In screening for natural, antidiabetic principles Yoshikawa et al discovered that a water-soluble fraction (25-100 mg/kg p.o.) prepared from the roots and stems of S. reticulata strongly inhibited elevations in rats’ serum glucose levels after the administration of sucrose or maltose but not glucose. In addition, the fraction inhibited rat intestinal maltase (an enzyme that hydrolyzes maltose into two molecules of glucose) and sucrase (an enzyme that hydrolyzes sucrose into its components, glucose and fructose) in an in vitro test. The IC50 values were 35 g/ml for maltase (substrate: maltose 37 mM) and 26 g/ml for sucrase (substrate: sucrose 37 mM). Because S. reticulta extract did not have any affect on alloxan-induced hyperglycemia in mice, the antidiabetic property of
S. reticulata
was attributed to its intestinal a-glucosidase inhibitory activity. a-Glucosidase inhibitors retard the digestion and hence absorption of carbohydrates in the small intestine that prevents the increase in blood glucose concentration after a carbohydrate load.

To confirm this activity, Yoshikawa et al. conducted an additional study, a bioassay-guided separation. A water-soluble fraction prepared from the dried roots of S. reticulata was further separated by chromatographic methods to yield salacinol. Salacinol showed competitive inhibition for intestinal a-glucosidase in vitro that was stronger than that of the water-soluble fraction. IC50 values of 3.2 g/ml for maltase, 0.84 g/ml for sucrase, and 0.59 g/ml for isomaltase (substrate: isomaltose 3.7 mM) were obtained. The following results were observed upon comparison of salacinol with acarbose, a clinically used a-glucosidase inhibitor.

  • Salacinol’s inhibitory activity against maltase and sucrase was almost
    equal to that of acarbose, but more potent than acarbose against isomaltase.

  • Male rats were fasted for 20-24 hour before test compounds were
    administered orally. Thirty minutes after administration of the test
    compounds, sucrose (1 g/kg body weight) was given orally. Salacinol
    (1.3-10 mg/kg p.o.) inhibited the increase of serum glucose levels in
    sucrose-loaded rats more strongly than acarbose as shown
    in Figure 2.

 


Figure 2. Comparison of the inhibitory effects of salacinol, water-soluble fraction of
S. reticulata
, and acarbose on increases in the serum glucose levels of sucrose-loaded rats (** p < 0.01, N = 5-7).

 

A bioassay-guided study conducted with kotalanol yielded similar results. However, kotalanol furnished more potent inhibitory activity against sucrase than either salacinol or acarbose. The IC50 values for kotalanol were 0.58 g/ml for sucrase, 2.8 g/ml for maltase, and 1.9 g/ml for isomaltase; for salacinol the values were 0.84 g/ml for sucrase, 3.2 g/ml for maltase, and 0.59 g/ml for isomaltase; and for acarbose the values were 1.1 g/ml for sucrase, 1.3 g/ml for maltase, and 100 g/ml for isomaltase.

In another chemical investigation of Salacia reticulata, Yoshikawa et al isolated mangiferin (a xanthone), three catechins (-)-epicatechin, (-)-epigallocatechin, and (-)-4’-O-methylepigallocatechin) and two catechin dimers (-)-epiafzelechin-(4barrow.gif (80 bytes)8)-(-)-4’-O-methylepigallocatechin and (-)-epiafzelechin-(4barrow.gif (80 bytes)8)-(-)-4’-O-methylepigallo-catechin. The inhibitory activity of these constituents against carbohydrate metabolizing enzymes, sucrase, maltase, isomaltase, a-amylase, and aldose reductase, were examined and compared to salacinol and kotalanol. Like kotalanol and salacinol, mangiferin furnished a-glucosidase inhibitory activity as shown in Table 1. It inhibited both sucrase and isomaltase. In addition, mangiferin furnished a property not seen with kotalanol and salacinol; mangiferin inhibited aldose reductase activity. Aldose reductase, an enzyme of the polyol pathway, represents a biochemical link between hyperglycemia and diabetic complications of the eye and other major organ systems.

Table 1 : Inhibitory Effects of Constituents from S. reticulata on a-Glucosidases, a-Amylase, and Aldose Reductase (ARase) Activities Values in parentheses represent inhibition at 300 g/ml. NT indicates "not tested''.

IC50(mg/ml)

Constituent Sucrase Maltase Isomaltase a-Amylase ARase
Mangiferin 87 >300(42) 216 >300(16) 1.4
Salacinol 0.84 3.2 0.59 NT >6>
Kotalanol 0.58 2.8 1.9 NT >6

A recent research report revealed that Salaretin (Salacia reticulata Extract) effectively inhibits a-amylase, the enzyme that catalyzes the breakdown of dietary starch to simple sugars, thereby potentially inhibiting starch digestion. In this study, the extract was found to inhibit alpha amylase (derived from porcine pancreas) in a dose-dependent manner, with 68.75% inhibition at a concentration of 35 mcg/ml.


Figure 3: Dose-dependent inhibitory effect of Salaretin (Salacia reticulata Extract, SRE) on porcine alpha amylase activity

In studies on an animal model (KK-Ay mice) of type 2 diabetes, researchers found that mangiferin and its glucosides lowered blood glucose levels on oral administration. No effects were observed in normal mice and beneficial effects on hyperinsulinemia were observed in diabetic mice. The authors of this study concluded that mangiferin and its glucosides probably have beneficial effects on blood glucose levels through increasing insulin sensitivity.

Mangiferin was found to have beneficial effects in animal models of type 2 diabetes when coupled with exercise. KK-Ay mice subjected to an exercise regimen and treated with orally administered mangiferin (30 mg/kg) for two weeks, showed better lipid profiles than control mice subjected to the exercise regimen and receiving a matching placebo.

A recent study revealed that mangiferin, when administered orally for three weeks, significantly improved hyperinsulinemia and, on insulin tolerance test, reduced blood glucose levels of KK-Ay mice (animal model of type 2 diabetes), as compared to untreated controls. It was concluded that mangiferin probably decreases blood sugar levels through decreasing insulin resistance.

Karunanayake used male, Sprague-Dawley rats to evaluate the oral hypoglycemic activities of aqueous decoctions of three well-known, Ayurvedic plants widely used to treat diabetics, Salacia reticulata (Celastraceae), Aegle marmelos (Rutaceae), and Momordica charantia (Curcubitaceae). All plants were prepared in the manner in which they would be administered to diabetic patients as shown in Table 2.

Table 2. Oral Hypoglycemic Activity: Preparation of Medicinal Plants forAdministration in Normal Rats

Plant

Source

Method of Preparation

1. Salacia reticulata Dried and powdered root bark 250 g of the root bark was boiled in 1000 ml distilled water for 3 hours before reducing the volume to 100 ml in vacuo.
2. Aegle marmelos Dried and powdered root bark 250 g of the root bark was boiled in 1000 ml distilled water for 3 hours before reducing the volume to 100 ml in vacuo.
3. Momordica charantia Fleshy parts of dried fruits without the seeds 250 g of dried fruit flesh was cut into small pieces and macerated. The macerate squeezed through muslin cloth and centrifuged to obtain the supernatant used for the investigation.

The rats consumed a standard laboratory diet. Before experimentation, they were fasted overnight for 14-16 hours. Blood samples were collected from all rats for the determination of fasting blood glucose. Afterwards, the rats were divided into four groups each containing 6 rats. Group I received saline (1 ml / 100 g body weight) and groups II-IV were given one of the three medicinal plant preparations in dosages of 1 ml/100 g body weight. Blood samples were collected at one-hour intervals post-administration and assayed for blood glucose.

As shown in Figure 4, all plant extracts furnished highly potent blood glucose lowering activity. Compared to the control group, maximum reductions in blood glucose of 44% and 30% were observed 3 hours after the administration of Aegle marmelos and Salacia reticulata, respectively. Four hours after the administration of Momordica charantia, a maximum reduction of 45% was observed in the rats’ blood glucose levels. The hypoglycemic activity of Salacia reticulata persisted up to 5 hours (the endpoint of the experiment), while that of both Momordica charantia and Aegle marmelos seemed to last longer according to the readings at that time.


Figure 4 (A-D). Effect of aqueous extracts of Aegle marmelos, Momordica charantia, and Salacia reticulata on the fasting blood glucose levels of rats.


Serasinghe and coworkers orally administered an aqueous extract of Salacia reticulata prepared from the root bark to streptozotocin-induced diabetic rats. Before drug administration, blood samples were collected from rats fasted for 16 hours. Subsequently, selected rats either received water (control group) or different doses of Salacia reticulata extract. Blood samples were collected every hour for 5 hours to determine the change in blood plasma glucose levels of treated and control rats. Reductions in plasma glucose levels of more than 100 mg/dl were observed in 42.8%, 45.4%, and 87.5% of treated rats that received 0.5 g/kg, 1.0 g/kg, and 5.0 g/kg of the Salacia reticulata extract, respectively. The effects of the extract lasted for the 5-hour duration of the experiment. The blood glucose lowering effects of Salacia reticulata is persistent (Fig.5).


Figure 5. Effect of Salacia reticulata extract (5.0 g/Kg p.o.) on the plasma glucose levels of streptozotocin-induced diabetic rats (* = p <0.005).

Shimoda et al. investigated the effects of an aqueous extract prepared from the stems of S. reticulata extract (SRE) on postprandial hyperglycemia in rats and humans. In a dose-dependent manner, SRE suppressed an increase in serum glucose levels when fed with sucrose, maltose, and a-starch but not glucose or lactose. Of the sugars used to induce hyperglycemia, SRE was most effective against sucrose. In addition, SRE strongly inhibited the activities of a-glucosidases prepared from yeast and rat jejunum (IC50: 5 and 8 g/ml, respectively). It also inhibited the activity of a-amylase (IC50: 35 g/ml) but not that of b-glucosidase. The relative inhibitory effects of SRE against partially purified a-glucosidases from the rat jejenum was sucrase = isomaltase (IC50: 15 g/ml) > maltase (IC50: 7 g/ml). In the sucrose tolerance test conducted on human volunteers, administration of 200 mg p.o. SRE 5 minutes before sucrose loading (50 g) significantly suppressed postprandial hyperglycemia .

A double-blind placebo-controlled study performed in Japan revealed that blood sugar levels reduced significantly in humans with mild type 2 diabetes, receiving Salacia reticulata extract as part of their diet, as compared to control subjects receiving a matching placebo.

© Sabinsa Corporation. 2002