Diabetes mellitus is a growing threat to human health. Current pharmacological agents cause undesirable side-effects. Herbal remedies offer the potential for alternative treatment strategies that may prove more cost-effective and devoid of the undesirable side-effects. The purpose of this study was to evaluate the in vitro anti-diabetic activity of aqueous and methanol extracts of Sclerocarya birrea (A. Rich.) Hochst. (Anacardiaceae) and Ziziphus mucronata Willd. (Rhamnaceae), which are traditionally used for the treatment of diabetes mellitus in southern Africa.
Polyphenolic contents of extracts were quantified using the aluminium trichloride and Folin-Ciocalteau methods. The capacity of individual extracts to scavenge both the 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) and 2,2-diphenyl-1-picrylhydrazyl radicals was used as a measure of antioxidant activity. The inhibitory activities of the crude extracts of both plants on the enzymes, α-amylase and α-glucosidase, were determined using colorimetric assays. The effects of the crude extracts on cell viability was assessed in C2C12 myotubes, HepG2 hepatocarcinoma cells, 3T3-L1 adipocytes and RIN-m5F pancreatic β-islet cells, using the Sulforhodamine B assay. Fluorescence detection was used to investigate the effects of the crude extracts on glucose uptake in C2C12, HepG2 and 3T3-L1 cells. Insulin secretion was assessed in RIN-m5F cells, using ELISA.
Crude extracts of both plants contained flavonoids and phenols, but flavonoid content was predominantly higher. All the extracts displayed antioxidant activity, with the methanol extract of S. birrea possessing the most potent free radical scavenging ability (IC50 = 2.16 μg/ml). Aqueous and methanol extracts of S. birrea displayed significantly (p < 0.05) greater inhibition of α-amylase, than the positive control, acarbose. Only the methanol extract of Z. mucronata inhibited α-amylase activity. Furthermore, crude extracts of both plants also displayed potent α-glucosidase inhibitory activity. Most of the crude extracts had low toxicity, where concentrations of 100 μg/ml of crude extract of the plants did not induce 50% cell death.
Although no significant increase in insulin secretion from cultured RIN-m5F cells was noted, the crude extracts of both plants significantly (p < 0.05) increased glucose uptake in C2C12, HepG2 and 3T3-L1 cells, with efficacy significantly (p < 0.05) higher than the positive control, insulin.
From the results, the plant extracts appear to exert their hypoglycaemic effects independently of insulin, via an extra-pancreatic mechanism, possibly involving interactions with the different receptors. An additive hypoglycaemic effect originates from the inhibition of both α-amylase and α-glucosidase. The findings of the present study provide evidence that S. birrea and Z. mucronata possess in vitro anti-diabetic activity. Further investigations are required to elucidate the mechanism(s) of action of the crude extracts using more targeted in vitro assays.