The Hypoglycaemic and Antioxidant Constituents of the Stem of Adenia Lobata Engl (Passifloraceae) and the Stem Bark of Morinda Lucida Benth (Rubiaceae)
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Date
2012-06-15
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Abstract
The hypoglycaemic and antioxidant activities of two Ghanaian medicinal plants namely Adenia lobata Engl (Passifloraceae) and Morinda lucida Benth (Rubiaceae), used to treat diabetes mellitus in traditional medicine, have been investigated. The dried stem powder of A. lobata was successively extracted by Soxhlet with petroleum ether and 70% ethanol to obtain the crude petroleum ether (PEAL: yield =1.1w/w %) and ethanol (EEAL: yield = 5.4 w/w %) extracts. The dried stem bark powder of M. lucida was similarly extracted with petroleum ether, ethylacetate and 70% ethanol to obtain the crude petroleum ether (PEML: yield = 0.9%), ethylacetate (EAEML: yield = 4.0%) and ethanol (EEML: yield = 4.6%) extracts. The extracts were initially assessed for their hypoglycaemic and antioxidant activities and later subjected to chromatographic separation to isolate their chemical constituents. The isolated compounds were identified using NMR spectroscopic methods with reference to literature, and also assessed for their hypoglycaemic activities.
The hypoglycaemic activity of PEAL and EEAL were determined in streptozotocin-induced diabetic rats (80 mg/kg body weight). Five groups of diabetic rats were given 150, 300 and 600 mg/kg body weight of PEAL and EEAL orally once daily for 20 days. Glibenclamide (5 mg/kg body weight) was used as positive control while distilled water (5 ml) acted as the normal diabetic control. The blood glucose levels were monitored initially for 6 hours and subsequently over 20 days. Both extracts exhibited statistically significant (p< 0.001) hypoglycaemic activity throughout the study period, with EEAL showing the greatest activity. EEAL at 600 mg/kg body weight after 6 hours produced 50.3% reduction whereas glibenclamide gave 52.7% reduction over the initial blood glucose levels of the diabetic rats. Again, both EEAL at 600 mg/kg body weight and glibenclamide caused the blood glucose levels of the diabetic rats to fall by 81.0% and 82.1% respectively after the full 20 days.
The same protocol was followed to establish the hypoglycaemic activity of PEML, EAEML and EEML in the diabetic rats using doses of 100 mg/kg, 200 mg/kg and 400 mg/kg. The most significant reduction (51.2%) by the extracts after six hours was caused by EEML at a dose of 400 mg/kg body weight and glibenclamide caused 53.0% reduction in blood glucose levels of the diabetic control rats. However, after day 20, at 400 mg/kg body weight, PEML exhibited the most significant reduction (77.7%) of the blood glucose levels compared to EEML (60.7%).
The antioxidant properties of the petroleum ether and ethanol extracts of A. lobata (PEAL, EEAL) and M. lucida (PEML, EEML) were evaluated using five assays; total phenolic content, total antioxidant capacity, reducing power, DPPH scavenging effect and lipid peroxidation activity. In all these assays, the antioxidant properties increased with increasing concentration of the extracts. The IC50 values for DPPH scavenging effect for PEAL and EEAL were 2.106 ± 0.008 and 0.5210 ± 0.006 respectively while that of N-propyl gallate was 0.2917 ± 0.005. The IC50 values for lipid peroxidation activity for PEAL and EEAL were 8.593 ± 0.090, 0.5985 ± 0.008 respectively and that of N-propyl gallate was 0.4217 ± 0.006. The IC50 values for DPPH scavenging activity for PEML and EEML were 7.426 ± 0.007 and 0.4403 ± 0.005 respectively whereas that of lipid peroxidation activity gave 3.554 ± 0.009 for PEML and 1.928 ± 0.007 for EEML. Thus, for both A. lobata and M. lucida, the ethanolic extracts showed better antioxidant potential than the petroleum ether extracts.
The 70% ethanol extract of A. lobata (EEAL) and the petroleum ether extract of M. lucida (PEML) which showed the most significant hypoglycaemic activity were subjected to series of column chromatographic separation using silica gel as stationary phase and eluting with petroleum ether, ethylacetate, ethanol, and their mixtures in gradient elution. Two compounds, palmitic acid and -hydroxy--valerolactone were isolated from A. lobata extract and stigmasterol was isolated from M. lucida extract. These were characterized using 1H, 13C NMR, COSY, HSQC and HMBC Spectroscopy with comparison to literature.
All the compounds were also evaluated for hypoglycaemic activity over 6 hours. The compounds at various doses were administered orally to diabetic rats and the most significant hypoglycaemic effects were caused by the highest doses of the compounds used in the experiments. The palmitic acid (180 mg/kg body weight), -hydroxy--valerolactone (180 mg/kg body weight) and stigmasterol (100 mg/kg body weight) caused 30.4%, 50.9% and 40.5% reduction respectively over the initial blood glucose levels in the diabetic rats.
The results of these studies have shown that the extracts of stems of A. lobata and stem bark of M. lucida possess hypoglycaemic activity in diabetic animal model and also display antioxidant properties. The compounds, palmitic acid, -hydroxy--valerolactone and stigmasterol also possess hypoglycaemic activity. These findings may justify the traditional use of these medicinal plants in the management of diabetes mellitus.
Description
A thesis submitted in fulfillment of the requirements for the degree
of
Doctor of Philosophy
in the Department of Pharmacognosy
Faculty of Pharmacy and Pharmaceutical Sciences,
College of Health Science,2012