Role of the thiosugar ring in the inhibitory activity of salacinol, a potent natural α-glucosidase inhibitor

Herein, ring-cleaved (24) and truncated (25) analogues of an azasugar, 1-deoxynojirimycin (23), exhibited inhibitory activity (Ki = 4-10 μM) equal to that of the parent compound (1, Ki = 14 μM). Based on this structure-activity relationship (SAR), four ring-cleaved (26a-26c and 27c) and three truncated (28a-28c) analogues of salacinol (1), a potent thiosugar-ring-containing α-glucosidase inhibitor, were synthesised. Bioassay results revealed that all the synthetics were inactive, indicating that the 5-membered thiosugar ring of 1 played an essential role in the potent activities of sulfonium-type inhibitors. The present findings are interesting and important in understanding the function of salacinol, considering that the observed inhibitory activity trend was contrary to the SAR observed in aza-compounds (23, 24, and 25) in a previous study, which suggested that the cyclic structure did not contribute to their strong inhibitory activity.

Design and Synthesis of Sulfonium Derivatives: A Novel Class of α-Glucosidase Inhibitors with Potent In Vivo Antihyperglycemic Activities

We report the first attempt of double-spot structural modification on a side-chain moiety of sulfonium-type α-glucosidase inhibitors isolated from genus Salacia. A series of sulfonium salts with benzylidene acetal linkage at the C3' and C5' positions were designed and synthesized. In vitro enzyme inhibition evaluation showed that compounds with a strong electron-withdrawing group attached at the ortho position on the phenyl ring present stronger inhibitory activities. Notably, the most potent inhibitor 21b (1.0 mpk) can exhibit excellent hypoglycemic effects in mice, which can still compete with those of acarbose (20.0 mpk). Molecular docking of 21b demonstrated that besides conventional interacting patterns, the newly introduced benzylidene acetal moiety plays an important role in anchoring the whole molecule in a concave pocket of the enzyme. The successful identification of 21b as a lead compound for new drug discovery may provide a means for structure modification and diversification of the distinguished sulfonium-type α-glucosidase inhibitors.

Pharmaceutical Food Science: Search for Bio-Functional Molecules Obtained from Natural Resources to Prevent and Ameliorate Lifestyle Diseases

In this review, our resent pharmaceutical food science research for bio-functional molecules obtained from natural resources that contribute to i) suppression of postprandial blood glucose elevation and/or improvement of glucose tolerance and ii) reduction of visceral fat accumulation and improvement of lipid metabolism were summarized. Based on studies using MONOTORI science, salacinol (1), neokotalanol (4), and trans-tiliroside (20) have been approved or notified by the Consumer Affairs Agency in Japan as functional substances in food with health claims, Food for Specified Health Use and Food with Functional Claims.

The chemistry and biology of natural ribomimetics and related compounds

Natural ribomimetics represent an important group of specialized metabolites with significant biological activities. Many of the activities, e.g., inhibition of seryl-tRNA synthetases, glycosidases, or ribosomes, are manifestations of their structural resemblance to ribose or related sugars, which play roles in the structural, physiological, and/or reproductive functions of living organisms. Recent studies on the biosynthesis and biological activities of some natural ribomimetics have expanded our understanding on how they are made in nature and why they have great potential as pharmaceutically relevant products. This review article highlights the discovery, biological activities, biosynthesis, and development of this intriguing class of natural products.

A review of antidiabetic active thiosugar sulfoniums, salacinol and neokotalanol, from plants of the genus Salacia

During our studies characterizing functional substances from food resources for the prevention and treatment of lifestyle-related diseases, we isolated the active constituents, salacinol (1) and neokotalanol (4), and related thiosugar sulfoniums, from the roots and stems of the genus Salacia plants [Celastraceae (Hippocrateaceae)] such as Salacia reticulata Wight, S. oblonga Wall., and S. chinensis L., and observed their antidiabetic effects. These plant materials have been used traditionally in Ayurvedic medicine as a specific remedy at the early stage of diabetes, and have been extensively consumed in Japan, the United States, and other countries as a food supplement for the prevention of obesity and diabetes. Here, we review our studies on the antidiabetic effects of plants from the genus Salacia, from basic chemical and pharmacological research to their application and development as new functional food ingredients.

Ligand compatibility of salacinol-type α-glucosidase inhibitors toward the GH31 family

We show that salacinol-type α-glucosidase inhibitors are ligand-compatible with the GH 31 family. Salacinol and its 3′-O-benzylated analogs inhibit human lysosomal α-glucosidase at submicromolar levels. Simple structure-activity relationship studies reveal that the salacinol side-chain stereochemistry significantly influences binding to GH31 α-glucosidases.

Salacinol-type α-glucosidase inhibitors are ligand-compatible with the GH 31 family. Salacinol and its 3′-O-benzylated analogs inhibit human lysosomal α-glucosidase at submicromolar levels.

Elongation of the side chain by linear alkyl groups increases the potency of salacinol, a potent α-glucosidase inhibitor from the Ayurvedic traditional medicine "Salacia," against human intestinal maltase

Four chain-extended analogs (12a-12d) and two related de-O-sulfonated analogs (13a and 13c) by introducing alkyl groups (a: R = C₃H₇, b R = C₆H₁₃, c: R = C₈H₁₇, d: R = C₁₀H₂₁) to the side chains of salacinol (1), a natural α-glucosidase inhibitor from Ayurvedic traditional medicine "Salacia", were synthesized. The α-glucosidase inhibitory activities of all the synthesized analogs were evaluated in vitro. Against human intestinal maltase, the inhibitory activities of 12a and 13a with seven-carbon side chain were equal to that of 1. In contrast, analogs (12b-12d, and 13c) exhibited higher level of inhibitory activity against the same enzyme than 1 and had equal or higher potency than those of the clinically used anti-diabetics, voglibose, acarbose, and miglitol. Thus, elongation of the side chains of 1 was effective for specifically increasing the inhibitory activity against human intestinal maltase.

Recent Progress in the Construction of Natural De-O-Sulfonated Sulfonium Sugars with Antidiabetic Activities

A group of sulfonium salts equipped with a polyhydroxylated side-chain structure have been isolated and identified as potent α-glycosidase inhibitors. Consequently, they have become an attractive target in diverse research disciplines, including organic synthesis, drug discovery, and chemical biology. To this end, the development of practical and effective synthetic strategies, especially for more bioactive de-O-sulfonated sulfonium salts, is a significant research area in organic synthesis. An ideal synthetic methodology should provide easily accessible intermediates with high chemical stability for the key coupling reaction to diastereoselectively construct the sulfonium cation center. This minireview summarizes recently developed strategies applied in the construction of natural de-O-sulfonated sulfonium sugars: 1) acid-catalyzed de-O-sulfonation of sulfonium sulfate inner salts, 2) a coupling reaction between side-chain fragments containing leaving groups and a thiosugar, 3) a coupling reaction between side-chain fragments containing epoxide structures and a thiosugar, and 4) a two-step sequential S_N 2 nucleophilic substitution between side-chain fragments containing thiol groups and a diiodide derivative.

Practical Route to Neokotalanol and Its Natural Analogues: Sulfonium Sugars with Antidiabetic Activities

An efficient and divergent approach toward the synthesis of all four de-O-sulfonated sulfonium type α-glucosidase inhibitors, originally isolated from plants of genus Salacia, is reported for the first time. The key strategy features a coupling reaction between thiol derivatives and a diiodide counterpart. The newly designed thiol coupling partner presents high chemical stability, while the diiodide partner could be easily obtained with increased overall yields compared with conventional routes. The intermolecular nucleophilic substitution reaction followed by a diastereoselective intramolecular cyclization provided the target five-member sulfonium salt structure, which was connected in an α-orientation to a polyhydroxylated side-chain moiety.

Diastereoselective Synthesis of Salacinol-Type α-Glucosidase Inhibitors

A facile and highly diastereoselective approach toward the synthesis of potent salacinol-type α-glucosidase inhibitors, originally isolated from plants of the genus "Salacia", was developed using the S-alkylation of thiosugars with epoxides in HFIP (∼90%, dr, α/β = ∼ 26/1). The dr ratio of the product was significantly improved by the protocol as compared to that of the conventional S-alkylation of thiosugars (dr, α/β = ∼ 8/1). The protocol could be used for gram scale synthesis of the desired compounds. The 3'-O-benzylated salacinol analogs, which are the most potent in vitro inhibitors to date, were synthesized and evaluated in vivo; all analogs suppressed blood glucose levels in maltose-loaded mice, at levels comparable to those of the antidiabetic agent, voglibose.

Aspects of extraction and biological evaluation of naturally occurring sugar-mimicking sulfonium-ion and their synthetic analogues as potent α-glucosidase inhibitors from Salacia: a review

A review of the selective inhibitory activities of sulfonium compounds ofSalaciaagainst intestinal α-glucosidases, structural features important for effective inhibition and the toggling approach for controlling starch digestion and glucose release.

Hydrophobic substituents increase the potency of salacinol, a potent α-glucosidase inhibitor from Ayurvedic traditional medicine 'Salacia'

Using an in silico method, seven analogs bearing hydrophobic substituents (8a: Me, 8b: Et, 8c: n-Pent, 8d: n-Hept, 8e: n-Tridec, 8f: isoBu and 8g: neoPent) at the 3'-O-position in salacinol (1), a highly potent natural α-glucosidase inhibitor from Ayurvedic traditional medicine 'Salacia', were designed and synthesized. In order to verify the computational SAR assessments, their α-glucosidase inhibitory activities were evaluated in vitro. All analogs (8a-8g) exhibited an equal or considerably higher level of inhibitory activity against rat small intestinal α-glucosidases compared with the original sulfonate (1), and were as potent as or higher in potency than the clinically used anti-diabetics, voglibose, acarbose or miglitol. Their activities against human maltase exhibited good relationships to the results obtained with enzymes of rat origin. Among the designed compounds, the one with a 3'-O-neopentyl moiety (8g) was most potent, with an approximately ten fold increase in activity against human maltase compared to 1.

Salacia chinensis L. extract ameliorates abnormal glucose metabolism and improves the bone strength and accumulation of AGEs in type 1 diabetic rats

Although extracts of the roots and stems of Salacia chinensis have been used in folk medicines for chronic diseases such as rheumatism, irregular menstruation, asthma and diabetes mellitus, little is known about the mechanism by which Salacia chinensis extract (SCE) ameliorates these diseases. To clarify whether SCE ameliorates the progression of lifestyle-related diseases, the inhibitory effect of SCE on the formation of advanced glycation end products (AGEs) was analyzed in a rat model of streptozotocin-induced diabetes. Although the oral administration of SCE did not ameliorate the diabetes-induced decrease in body weight, it ameliorated the increase in glycoalbumin levels in diabetic rats. An analysis by liquid chromatography tandem mass spectrometry (LC-MS/MS) demonstrated that the levels of N(ε)-(carboxymethyl)lysine (CML) were highest in the femurs and that they increased by the induction of diabetes. The administration of SCE also ameliorated the decreased femur strength and the accumulation of CML. Furthermore, when all of the carbohydrates in the chow of diabetic rats were replaced with free glucose, the administration of SCE significantly ameliorated a diabetes-induced increase in glycoalbumin and decrease in serum creatinine level and body weight. This study provides evidence to support that SCE ameliorates diabetes-induced abnormalities by improving the uptake of glucose by various organs.

A Fortuitously Straightforward Synthesis of 4-Acetoxy-2-Propyltetrahydrothiophene

4-Acetoxy-2-propyltetrahydrothiophene was synthesised from 1-hepten-4-ol by a three-step route involving epoxidation and mesylation to 1,2-epoxy-4-heptyl mesylate and then reaction with thioacetate. An acetoxylated cyclic product was formed instead of the expected thioacetate, and a mechanism for its formation using an intramolecular transesterification is proposed.

Salacinol and related analogs: new leads for type 2 diabetes therapeutic candidates from the Thai traditional natural medicine Salacia chinensis

The antidiabetic effect of a hot water extract of stems of Salacia chinensis (SCE) was evaluated in vivo in KK-A^y mice, a typical type 2 diabetes mellitus mice model. Administration of CE-2 dietary feed containing 0.25 and/or 0.50% of SCE for three weeks to KK-A^y mice significantly suppressed the elevation of both blood glucose and HbA1c levels without significant changes in body weight or food intake. Glucose tolerance was improved by administration to KK-A^y mice for 27 days of AIN93M purified dietary feed containing 0.12% of SCE. No suppressive effect with respect to HbA1c level was observed when AIN93M/Glc dietary feed in which all digestible glucides were replaced with glucose was administered with SCE. Thus, α-glucosidase inhibitory activity approved as the mechanism of action of the antidiabetic effect of SCE by in vitro investigation was reconfirmed also in in vivo studies. Evaluation of the α-glucosidase inhibitory activity of the active constituents, salacinol (1), kotalanol (3), and neokotalanol (4), by employing human α-glucosidases revealed that these compounds inhibited them as potently (IC₅₀ = 3.9–4.9 μM for maltase) as they inhibited rat small intestinal α-glucosidase. The principal sulfonium constituents (1–4) were highly stable in an artificial gastric juice. In addition, 1–4 were hardly absorbed from the intestine in an experiment using the in situ rat ligated intestinal loop model. The results indicate that these sulfoniums are promising leads for a new type of anti-diabetic agents.

Evaluation of Salacia species as anti-diabetic natural resources based on quantitative analysis of eight sulphonium constituents: a new class of α-glucosidase inhibitors

INTRODUCTION

Stems and roots of Salacia genus plants have been used in Ayurveda as a specific remedy for early stage diabetes. Previous investigations identified four sulphonium sulphates, that is, salacinol (1), kotalanol (3), ponkoranol (5) and salaprinol (7), as the compounds responsible for the anti-diabetic activity. Their desulphonates (2, 4, 6 and 8) were also isolated as active constituents. Two separate quantitative analytical protocols, that is, for 1 and 3 and for 2 and 4, have been developed recently.

OBJECTIVE

To: validate the two analytical protocols with respect to all eight sulphoniums; evaluate the quality of a variety of Salacia samples collected in different geographical regions, that is, Thailand, Sri Lanka and India; and determine their distribution in each part of the plant, that is, stems/roots, leaves and fruits.

METHODS

Analyses of four sulphonium sulphates in 32 Salacia extracts were carried out on an Asahipak NH2P-50 column, and those of the corresponding desulphonates were conducted on an Inertsil ODS-3 column.

RESULTS

Neokotalanol (4) was the major constituent in Salacia samples from Thailand, whereas 1 was the primary constituent in extracts of the stems/roots of plants from Sri Lanka and India. These sulphoniums were only present in trace amounts in leaves and fruits of the plants.

CONCLUSION

Two analytical protocols were successfully applied to analyse 32 Salacia samples, and revealed that sulphoniums (1-8) had characteristic distributions due to the plant part and/or due to geographical region.

Naturally occurring sulfonium-ion glucosidase inhibitors and their derivatives: a promising class of potential antidiabetic agents

In humans, four different enzymes mediate the digestion of ingested carbohydrates. First salivary and pancreatic α-amylases, the two endoacting retaining glucosidases, break down the complex starch molecules into smaller linear maltose-oligomers (LM) and branched α-limit dextrins (αLDx). Then two retaining exoglucosidases, maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI), convert those molecules into glucose in the small intestine. The small intestinal brush-border epithelial cells anchor MGAM and SI, and each contains a catalytic N- and C-terminal subunit, ntMGAM, ctMGAM, ntSI, and ctSI, respectively. All four catalytic domains have, to varying extents, α-1,4-exohydrolytic glucosidase activity and belong to the glycoside hydrolase family 31 (GH31). ntSI and ctSI show additional activity toward α-1,6 (isomaltose substrates) and α-1,2 (sucrose) glycosidic linkages, respectively. Because they mediate the final steps of starch digestion, both MGAM and SI are important target enzymes for the treatment of type-2 diabetes. Because of their potent inhibitory activities against the mammalian intestinal α-glucosidases, sulfonium-ion glucosidase inhibitors isolated from the antidiabetic herbal extracts of various Salacia species have received considerable attention recently. Thus far, researchers have isolated eight sulfonium-ion glucosidase inhibitors from Salacia species: salaprinol, salacinol, ponkoranol, kotalanol, and four of their corresponding de-O-sulfonated compounds, the structures of which comprise a 1,4-anhydro-4-thio-d-arabinitol and a polyhydroxylated acyclic side chain. Some of these compounds more strongly inhibit human intestinal α-glucosidases than the currently available antidiabetic drugs, acarbose and miglitol, and could serve as lead candidates in the treatment of type-2 diabetes. In this Account, we summarize progress in the field since 2010 with this class of inhibitors, with particular focus on their selective inhibitory activities against the intestinal glucosidases. Through structure-activity relationship (SAR) studies, we have modified the natural compounds to derive more potent, nanomolar inhibitors of human MGAM and SI. This structural optimization also yielded the most potent inhibitors known to date for each subunit. Furthermore, we observed that some of our synthetic inhibitors selectively blocked the activity of some mucosal α-glucosidases. Those results led to our current working hypothesis that selective inhibitors can dampen the action of a fast digesting subunit or subunits which places the burden of digestion on slower digesting subunits. That strategy can control the rate of starch digestion and glucose release to the body. Decreasing the initial glucose spike after a carbohydrate-rich meal and extending postprandial blood glucose delivery to the body can be desirable for diabetics and patients with other metabolic syndrome-associated diseases.

Sulfonium ions as inhibitors of the mycobacterial galactofuranosyltransferase GlfT2

The mycobacterial cell wall possesses a core galactan moiety composed of approximately 30 galactofuranosyl residues attached via alternating β-(1→5) and β-(1→6) linkages.

The biosynthesis of nitrogen-, sulfur-, and high-carbon chain-containing sugars

Carbohydrates serve many structural and functional roles in biology. While the majority of monosaccharides are characterized by the chemical composition (CH2O)n, modifications including deoxygenation, C-alkylation, amination, O- and N-methylation, which are characteristic of many sugar appendages of secondary metabolites, are not uncommon. Interestingly, some sugar molecules are formed via modifications including amine oxidation, sulfur incorporation, and "high-carbon" chain attachment. Most of these unusual sugars have been identified over the past several decades as components of microbially produced natural products, although a few high-carbon sugars are also found in the lipooligosaccharides of the outer cell walls of Gram-negative bacteria. Despite their broad distribution in nature, these sugars are considered "rare" due to their relative scarcity. The biosynthetic steps that underlie their formation continue to perplex researchers to this day and many questions regarding key transformations remain unanswered. This review will focus on our current understanding of the biosynthesis of unusual sugars bearing oxidized amine substituents, thio-functional groups, and high-carbon chains.

Modulation of starch digestion for slow glucose release through "toggling" of activities of mucosal α-glucosidases

Starch digestion involves the breakdown by α-amylase to small linear and branched malto-oligosaccharides, which are in turn hydrolyzed to glucose by the mucosal α-glucosidases, maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI). MGAM and SI are anchored to the small intestinal brush-border epithelial cells, and each contains a catalytic N- and C-terminal subunit. All four subunits have α-1,4-exohydrolytic glucosidase activity, and the SI N-terminal subunit has an additional exo-debranching activity on the α-1,6-linkage. Inhibition of α-amylase and/or α-glucosidases is a strategy for treatment of type 2 diabetes. We illustrate here the concept of "toggling": differential inhibition of subunits to examine more refined control of glucogenesis of the α-amylolyzed starch malto-oligosaccharides with the aim of slow glucose delivery. Recombinant MGAM and SI subunits were individually assayed with α-amylolyzed waxy corn starch, consisting mainly of maltose, maltotriose, and branched α-limit dextrins, as substrate in the presence of four different inhibitors: acarbose and three sulfonium ion compounds. The IC(50) values show that the four α-glucosidase subunits could be differentially inhibited. The results support the prospect of controlling starch digestion rates to induce slow glucose release through the toggling of activities of the mucosal α-glucosidases by selective enzyme inhibition. This approach could also be used to probe associated metabolic diseases.

Isolation, structure identification and SAR studies on thiosugar sulfonium salts, neosalaprinol and neoponkoranol, as potent α-glucosidase inhibitors

Two hitherto missing members of sulfonium salts family in Salacia genus plants as a new class of α-glucosidase inhibitors, neoponkoranol (7) and neosalaprinol (8), were isolated from the water extracts, and their structures were unambiguously identified. For further SAR studies on this series of sulfonium salts, several epimers of 7 and 8 were synthesized, and their inhibitory activities against rat small intestinal α-glucosidases were evaluated. Among them, 3'-epimer of 7 was found most potent in this class of molecules, and revealed as potent as currently used antidiabetics, voglibose and acarbose.

Effects of Salacia chinensis extract on reproductive outcome in rats

Salaciachinensis is a traditional South and Southeast Asian herb medicine and has been reported to have an antidiabetic function via α-glucosidases inhibitory activity. In this study, the effects of S. chinensis extract (SCE) on reproductive functions of F0 males and females and the effects on survival and growth of F1 offspring were examined using Sprague-Dawley rats. SCE was administered at dose levels of 0, 500, 1000 and 2000 mg/kg/day orally to groups consisting of 25 males and 25 females. Males were dosed once a day in the morning from 8 weeks before mating, throughout the mating period and until the day before necropsy and females were dosed once a day in the morning for 2 weeks before mating and through the mating, gestation and lactation periods (until day 20 of lactation). In all SCE treatment groups, no toxic signs were noted on reproductive outcome such as estrous cycle of F0 females or any parameters for reproductive function or survival, growth, sensory reflex or function development of F1 pups. Therefore, we concluded that SCE has no effects on the reproductive outcome even at a remarkably high dosage level, 2000 mg/kg/day, in Sprague-Dawley rats.

Quantitative analysis of neosalacinol and neokotalanol, another two potent α-glucosidase inhibitors from Salacia species, by LC-MS with ion pair chromatography

A quantitative analytical method for the highly polar sulfonium pseudo-sugar constituents neosalacinol (3) and neokotalanol (4), another two potent α-glucosidase inhibitors isolated from Ayurvedic traditional medicine Salacia species, was developed by employing an ion pair reagent upon chromatographic separation. The optimum conditions for separation and detection of these two constituents were achieved on an ODS column (3-µm particle size, 2.1-mm i.d. × 100 mm) with 5 mM undecafluorohexanoic acid-MeOH (99:1, v/v) as the mobile phase and using MS equipped with an electrospray ionization source. More than ten samples of Salacia from different origins were analyzed, and the results indicated that the assay was reproducible and precise and could be readily utilized for evaluation of α-glucosidase inhibitory activity of Salacia species. By combining this assay with the quantitative analytical method previously developed for salacinol (1) and kotalanol (2), a more precise and strict evaluation of α-glucosidase inhibitory activities of extracts from Salacia species (R = 0.959 for maltase and 0.795 for sucrase) was achieved.

Docking and SAR studies of salacinol derivatives as alpha-glucosidase inhibitors

Salacinol is a potent alpha-glucosidase inhibitor isolated from Salacia reticulata, and a good lead compound for an antidiabetic drug. It is essential to clarify the binding state of salacinol to alpha-glucosidase for efficient optimization study using structure-based drug design. Redocking simulations of two inhibitors, acarbose and casuarine whose complex structures are known, were performed to assure the appropriate docking pose prediction. The simulation reproduced both experimental binding states with accuracy. Then, using the same simulation protocol, the binding mode of salacinol and its derivatives has been predicted. Salacinol bound to the protein with a similar binding mode as casuarine, and the predicted structures could explain most of the structure-activity relationships of salacinol derivatives.