed at investigating the effect of the aqueous extract of the Dioscorea dumetorum tuber on blood triacylglycerol, cholesterol and β-hydroxybutyrate levels in alloxan-induced diabetic rats.

Experimental

Plant Material

Dioscorea dumetorum tubers collected from a farm at Agenebode, Edo State, Nigeria were peeled, thinly sliced, sun-dried for 6 hr and then oven-dried at 40 ^oC till a constant dry weight was recorded. The powdered tuber (50 g) was boiled in 1 litre of distilled water for 10 min and the suspension was filtered. The filtrate was then evaporated to dryness on a rotary evaporator (Gallenkamp, Germany) at 40 ^oC. The resulting gummy light brown residue, which weighed 4.03 g (8.1% w/w of dry starting material), was stored overnight at - 4 ^oC. A 2.5 g portion was reconstituted in 50 ml distilled water (40^oC) and used as stock crude drug at room temperature (25 ± 2 ^oC).

Animals

Twenty-four albino rats of the Wistar strain weighing between (180-250 g) were obtained from the Animal House Unit, Department of Pharmacology, University of Benin, Benin City, Nigeria. The animals were divided into three experimental groups of 8 rats per group, housed in standard rat cages and left to acclimatize to laboratory conditions for two weeks before the commencement of the experiment. Groups 1 and 2 rats were diabetic control and test rats, respectively, while Group 3 rats were normal control. All the rats were maintained on rat pellet food (Pfizer Feeds Plc., Nigeria) and tap water ad libitum.

Induction of diabetes in rats

Rats injected with alloxan monohydrate had several fold increases in the levels of urine and plasma glucose. These were significantly higher (p < 0.05) than values obtained for normal control rats (Table 1). Plasma and urine glucose concentrations in excess of 115 mg/100ml and 0.25 g/24 hr, respectively, confirmed the diabetic state^24,25 of the rats used before treatment with the plant extract. The control animals (normal) did not show any significant change in plasma triacylglycerol, cholesterol and β-hydroxybutyrate levels. However, there were significant increases in plasma triacylglycerol, cholesterol and β-hydroxybutyrate concentrations in the diabetic rats when compared with normal control (Tables 2, 3 and 4).

Following one week oral administration of the extract to the diabetic rats (group 2), the percentage reduction in plasma levels of triacylglycerol, cholesterol and β-hydroxybutyrate were 46.63%, 51.54% and 59.43%, respectively.

The phytochemical screening of the aqueous extract of Dioscorea dumetorum revealed the presence of flavonoids, alkaloids, saponins and cardiac glycosides.

Discussion

The results of this study clearly indicate that the water extract of Dioscorea dumetorum exhibits not only hypoglycaemic effects but also hypocholesterolaemic and hypotriacylglycerolaemic effects in alloxan - induced diabetic rats. The extract also significantly reduced the plasma levels of β-hydroxybutyrate concentrations.

In alloxan-induced diabetes, the β - cells of the islets of Langerhans, which produce insulin, are destroyed²⁶. In severe insulin deficiency, there is accelerated lipolysis^{10, 11}, resulting in the elevated plasma triacylglycerol levels obtained in untreated diabetic rats (Table 2). The excess acetyl CoA produced is a key substrate in the biosynthesis of cholesterol^{10, 11}, thus leading to the high plasma cholesterol levels observed in untreated diabetic rats (Table 3). High plasma levels of β-hydroxybutyrate in untreated diabetic rats (Table 4) is due to the fact that, in the absence of carbohydrate metabolism, little of the acetyl CoA is metabolized by the tricarboxylic acid cycle, while the remainder is converted to ketone bodies (ketonaemia), and some excreted in urine (ketonuria)^{10, 11}. These observations are consistent with previous findings that spontaneous diabetes in man and the experimentally induced condition in animals are characterized by increases in the concentrations of the above biochemical parameters^{27, 28}.

The hypolipidaemic and hypocholesterolaemic effects of the extract, observed in the diabetic treated rats (Tables 2 and 3), may be due in part to the presence of anti-nutrients. Anti-nutrients commonly found in plant foods such as phytic acid, lectins, phenolic compounds, amylase inhibitors, and saponins have been reported to reduce blood glucose and insulin responses to starchy foods and/or reduce plasma cholesterol and triglycerides²⁹. Saponin is known to possess blood cholesterol-lowering activity^{29, 30}. The cholesterol-lowering mechanism proposed for saponin is that, it binds cholesterol in the intestinal lumen, and so, cholesterol is less readily reabsorbed. In addition, it may also bind with bile acids, causing a reduction in its enterohepatic circulation, increasing its faecal excretion³⁰. Increased bile acid excretion is offset by enhanced synthesis from cholesterol in the liver and consequent lowering of the plasma cholesterol³¹.

In the present study, phytochemical screening of the aqueous extract of Dioscorea dumetorum showed the presence of saponins, flavonoids and cardiac glycosides in addition to alkaloids which has previously been extracted and characterized in the yam tuber^4,5,7. Steroids³² and sapogenin⁴ have also been identified in the yam tuber extract but the presence of these chemical agents was not investigated in this study.

Conclusion

Elevated blood cholesterol, triacylglycerol, β-hydroxybutyrate which occur in diabetic conditions can significantly be reduced by the oral administration of the aqueous extract of Dioscorea dumetorum tuber. This finding provides some biochemical basis for the use of the tuber in the management of patients with diabetes and confirm its role as a traditional antidiabetic remedy.

Acknowledgements

The author wishes to express her profound gratitude to Dr. Eric Omogbai of the Department of Pharmacology for his technical assistance and Professors A.U. Osagie and E.A.C. Nwanze for their fatherly encouragement.

References