There are many reports of herbal extracts being used in Ayurvedic literature as antidiabetic which are directly or indirectly used for the preparation of many modern drugs. However, these plants have not gained much importance as medicines and one of the factors is lack of specific standards being prescribed for herbal medicines and supportive animal/clinical trials [
15]. In the present study, we investigate six known plants, namely
A. indica;
S. cumini;
O. tenuflorum;
M. koenigii;
L. usitatissimum and
B. spectabilis having antidiabetic properties for their possible glucosidase inhibitory activity. Plants are known to produce a large variety of glucosidase inhibitors that provides protection against insects and microbial pathogens [
16,
17] therefore plant extracts were analyzed for
α-amylase inhibitory activity.
Pancreatic and intestinal glucosidases are the key enzymes of dietary carbohydrate digestion and inhibitors of these enzymes may be effective in retarding glucose absorption to suppress PPHG. In diabetic state, excessive hepatic glycogenolysis and gluconeogenesis is associated with decreased utilization of glucose by tissues being the fundamental mechanism underlying hyperglycemia [
18]. The liver glucosidase inhibitors, inhibits
α-1,6-glucosidase of glycogen-debranching enzymes in the liver and reduces the glycogenolytic rate which increases the accumulation of glycogen stores in the liver [
19,
20]. Inhibition of these enzyme systems decreases the current blood glucose levels in diabetic patient (as a short term effect) and shows a small reduction in hemoglobin
A1c level (as a long term effect) [
21].
Earlier studies indicated that
M. koenigii possess hypoglycemic activity, decreased glycogenolysis and gluconeogenesis properties and also finds its application as an adjuvant to dietary therapy and drug treatment for controlling Diabetes Mellitus [
22–
24]. Our investigation reported
M. koenigii as a good glucosidases inhibitor showing inhibition against porcine pancreatic
α-amylase as well as murine pancreatic and intestinal glucosidases. The chloroform extract of
M. koenigii showed significant inhibition (IC
50 values of 1.96, 1.06 and 2.68
μg

ml
−1) with porcine pancreatic
α-amylase (56.40%) as well as murine pancreatic and intestinal glucosidases, respectively, which is significantly higher than acarbose. Interestingly the phenolic content of this extract was very low, so the inhibitory activity may be because of specific inhibitory compounds. Methanolic extract showed inhibition (IC
50 value of 1.96
μg

ml
−1) only with murine intestinal glucosidase as shown in .
Similarly
O. tenuflorum shows inhibition of
α-amylase known to possess antidiabetic properties [
25,
26]. The three extracts of
O. tenuflorum shows inhibitory activity against porcine
α-amylase, murine pancreatic and liver glucosidases. Chloroform extract possess more potent inhibitory activity (IC
50 value of 1.76
μg

ml
−1) with murine pancreatic glucosidases. The inhibitory activity of aqueous extract (IC
50 values of 1.55 and 9.86
μg

ml
−1) was observed with porcine
α-amylase and with murine liver glucosidases, respectively, which is significantly more than acarbose.
Syzygium cumini is known to possess properties of normoglycemia and lowers blood glucose levels by improving oral glucose tolerance [
27,
28]. Our study suggests
S. cumini as a good inhibitor of glucosidases. Inhibitory activity of chloroform extract (IC
50 values of 4.28, 3.72 and 5.60
μg

ml
−1) was observed against porcine
α-amylase, murine pancreatic and murine intestinal glucosidases, respectively. The methanolic extract of
S. cumini showed inhibition with only murine liver glucosidases (IC
50 = 2.68
μg

ml
−1).
Bougainvillea spectabilis on the other hand is being used for the treatment of diabetes in the islands of West Indies and Asia. It is claimed to exert insulin-like effects and contain D-pinitol (3-O-methylchiroinositol) [
29–
31]. Our studies indicated
B. spectabilis chloroform extract has effective inhibition (IC
50 values of 3.20 and 2.06
μg

ml
−1) on porcine pancreatic
α-amylase and murine pancreatic glucosidases. The aqueous extract showed inhibition (IC
50 = 11.16
μg

ml
−1) with murine pancreatic glucosidases and methanolic extract (IC
50 = 9.26
μg

ml
−1) showed inhibition with murine liver glucosidase. The effects could be possibly because of the high-phenolic contents present in the crude extracts.
Azadirachta indica is one of the widely used antidiabetic plants, which possess hypolipidaemic, hypoglycaemic, immunostimulant and hepatoprotective properties [
32–
34]. Aqueous extracts of neem leaves was reported to reduce blood glucose and this effect could be due to blocking the action of epinephrine on glycogenolysis and peripheral utilization of glucose [
35]. It is not a glucosidase inhibitor as evidenced by no significant inhibition with porcine pancreatic
α-amylase. However, methanolic (IC
50 values of 2.60 and 1.80
μg ml
−1) and aqueous extract (IC
50 values of 3.17 and 6.21
μg ml
−1) showed inhibition with murine liver and intestinal glucosidases, respectively, which is significantly more than acarbose.
Linum usitatissimum is known to possess plasma triglycerides and cholesterol lowering properties. This plant being a dietary component is also known to have hypolipidemic effect [
36]. We investigate its role as glucosidase inhibitor. The aqueous (IC
50 value of 3.21
μg

ml
−1) and methanolic extract (IC
50 value of 18.62
μg ml
−1) has shown inhibition with murine pancreatic glucosidases which is significantly more than acarbose.
Diabetes being multifactorial disease the treatment choice differs from patient to patient. Therefore, it is important to find out the biological activity of these herbal extracts. Theoretically, these glucosidase inhibitors from plants act through a variety of mechanism. Presently it is not possible either to pin point the exact mechanism of inhibition of these extracts or to identify the active principle(s) responsible for such effect in this study. Nevertheless, the present study identifies the M. koenigii and O. tenuflorum as good glucosidase inhibitors of pancreatic and intestinal enzymes. They can be further studied and may be used as dietary supplement for controlling PPHG in type II diabetes.