Evaluation of in vitro antioxidant and antidiabetic potential of extracts from Phaseolus vulgaris L. seeds (Black turtle beans)

Introduction: Phaseolus vulgaris L also known as common beans or black turtle beans are known worldwide as the most important legume for direct human consumption. Many parts of the plant are known to have important pharmacological potential against many diseases including diabetes. Despite the importance of this legume, P. vulgaris remains an underutilized and under-researched legume in Nigeria. Its therapeutic potential is being overlooked and undermined due to insufficient data on its bioactivity. These bioactive compounds present in some plant derived foods are found as fraction, crude extract, and isolated bioactive compounds that have been screened for antioxidative and antidiabetic potential.  Several plant-derived foods and isolated bioactive compounds with potential antidiabetic properties are very limited.Objective: To investigate and estimate the antioxidative and antidiabetic effect of the different solvent extracts of P. vulgaris seed in vitro. Methods: Samples were subjected to antioxidant assays using 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging activity, ferric reducing power and 2,2-azino-bis (3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) Anti-diabetic potential in vitro was estimated by evaluating various solvent extracts on α-amylase and α-glucosidase for any inhibitory effects at doses ranging from (100-500μg/ml). Characterization of possible bioactive constituent in the different solvent extract was done using FTIR spectroscopy.Results: Aqueous extract showed a higher number of total polyphenol (11.3 ± 0.01mg/gGAE) and anthocyanin content of 76.34 ± 1.12 mg/g when compared with the other solvent extract. This was followed closely by the ethanol extract with a value of 7.3±0.01Mg and 74.53 ± 0.24 Mg/g. Ascorbic acid had a significantly higher (P<0.05) activity in the antioxidant assays used. However, among the solvent extracts tested, ethanol extract displayed highest (P<0.05) for ferric reducing power activity, (80.78±0.6mg/ml), ethyl acetate, aqueous and ethanol extracts had similar DPPH activities (12.92±2.30 mg/ml, 12.59±2.33 mg/ml and 12.54±2.30mg/ml) respectively. Dichloromethane, hexane, ethanol and ethyl acetate had similar ABTS activities. (5.69±2.86 mg/ml; 6.92±0.14 mg/ml; 10.10±1.11 mg/ml; 10.76±2.98 mg/ml) respectively. All solvent extracts displayed similar inhibitory activities against α amylase. However, ethyl acetate, aqueous and ethanol extracts showed significantly (P<0.05) higher values for α-glucosidase (3.07±0.61mg/ml; 2.82±0.14mg/ml; 2.60±0.61mg/ml). The Fourier Transform infra-red spectrophotometer (FTIR) of the extracts disclosed that the presence of polyphenol and flavonoids were due to the OH stretching and the terpenes were due to the C-H group. Conclusion: In conclusion, different solvent extracts from the seed of Phaseolus vulgaris have demonstrated low antioxidative but very promising anti –diabetic activities in vitro. The ethanol extract however displayed higher activity than other solvent extracts, FT-IR results of ethanol extracts revealed the presence of flavonoids, anthocyanins and phenolics. This study may further suggest that seeds of Phaseolus vulgaris signify a functional food as well as a nutraceutical in terms of managing of Type 2 diabetes.Keyword: Phaseolus vulgaris, α amylase, α glucosidase, antioxidative