α- and β-Glucosidase inhibitors: chemical structure and biological activity

Mexican antidiabetic herbs: valuable sources of inhibitors of α-glucosidases

Type II-diabetes mellitus (TII-DM) has been regarded as one of the most important public health problems in all nations in the 21st century. Although allopathic therapies remain the most important for the initial management of TII-DM, herbal remedies have gained wide acceptance for treating this condition. These alternative therapies are particularly valued in countries such as Mexico, rich in medicinal plants strongly attached to the cultural values of the population. Medicinal plants are prized sources of α-glucosidase inhibitors, which delay the liberation of glucose from complex carbohydrates, retarding glucose absorption, and thus controlling the characteristic hyperglycemia of TII-DM. Among the plant species used for treating diabetes in Mexico only 38 have been analyzed for their inhibitory activity of α-glucosidases. Most of these studies, reviewed in the present work, have focused on the evaluation of different types of extracts on the activity of α-glucosidases from diverse sources. Four species have been thoroughly analyzed in order to discover novel α-glucosidase inhibitors, namely, Hintonia latiflora and Hintonia standleyana (Rubiaceae), Ligusticum porteri (Apiaceae), and Brickellia cavanillesii (Asteraceae). Their ethnomedical uses, pharmacological and toxicological studies, chemical composition, and antihyperglycemic principles with α-glucosidase inhibitory activity are summarized.

A novel method for screening beta-glucosidase inhibitors

BACKGROUND

Few beta-glucosidase inhibitors have so far been reported from microorganisms due to the practical difficulties in performing the inhibition tests and subsequent interpretation of results. In an effort to investigate marine microbial extracts for β-glucosidase inhibitors, we developed a new protocol, using esculin as substrate in an agar plate based assay, to screen a large number of microbial extracts in a short span of time.

RESULTS

With the new method, pale yellowish zones against the blackish brown background could be visually observed with more clarity in sample extracts where β-glucosidase inhibitor was present. The new method was compared with the closest existing method and established beyond doubt. This agar plate based procedure required about one hour for minimum 12 samples and the throughput increases with the size of the agar gel plate used.

CONCLUSIONS

The new protocol was simple, rapid and effective in detecting beta-glucosidase inhibitors in microbial extracts.

Antidiabetic components of Cassia alata leaves: identification through α-glucosidase inhibition studies

CONTEXT

Cassia alata Linn. [syn. Senna alata (L.) Roxb.] (Caesalpiniaceae) is used for treating various disease conditions including diabetes but its mechanism(s) of action and active principles remain to be elucidated.

OBJECTIVE

The antidiabetic principles were identified using an in vitro α-glucosidase inhibition study.

MATERIALS AND METHODS

The methanol extract of leaves of C. alata, which showed potent α-glucosidase inhibitory activity (IC₅₀, 63.75 ± 12.81 µg/ml), was fractionated. Active fractions were taken for further analysis by a variety of techniques including HPLC and Combiflash chromatography. The identity of the isolated compounds was established by spectroscopic analysis while their potential antidiabetic activity was assessed by in vitro enzyme inhibition studies.

RESULTS

The α-glucosidase inhibitory effect of the crude extract was far better than the standard clinically used drug, acarbose (IC₅₀, 107.31 ± 12.31 µg/ml). A subsequent fractionation of the crude extract was made using solvents of ascending polarity (petroleum ether, chloroform, ethyl acetate, n-butanol and water). The ethyl acetate (IC₅₀, 2.95 ± 0.47 µg/ml) and n-butanol (IC₅₀, 25.80 ± 2.01 µg/ml) fractions which contained predominantly kaempferol (56.7 ± 7.7 µM) and kaempferol 3-O-gentiobioside (50.0 ± 8.5 µM), respectively, displayed the highest carbohydrate enzyme inhibitory effect.

DISCUSSION

One of the possible antidiabetic mechanisms of action of C. alata is by inhibiting carbohydrate digestion. This is the first report on α-glucosidase activity of kaempferol 3-O-gentiobioside.

CONCLUSION

Considering the activity profile of the crude extract and isolated bioactive compounds, further in vivo and clinical studies on C. alata extracts and compounds are well merited.

In vitro α-glucosidase inhibition and antioxidative potential of an endophyte species (Streptomyces sp. loyola UGC) isolated from Datura stramonium L

Endophytic actinomycetes isolated from Datura stramonium L. was evaluated for its effects against in vitro α-glucosidase inhibition, antioxidant, and free radical scavenging activities. Based on microbial cultural characteristic and 16S rRNA sequencing, it was identified as Streptomyces sp. loyola UGC. The methanolic extract of endophytic actinomycetes (MeEA) shows remarkable inhibition of α-glucosidase (IC50 730.21 ± 1.33 μg/ml), scavenging activity on 2,2-diphenyl-picrylhydrazyl (DPPH) (IC50 435.31 ± 1.79 μg/ml), hydroxyl radical (IC50 350.21 ± 1.02 μg/ml), nitric oxide scavenging (IC50 800.12 ± 1.05 μg/ml), superoxide anion radical (IC50 220.31 ± 1.47 μg/ml), as well as a high and dose-dependent reducing power. The MeEA also showed a strong suppressive effect on rat liver lipid peroxidation. Antioxidants of β-carotene linoleate model system revels significantly lower than BHA. The total phenolic content of the extract was 176 mg of catechol equivalents/gram extract. Perusal of this study indicates MeEA can be used as natural resource of α-glucosidase inhibitor and antioxidants.

Antihyperglycemic, antilipid peroxidation, and insulin secretory activities of Otostegia persica shoot extract in streptozotocin-induced diabetic rats and in vitro C187 pancreatic β-cells

CONTEXT

Otostegia persica Boiss (Lamiaceae) contains antioxidant agents and is used in traditional medicine for treatment of diabetes mellitus complications.

OBJECTIVES

The acute antihyperglycemic, antilipid peroxidation, and insulin secretory activities of methanol extract of O. persica aerial parts were investigated.

MATERIALS AND METHODS

The extract [200, 300, 400 mg/kg body weight (b.w.)] was given orally to rats and glucose (2 g/kg b.w. orally) was administered 30 min later. Glucose and insulin serum levels were measured before and 30, 60, 120, and 240 min after administration of the test samples in normal and diabetic rats. The in vitro insulin secretory activity of extract was evaluated in C187 pancreatic β-cells and its antilipid peroxidation effect was determined by measuring malondialdehyde (MDA) and glutathione (GSH) levels in rat livers after 240 min. The identification of the major phytoconstituents of the extract was carried out using gas chromatography-mass spectrometry.

RESULTS

The extract (300 mg/kg b.w.) significantly decreased the serum glucose level in diabetic rats at 1 h (494 ± 13.4 vs. 426 ± 12.9), 2 h (472.8 ± 17.8 vs. 396 ± 22), and 4 h (438.8 ± 25 vs. 346 ± 19) after treatment. Accordingly, the serum insulin level increased at the same times. The extract significantly increased glucose-induced insulin secretion in C187 β-cells. Moreover, the extract significantly decreased MDA and increased GSH levels in the liver of diabetic rats. Phytochemical analysis revealed thymol as the major phytoconstituent in the extract.

DISCUSSION AND CONCLUSION

O. persica shoot extract has antihyperglycemic, antilipid peroxidation, and insulin secretory properties.

Inhibitory effects of epigallocatechin gallate and its glucoside on the human intestinal maltase inhibition

Human intestinal maltase (HMA) is an α-glucosidase responsible for the hydrolysis of α-1,4-linkages from the non-reducing end of malto-oligosaccharides. HMA has become an important target in the treatment of type-2 diabetes. In this study, epigallocatechin gallate (EGCG) and EGCG glucoside (EGCG-G1) were identified as inhibitors of HMA by an in vitro assay with IC₅₀ of 20 ± 1.0 and 31.5 ± 1.0 μM, respectively. A Lineweaver-Burk plot confirmed that EGCG and EGCG-G1 were competitive inhibitors of maltose substrate against HMA and inhibition kinetic constants (K_i) calculated from a Dixon plot were 5.93 ± 0.26 and 7.88 ± 0.57 μM, respectively. Both EGCG and EGCG-G1 bound to the active site of HMA with numerous hydrophobic and hydrogen bond interactions.

Synthesis and glycosidase inhibitory activity of novel (2-phenyl-4H-benzopyrimedo[2,1-b]-thiazol-4-yliden)acetonitrile derivatives

A series of (2-phenyl-4H-benzopyrimodo[2,1-b][1,3]thiazol-4-yliden-4-yliden)acetonitrile derivatives have been prepared by ring transformation reaction of 4-(methylthio)-2-oxo-6-aryl-2H-pyrane-3-carbonitriles. The yield of ring transformation product is moderate to good. Furthermore the glycosidase inhibitory activities were tested by using α-amylase and α-glucosidase pancreatic, intestinal and liver enzymes, responsible for hyperglycemia in type II diabetes. The results revealed that all compounds exhibit significant glycosidase inhibitory activity.

Conformationally-locked N-glycosides with selective β-glucosidase inhibitory activity: identification of a new non-iminosugar-type pharmacological chaperone for Gaucher disease

A series of conformationally locked N-glycosides having a cis-1,2-fused pyranose-1,3-oxazoline-2-thione structure and bearing different substituents at the exocyclic sulfur has been prepared. The polyhydroxylated bicyclic system was built in only three steps by treatment of the corresponding readily available 1,2-anhydrosugar with KSCN using TiO(TFA)(2) as catalyst, followed by S-alkylation and acetyl deprotection. In vitro screening against several glycosidase enzymes showed highly specific inhibition of mammalian β-glucosidase with a marked dependence of the potency upon the nature of the exocyclic substituent. The most potent representative, bearing an S-(ω-hydroxyhexadecyl) substituent, was further assayed as inhibitor of the human lysosomal β-glucocerebrosidase and as pharmacological chaperone in Gaucher disease fibroblasts. Activity enhancements in N370S/N370S mutants analogous to those achieved with the reference compound ambroxol were attained with a more favorable chaperone/inhibitor balance.

Concise synthesis of (+)-conduritol F and inositol analogues from naturally available (+)-proto-quercitol and their glucosidase inhibitory activity

An effective synthesis of (+)-conduritol F, (+)-chiro- and (+)-epi-inositols from naturally available (+)-proto-quercitol is described. This synthetic method provides a concise synthesis of cyclitols in enantiomerically pure form. Of the synthesized cyclitols, (+)-conduritol F potently inhibits type I α-glucosidase with an IC(50) value of 86.1 μM, which is five times greater than the standard antidiabetic drug, acarbose.

α-Glucosidase inhibition and antihyperglycemic activity of prenylated xanthones from Garcinia mangostana

An ethanol extract of the fruit case of Garcinia mangostan, whose most abundant chemical species are xanthones, showed potent α-glucosidase inhibitory activity (IC(50)=3.2 μg/ml). A series of isolated xanthones (1-16) demonstrated modest to high inhibition of α-glucosidase with IC(50) values of 1.5-63.5 μM. In particular, one hitherto unknown xanthone 16 has a very rare 2-oxoethyl group on C-8. Kinetic enzymatic assays with a p-nitrophenyl glucopyranoside indicated that one of them, compound (9) exhibited the highest activity (K(i)=1.4 μM) and mixed inhibition. Using, a physiologically relevant substrate, maltose, as substrate, many compounds (6, 9, 14, and 15) also showed potent inhibition which ranged between 17.5 and 53.5 μM and thus compared favorably with deoxynojirimycin (IC(50)=68.8 μM). Finally, the actual pharmacological potential of the ethanol extract was demonstrated by showing that it could elicit reduction of postprandial blood glucose levels. Furthermore, the most active α-glucosidase inhibitors (6, 9, and 14) were proven to be present in high quantities in the native seedcase by a HPLC chromatogram.

Screening of endophytic fungi having ability for antioxidative and alpha-glucosidase inhibitor activities isolated from Taxus sumatrana

Endophytic microbes are considered as an important source of natural products. They show antibiotic, anticancer, antioxidative and antidiabetic activities. Therefore, there are many reports on the isolation and bioactivity screening of endophytic fungi from various plants including Taxus species. Taxus sumatrana (Miq.) de Laub is found in Indonesia. The objective of this study is to conduct an in vitro screening of 14 endophytic fungi isolated from Taxus sumatrana having antioxidative and alpha-glucosidase inhibitor activities. Each endophytic fungus was cultured for 7 days and the fungal mycelium and medium were extracted with methanol and ethyl acetate, respectively, to produce each extract. The antioxidative activity of each extract was tested by DPPH free radical scavenging activity and beta-carotene bleaching assays, whereas antidiabetic activity was tested based on alpha-glucosidase inhibitor activity. The screening results showed that fungal mycelia of TSC 13 had the best alpha-glucosidase inhibitor activity and TSC 24 had the best antioxidative activity. Isolation of bioactive compounds from TSC 13 and TSC 24 is being conducted. This is the first report that endophytic fungi isolated from T. sumatrana exhibited anti alpha-glucosidase inhibitory and anti oxidative activities.

Synthesis and α-glucosidase inhibitory mechanisms of bis(2,3-dibromo-4,5-dihydroxybenzyl) ether, a potential marine bromophenol α-glucosidase inhibitor

Bis(2,3-dibromo-4,5-dihydroxybenzyl) ether (BDDE), derived from the marine algae, is a potential α-glucosidase inhibitor for type 2 diabetes treatment. In the present study, a synthetic route was established as a valid approach to obtain BDDE. Fluorescence spectra, circular dichroism spectra and molecular docking methods were employed to elucidate the inhibitory mechanisms of BDDE against α-glucosidase. The results showed that BDDE could be prepared effectively and efficiently with the established synthetic methods. Synthetic BDDE bound with α-glucosidase and induced minor conformational changes of the enzyme. The docking results indicated the interaction between BDDE and α-glucosidase was driven by both hydrophobic forces and hydrogen bonds. The docked BDDE molecule was completely buried in the α-glucosidase binding pocket with part of the molecule reaching the catalytic center and overlapping with the position of glucose, and the rest of the molecule extending towards protein surface. This study provides useful information for the understanding of the BDDE-α-glucosidase interaction and for the development of novel α-glucosidase inhibitors.