Novel α-glucosidase inhibitors from Macaranga tanarius leaves

One of the hyperglycaemic remedies is glucose absorption reduction by suppressing carbohydrate digestion due to utilisation of α-glucosidase inhibitors (AGIs). Determination of prospecting herbs done in vitro by using enzyme assay resulted in the finding of Macaranga tanarius, which showed a potent inhibitory activity. An EtOAc-soluble extract of M. tanarius leaves was chromatographed by a Diaion HP-20 column and the active fractions were further purified with high performance liquid chromatography (HPLC) to isolate active principles against α-glucosidase. Five ellagitannins were successfully isolated and identified. Structure determination revealed that these isolated compounds were mallotinic acid (IC₅₀ > 5.00 mM), corilagin (IC₅₀ = 2.63 mM), chebulagic acid (IC₅₀ = 1.00 mM), and two novel compounds named macatannins A (IC₅₀ = 0.80 mM) and B (IC₅₀ = 0.55 mM). AGIs play an important role for the treatment of diabetes, therefore this research results may suggest novel alternatives for diabetes treatment management.     

Comparative evaluation of rosmarinic acid,methyl rosmarinate and pedalitin isolated from Rabdosia serra (MAXIM.) HARA as inhibitors of tyrosinase and α-glucosidase

Rabdosia serra has been used in traditional Chinese medicine for centuries. In order to illustrate the pharmaceutical activity of R. serra as hypoglycaemic and skin-whitening agents, rosmarinic acid (confirmed as the major compound in R. serra), methyl rosmarinate and pedalitin isolated from R. serra were evaluated for their inhibitory effects and mechanisms on tyrosinase and α-glucosidase. The inhibitory effects on both tyrosinase and α-glucosidase were in decreasing order, pedalitin > methyl rosmarinate > rosmarinic acid. The IC₅₀ values for the tyrosinase and α-glucosidase activity inhibited by pedalitin were 0.28 and 0.29 mM, respectively. Both rosmarinic acid and methyl rosmarinate were considered as noncompetitive inhibitors of tyrosinase, while pedalitin was suggested to be a mixed-type inhibitor of tyrosinase. In the assay of α-glucosidase inhibition, rosmarinic acid was found to be a competitive inhibitor, whereas both methyl rosmarinate and pedalitin were considered as mixed-type inhibitors.     

In vitro screening of phenolic compounds, potential inhibition against α-amylase and α-glucosidase of culinary herbs in Thailand

Herbs that are commonly used in Thai dishes were selected for evaluation in order to determine their phenolic compounds, antioxidant activity, and in vitro potential inhibition against α-amylase and α-glucosidase. The total phenolic content ranged from 2.89 to 75.26 mg GAE/g dw. The three aqueous herb extracts with the highest total phenolic content were yellow-berried nightshade (Solanum xanthocarpum), holy basil (Ocimum sanctum) and acacia (Acacia pennata). The antioxidant activity was expressed as percent inhibition of DPPH, ranging from a high of 85% in bitter gourd (Monordica charantia) to a low of 0% in garlic (Allium sativum) and shallot (Allium cepa). A high correlation (r = 0.70, p < 0.05) was observed between total phenolic content and antioxidant activity for the herb extracts in the Solanaceae family while, an insignificant high and negatively correlation (r = −1.00, p > 0.05) for the herb extracts in the Cucurbitaceae and Umbelliferae families was observed. All of the herbs had caffeic content, varying from 0.00 to 23.93 mg/g dw. Peppermint (Mentha canalenisa), galangal (Languas galangal) and holy basil (O. sanctum) had a significant p-coumaric acid content. The first and second obtained principal component represented 60% of the total variation. The potential inhibition against α-amylase for the herbs extracts ranged from 0% to 58%, while the highest percentage was observed in the acacia (A. pennata) extract. The potential inhibition against α-glucosidase varied from 7% to 100%. A high correlation (r = 0.68, p < 0.05) was observed between α-amylase inhibition and caffeic acid content for aqueous extracts in the Alliaceae, Cucurbitaceae, and Leguminosae families, while a high correlation (r = 0.49, p < 0.05) was observed between α-glucosidase inhibition and total phenolic content for the aqueous extracts of all herbs. Therefore, specific culinary herbs showed to have a potential use for dietary management during the early stages of hyperglycaemia.     

Inhibitory potential of trilobatin from Lithocarpus polystachyus Rehd against α-glucosidase and α-amylase linked to type 2 diabetes

The chemical structure of the sweet compound from Lithocarpuspolystachyus Rehd was identified as trilobatin on the basis of HPLC, EIS-MS and NMR analyses. The inhibitory activities of trilobatin against α-glucosidase and α-amylase were evaluated, and the inhibition mechanism was analysed with Lineweaver–Burk plots. Also the antioxidant activity evaluation of trilobatin was conducted by DPPH radical scavenging assay. Comparing with acarbose, trilobatin showed a strong inhibitory activity against α-glucosidase and a moderate inhibitory activity against α-amylase. The Lineweaver–Burk plots analysis elucidated that trilobatin inhibited the enzyme non-competitively. DPPH scavenging activity of trilobatin (IC₅₀ = 0.57 mg/ml) was higher than rutin (IC₅₀ = 0.72 mg/ml), which indicated that trilobatin had a moderate antioxidant potential. These results suggest that trilobatin is a potential effective α-glucosidase inhibitor for management of postprandial hyperglycemia with less side effect, and provide strong rationale for further animal and clinical studies.     

α-Glucosidase inhibitors from the bulb of Eleutherine americana

One effective way to treat diabetes is by suppressing carbohydrate digestion due to the utilisation of α-glucosidase inhibitors (AGIs). The determination of prospective herbs, done in vitro by using enzyme assay, resulted in the finding of Eleutherine americana, which showed a potent inhibitory activity. A 50% aqueous methanol-soluble extract of bulbs of E. americana was chromatographed successively and the active fractions were further purified with preparative high performance liquid chromatography (HPLC) to isolate active compounds against α-glucosidase. Structure determination by mass and NMR analysis revealed that these isolated compounds were eleutherol (1), eleutherinoside A (2), and eleuthoside B (3) based on comparisons with the reference data. Considering the amount and the inhibitory activity of each naphthalene in the whole extracts, the bulb of E. americana inhibitory activity against α-glucosidase might be a result of compound 2 (IC₅₀ = 0.5 mM, yield = 5 mg/50 g plant sample, with a characteristic structure which has never been found in other AGIs. AGIs play an important role for the treatment of diabetes, therefore these results may suggest novel alternatives for diabetes treatment management.     

Flavan-3-ol contents,anti-oxidative and α-glucosidase inhibitory activities of Cynomorium songaricum

Flavan-3-ol oligomers from the stems of Cynomorium songaricum were found to show potent SOD-like activity with one of the dimers being most potent. The flavan-3-ols also showed inhibitory activity on α-glucosidase, implying the beneficial effects of this herb on diabetes patients. The total (extractable and non-extractable) tannin content of this herb was found to be as high as 18.3%. The contents of catechin, flavan-3-ol oligomers and extractable tannins using 70% acetone, methanol or water were analysed. It was found that 70% acetone was the most efficient extract solvent amongst these three solvents, especially for higher flavan-3-ol oligomers.     

Enzymatic properties and transglycosylation of α-galactosidase from Penicillium oxalicum SO

Penicillium oxalicum SO α-galactosidase demonstrated weak hydrolysing activity but a high rate of transglycosylation in the reaction with melibiose, where the major product was 6-α-galactosyl melibiose. The transfer ratio was 83.6% and was maintained over a long reaction time of 80 h. The molecular weight was estimated to be 124,000 by SDS–PAGE. The optimal pH was ∼3 and a stable pH, with a range of 2.4–9.5, was found. The optimal temperature was ∼60 °C and the activity was stable below 60 °C. With respect to acceptor specificity, mono-alcohols, sugar alcohols and sugars were poor acceptors, but the di-alcohol ethylene glycol and the tri-alcohol glycerin were good acceptors. The percentage of transglycosylation to glycerin increased up to 41.7%, as that to melibiose decreased, with the initial glycerin concentration of 40%. The production of α-d-galactosylglycerol was 293 mg for each gram of melibiose used by the enzymatic reaction.     

Substrate specificities and mechanism of action of multiple α-galactosidases from Streptomyces griseoloalbus

α-Galactosidase or melibiase is a versatile enzyme with many potential biotechnological and industrial applications. The substrate specificities of three α-galactosidases – α-Gal I, α-Gal II, and α-Gal III – from Streptomyces griseoloalbus were studied using different galactose-containing natural substrates like melibiose, raffinose and stachyose. The kinetic parameters K_m and V_max were determined from the Lineweaver–Burk plot. α-Gal I showed highest affinity towards melibiose where as α-Gal II and α-Gal III showed highest affinity towards stachyose. The ¹H NMR studies showed that all the three α-galactosidases had a retaining mechanism of hydrolysis.     

Compounds from Angelica keiskei with NQO1 induction, DPPH scavenging and α-glucosidase inhibitory activities

A LC-MS method, which GSH was used as substrate, was employed to reveal the compounds with NQO1 induction activity from Angelica keiskei. Some compounds, proposed as isobavachalcone, xanthoangelol and 4-hydroxyderricin, have NQO1 induction activity. To make the actual structures and bioactivities of these compounds clear, 23 compounds, including above mentioned compounds and two new compounds 4-hydroxy-3,5,5-trimethyl-4-(1,2,3-trihydroxybutyl)cyclohex-2-enone (18) and (Z)-2-(3-hydroxypent-1-ynyl)-3-(non-1-enyl)oxiran-2-ol (23), were isolated from the 95% ethanol extract of A. keiskei. The bioassay results suggested the compounds had notable NQO1 induction activity. The radical scavenging and α-glucosidase inhibition activities of the isolated compounds were also tested. Compounds (E)-1-(2,4-dihydroxy-3-(3-methylbut-2-enyl)phenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (1), (E)-1-(3-((E)-3,7-dimethylocta-2,6-dienyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (2), 1-(3-hydroxy-10,13-dimethyl-2,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclopenta[α]phenanthren-17-yl)ethanone (17), 4-hydroxy-3,5,5-trimethyl-4-(1,2,3-trihydroxybutyl)cyclohex-2-enone (18) could scavenge DPPH radical by more than 20%. Compounds (E)-1-(2,4-dihydroxy-3-(3-methylbut-2-enyl)phenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (1), (E)-1-(3-((E)-3,7-dimethylocta-2,6-dienyl)-2,4-dihydroxyphenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (2), (E)-1-(2-hydroxy-4-methoxy-3-(3-methylbut-2-enyl)phenyl)-3-(4-hydroxyphenyl)prop-2-en-1-one (3), 7-hydroxy-6-(3-methylbut-2-enyl)-2H-chromen-2-one (5), 10-hydroxy-8,8-dimethyl-2-oxo-9,10-dihydropyrano[6,5-H]chromen-9-yl)-3-methylbut-2-enoate (9), 2-(4-hydroxyphenyl)-7-methoxy-8-(3-methylbut-2-enyl)-2,3-dihydrochromen-4-one (16), (10S,15R,Z)-10,15-dihydroxyheptadeca-8,16-dien-11,13-diynyl acetate (20), (3R,8S,Z)-heptadeca-1,9-dien-4,6-diyne-3,8-diol (21) exhibited excellent α-glucosidase inhibition activity.     

Two new saponins from tetraploid jiaogulan (Gynostemma pentaphyllum), and their anti-inflammatory and α-glucosidase inhibitory activities

Jiaogulan tea has been commercialised globally. This study investigated the chemical components and health properties of a new jiaogulan genotype, tetraploid Gynostemma pentaphyllum. Two new saponins, (23S)-21β-O-methyl-3β,20ξ-dihydroxy-12-oxo-21,23-epoxydammar-24-ene-3-O-[α-l-rhamnopyranosyl(1→2)][β-d-glucopyranosyl(1→3)]-α-l-arabinopyranoside (4) and 23β-H-3β,20ξ-dihydroxy-19-oxo-21,23-epoxydammar-24-ene-3-O-[α-l-rhamnopyranosyl(1→2)][β-d-xylopyranosyl(1→3)]-α-l-arabinopyranoside (5), together with one lactone, 3,5-dihydroxyfuran-2(5H)-one (1), and two flavonoids, rutin (2) and kaempferol 3-O-rutinoside (3), were characterised in the aerial parts of tetraploid jiaogulan. The chemical structures of the five isolated compounds were elucidated by NMR, HR-MS spectra and chemical degradation. The five compounds were also examined and compared with the methanol extract and n-butanol soluble fraction of the jiaogulan for their inhibitory activities on lipopolysaccharide (LPS)-induced IL-1β, IL-6 and COX-2 mRNA expression in RAW 264.7 mouse macrophages, and their in vitro α-glucosidase suppressing capacities. The results from this study may be used to promote the potential application of jiaogulan in functional foods.     

Identification and characterization of novel α-amylase and α-glucosidase inhibitory peptides from camel whey proteins

This study explores the inhibitory properties of camel whey protein hydrolysates (CWPH) toward α-amylase (AAM) and α-glucosidase (AG). A general full factorial design (3 × 3) was applied to study the effect of temperature (30, 37, and 45°C), time (120, 240, and 360 min), and enzyme (pepsin) concentration (E%; 0.5, 1, and 2%). The results showed that maximum degree of hydrolysis was obtained when hydrolysis was carried out at higher temperature (45°C; P < 0.05), compared with lower temperatures of 30 and 37°C. Electrophoretic pattern displays degradation of all protein bands upon hydrolysis by pepsin at various hydrolysis conditions applied. All the 27 CWPH generated showed significant AAM and AG inhibitory potential as indicated by their lower IC₅₀ values (mg/mL) compared with intact whey proteins. In total 196 peptides were identified from selected hydrolysates and 15 potential peptides (PepSite score > 0.8; http://pepsite2.russelllab.org/) were explored via in silico approach. Novel peptides PAGNFLMNGLMHR, PAVACCLPPLPCHM, MLPLMLPFTMGY, and PAGNFLPPVAAAPVM were identified as potential inhibitors for both AAM and AG due to their high number of binding sites and highest binding probability toward the target enzymes. CCGM and MFE, as well as FCCLGPVPP were identified as AG and AAM inhibitory peptides, respectively. This is the first study that reports novel AG and AAM inhibitory peptides from camel whey proteins. The future direction for this research involves synthesis of these potential AG and AAM inhibitory peptides in a pure form and investigate their antidiabetic properties in the in vitro, as well as in vivo models. Thus, CWPH can be considered for potential applications in glycaemic regulation.     

Anti-diabetic activity peptides from albumin against α-glucosidase and α-amylase

The objectives of this study were to identify novel peptides from albumin, and to evaluate and validate the anti-diabetic activity of peptides against α-glucosidase and α-amylase. In the research, albumin hydrolysate was purified and identified, tandem MS was adapted to characterise the amino acid sequences of peptides from the hydrolysate. In addition, anti-diabetic effects of the peptides with α-glucosidase and α-amylase inhibitory activity have been performed. The present work found eight novel peptides from albumin. Results also suggested that peptide KLPGF had α-glucosidase inhibitory activity with an IC(50) of 59.5±5.7μmoll(-1) and α-amylase inhibitory activity with an IC(50) of 120.0±4.0μmoll(-1). In conclusion, the results revealed that the peptide KLPGF was a potential anti-diabetic inhibitor.     

Biological activity of rice extract and the inhibition potential of rice extract,rice volatile compounds and their combination against α-glucosidase, α-amylase and tyrosinase

Rice is produced for consumption and traditional medicine. Rice is also used as an ingredient in cosmetic products. In this study, the author investigated the biological activity and inhibition potential against α-glucosidase, α-amylase and tyrosinase activity of rice extract (black rice [BR], red rice [RR] and white rice [WR]), rice volatile compounds, rice extract combined with volatile compounds, rice extract combined with standard inhibitors and volatile compounds combined with standard inhibitors. The results revealed that the free-radical scavenging capacity of rice extract is related to the phenolic content and flavonoids. BR showed the highest potential to inhibit α-glucosidase and α-amylase activity, whereas WR showed the highest potential to inhibit tyrosinase activity. Among rice volatile compounds, vanillin and vanillyl alcohol had the highest inhibition potential against α-glucosidase and α-amylase, respectively, whereas guaiacol had the highest inhibitory activity against tyrosinase. Molecular docking supported by the high binding efficiency was also obtained from vanillin and guaiacol when located at the active site of these enzymes. The combination of RR with acarbose (AB) had the highest inhibition potential and showed a synergic effect on both α-glucosidase and α-amylase. Interestingly, the combination of rice extract (BR, RR and WR) and vanillin and vanillyl alcohol had a synergic effect on α-amylase. Moreover, the combination of WR and vanillyl alcohol had the highest inhibition potential and showed a synergic effect on tyrosinase, whereas rice volatile compounds had a synergic effect on tyrosinase obtained from 2-pentylfuran/kojic acid (KA), vanillin/KA and vanillyl alcohol/KA.     

Purification and characterisation of α-glucosidase inhibitory peptides from defatted camellia seed cake

Enzymatically derived peptides from different proteins have displayed the potential to provide health benefits. Present research aims to obtain the functional peptides and improve the economic value of camellia seed cake via enzymatic hydrolysis. Camellia seed cake protein hydrolysates (CSCPH) prepared with alcalase demonstrated higher α-glucosidase inhibitory activity. After ultrafiltration, the α-glucosidase inhibitory activity of different components was compared. The component (CSCPH-IV) with the highest activity was selected for further separation and purification. The peptides were purified using reverse-phase high-performance liquid chromatography (RP-HPLC) twice and identified as GHSLESIK, GLTSLDRYK and SPGYYDGR by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Subsequently, the preliminarily inhibitory effect of these peptides on α-glucosidase activity was demonstrated by the molecular docking model. These results showed that three novel peptides isolated from the hydrolysates of camellia seed cake have the potential to be used as anti-diabetic compounds for the development of functional foods.     

Inhibitory effect on α-glucosidase by the fruits of Terminalia chebula Retz.

Mammalian α-glucosidase inhibitory activity by Terminalia chebula Retz. fruits was investigated. The aqueous methanolic extract was found to have potent rat intestinal maltase inhibitory activity, whereas neither intestinal sucrase nor isomaltase activity was inhibited by this extract. Using bioassay-guided separation, three active ellagitannins were identified as chebulanin (1), chebulagic acid (2) and chebulinic acid (3) and were shown to possess potent intestinal maltase inhibitory activity, with the IC₅₀ values of 690 μM, 97 μM and 36 μM, respectively. The intestinal maltase inhibitory activities of 2 and 3 were even higher than that of 1,2,3,4,6-penta-O-galloyl-β-d-glucose (PGG) (4, IC₅₀=140 μM), which is a known potent α-glucosidase inhibitor. Comparison of the activities of 1–4, 1,2,3-O-trigalloyl-β-d-glucose (5), neochebulagic acid (6) and corilagin (7) suggested that the positions of chebulloyl and galloyl groups mostly affected the potency. Kinetic studies revealed that 2, 3, and 4 inhibited maltose-hydrolyzing activity of intestinal α-glucosidase, noncompetitively. This is the first report on mammalian α-glucosidase inhibition by 1, 2 and 3 isolated from T. chebula fruits. These results suggest a use of the extract of T. chebula fruits for managing Type 2 diabetes.     

Rhus verniciflua Stokes flavonoid extracts have anti-oxidant,anti-microbial and α-glucosidase inhibitory effect

We isolated four compounds, fustin, gallic acid, 3′,4′,7-trihydroxyflavone, and fisetin from Rhus verniciflua. These compounds showed electron donation ability (87–94%) that was stronger than butylated hydroxyanisole (52%). Gallic acid (OD₇₀₀ = 1.98) showed the highest reducing power, and the other isolated compounds (OD₇₀₀ = 0.66–1.31) showed stronger activity than α-tocopherol (OD₇₀₀ = 0.21). The ethyl-acetate fraction had the highest phenolic content (723 mg GAE/g), followed by the 80% ethanolic extract (597 mg GAE/g). For Gram-negative bacteria, fisetin had the most potent anti-bacterial activity (MIC = 8 μg/ml) against Escherichia coli. 3′,4′,7-Trihydroxyflavone (106%), the 80% ethanolic extract (101%), and ethyl-acetate (98%) had the most powerful α-glucosidase inhibitory effect at 50 μg/ml. R. verniciflua extracts have potential as functional food additives.