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Inhibition of Sucrase-Isomaltase and Maltase-Glucoamylase Activity by the Natural Product Kotalanol: A Potential Oral Agent for Treatment of Type 2 Diabetes Mellitus

Deposition Information
Author: 
Markosian, Christopher
Semester: 
2015 - Spring - 01:090:297:3
Project: 
Final Project
School: 
Rutgers
Status: 
Reviewed
Reviewed by: 
Dr. Stephen K. Burley

I. Introduction

Insulin is a polypeptide chain hormone that normally triggers glucose uptake in cells. Individuals with Type 2 Diabetes Mellitus (DM) experience high levels of blood-glucose levels due to inadequate insulin production in response to dietary intake or insulin resistance. Five forms of non-insulin anti-diabetes drugs are available to treat Type 2 DM: enhancers of insulin action in the periphery, enhancers of insulin secretion by β-cells, suppressors of endogenous glucose production, blockers of gastrointestinal tract carbohydrate absorption, and blockers of glucose re-uptake in the kidney. Each of these treatment options targets one or more pathways to reduce excess glucose levels in the blood.


In the gastrointestinal tract, numerous α-glucosidase enzymes catalyze hydrolysis of dietary carbohydrates into glucose monomers that can enter the blood stream. Even normal glucose absorption can represent a risk for type 2 diabetes patients ingesting high carbohydrate meals. Drugs that block the action of α-glucosidases reduce glucose formation and absorption into the bloodstream, reducing the likelihood of glucose related damage to eyes, kidneys, and peripheral nerves.


Kotalanol, an Ayurvedic medicine for diabetes, is one such enzyme inhibitor. This natural product is found in the roots and stems of Salacia oblonga and Salacia reticulata (Yoshikawa et al., 1998), which are native to Sri Lanka and South India. Kotalanol inhibits the activity of sucrase-isomaltase (SI) and maltase-glucoamylase (MGAM), two α-glucosidases that are released into the small intestine. These two enzymes hydrolyze carbohydrates into glucose monomers. Unhydrolyzed carbohydrates simply pass through the terminal portion of the gastrointestinal system and are either digested by gut bacteria or excreted.


II. Function of SI and MGAM

SI and MGAM are two of the four intestinal glycoside hydrolase GH31 enzymes that split disaccharides into glucose monomers during digestion (Sim et al., 2010b). By recognizing an α-O-glycosidic bond, which connects two hexose monomers, α-glucosidases break down disaccharides via hydrolysis. Hydrolysis utilizes a water molecule to break the O-glycosidic bond.


SI hydrolyzes sucrose (a disaccharide composed of α-glucose and β-fructose connected by a 1,2-α-O-glycosidic bond, as shown in Figure 1) and α-limit dextrin (a disaccharide composed of two α-glucose monomers connected by a 1,6-α-O-glycosidic bond, as shown in Figure 2). The products of sucrose hydrolysis are α-glucose and β-fructose and the products of α-limit dextrin hydrolysis are maltose and α-glucose.

Figure 1. Hydrolysis of sucrose by sucrase-isomaltase. The 1,4-α-O-glycosidic bond is broken via hydrolysis. (A) Sucrose before hydrolysis. (B) Products of sucrose hydrolysis.

 

 

Figure 2. Hydrolysis of α-limit dextrin by sucrase-isomaltase. The 1,6-α-O-glycosidic bond is broken via hydrolysis. (A) α-limit dextrin before hydrolysis. (B) Products of α-limit dextrin hydrolysis.

 

MGAM breaks down maltose (an oligosaccharide composed of two α-glucose monomers connected by a 1,4-α-O-glycosidic bond, as shown in Figure 3) and amylose (a polysaccharide composed of numerous α-glucose monomers connected by 1,4-α-O-glycosidic bonds). Both substrates have the same O-glycosidic bond. Unlike SI, this enzyme is only capable of hydrolyzing one type of glycosidic bond. The products of hydrolysis are α-glucose and shortened amylose polysaccharides.

Figure 3. Hydrolysis of maltose by maltase-glucoamylase. The 1,4-α-O-glycosidic bond is broken via hydrolysis. (A) Maltose before hydrolysis. (B) Products of maltose hydrolysis.


III. Structures of SI and MGAM

Both SI and MGAM are found tethered within the small intestine by an O-glycosylated stalk (Sim et al., 2010b). The N-terminal domains of both enzymes (ntSI and ntMGAM) have been studied by X-ray crystallography.

 

ntSI

SI hydrolyzes both α-limit dextrins and sucrose. These enzymatic activities are segregated within the protein. The N-terminal domain of SI (ntSI) breaks down α-limit dextrins (1-4 α-O-glycosidic bonds) (Sim et al., 2010b), while the C-terminal domain of SI (ctSI) breaks down sucrose (1-2-α-O-glycosidic bond).


The structure of the N-terminal domain of SI (ntSI) was determined at 3.2Å resolution (Sim et al., 2010b). The DNA segment encoding residues 62-931 of full-length human sucrase-isomaltase (total of 1,827 residues) was amplified via polymerase chain reaction. This fragment was cloned into a pMT-TEVA Drosophila expression vector at the BstBI and PmeI restriction sites, creating a pMT-TEVA-ntSI plasmid. This plasmid was subsequently expressed in Drosophila S2 cells. The secreted ntSI protein was purified and crystallized using the hanging drop vapor diffusion method. X-ray diffraction data was collected and analyzed to reveal the three-dimensional (3D) atomic level structure of residues 29-898. 


Hereafter, ntSI residue numbers correspond to those in PDB ID 3lpo (not UniProt numbering). The conversion from UniProt to PDB numbering is -33 residues.


As shown in Figure 4, ntSI consists of five subdomains: the type-P trefoil subdomain (residues 29-80), the N-terminal subdomain (81-296), the catalytic barrel subdomain (297-681), the proximal C-terminal subdomain (682-759), and the distal C-terminal subdomain (760-898) (Sim et al., 2010b). Excluding the type-P trefoil, the other four subdomains are conserved in all GH31 enzymes.

 

Figure 4. Apo-ntSI, color coded as follows: red denotes α-helices, blue β-strands, pink type-P trefoil subdomain, yellow N-terminal subdomain, orange catalytic barrel subdomain, green proximal C-terminal subdomain, purple distal C-terminal subdomain, and white highlights selected residues. (A) apo-ntSI. (B) With a 180°-rotation on y-axis. (C) Active site.

 

ntMGAM

MGAM breaks down maltose (1-6 α-O-glycosidic bonds). The N-terminal domain of MGAM (ntMGAM) is responsible for binding to and hydrolyzing its substrate. The structure of the ntMGAM (residues 29-898) was determined at 2.0Å resolution (Sim et al., 2010a), using the same  method as for ntSI.


Hereafter, ntMGAM residue numbers correspond to those in PDB ID 2qly (not UniProt numbering). The conversion from UniProt to PDB numbering is -86 residues.


As shown in Figure 5, ntMGAM also consists of five subdomains: the type-P trefoil subdomain (residues 29-63), the N-terminal subdomain (64-296), the catalytic barrel domain (297-681), the proximal C-terminal subdomain (682-757), and the distal C-terminal subdomain (758-898).

Figure 5. Apo-ntMGAM, color coded as follows: red denotes α-helices, blue β-strands, pink type-P trefoil subdomain, yellow N-terminal subdomain, orange catalytic barrel subdomain, green proximal C-terminal subdomain, purple distal C-terminal subdomain, and white highlights selected residues. (A) apo-ntMGAM. (B) With a 180°-rotation on y-axis. (C) Active site.

 

Structure Similarity

Figure 6 displays an overlay of the structures of apo-ntSI and apo-ntMGAM (root-mean-square deviation or rmsd=0.6Å for 819 α-carbon atomic pairs). Conserved residues contributing to substrate recognition overlay with α-carbon rmsd=0.2Å [ntSI/ntMGAM: Asp231/Asp203, Asp355/Asp327, Arg555/Arg526, and Asp571/Asp542]. Some structural differences are apparent within a superficial loop (ntSI: residues 400-408; ntMGAM: 372-379) and the type-P trefoil domain.

Figure 6. Overlay of apo-ntSI (yellow) and apo-ntMGAM (blue).


IV. Structure of Kotalanol

The molecular formula of kotalanol is C12H24O12S2 (Yoshikawa et al., 1998). As illustrated in Figure 7, it is composed of a 1-deoxyheptosyl-3-sulfate anion and a 1-deoxy-4-thio-D-arabinofuranosyl sulfonium cation. Along with numerous hydroxyl groups capable of hydrogen bonding, kotalanol contains two charged components near the center of the molecule: SO3- and S+. The five-membered ring resembles the five-membered ring of β-fructose. However, a sulfur atom occurs in the location of the oxygen atom of the substrate.

Figure 7. The structure of kotalanol. (A) visualized in Chimera. (B) The arrangement of atoms in kotalanol.


V. Kotalanol binding to SI and MGAM

Entropic Considerations: Kotalanol binds tightly to active site clefts of ntSI and ntMGAM. In the absence of either substrate or inhibitor, these clefts bind water molecules. Some of the favorable free energy of kotalanol binding to the enzymes comes from the entropy change that occurs when the inhibitor displaces partially ordered water molecules from the cleft and expels them into the bulk solvent thereby increasing the entropy of the system.

 

ntSI Complex with Kotalanol

Kotalanol strongly inhibits the α-glucosidase activity of ntSI, but has no affect on isomaltase activity (Sim et al., 2010b).

The structure of the ntSI-kotalanol complex was determined at 2.15Å resolution using preformed ntSI crystals soaked in a 200µM solution of kotalanol dissolved in mother liquor (Sim et al., 2010b).


Kotalanol binds to ntSI in an active site cleft, where α-limit dextrins bind during enzymatic hydrolysis, as shown in Figure 8. When kotalanol occupies the cleft, α-limit dextrins are unable to bind. Table 1 enumerates observed intermolecular interactions between ntSI and kotalanol.

Figure 8. Comparison of the solvent-accessible molecular surfaces of apo-ntSI and kotalanol-bound ntSI. Blue denotes basic residues, red: acidic residues, and white: hydrophobic residues. (A) Base view of the solvent-accessible molecular surface of ntSI complexed with kotalanol. (B) With a 180°-rotation on y-axis. (C) With a 45°-rotation counterclockwise on z-axis. (D) Focused close-up view of the empty active site of ntSI. (E) Close-up view of the ntSI active site occupied by kotalanol.

 

Table 1. Interactions Between ntSI and Kotalanol

Direct Hydrogen Bonds

(Figure 9)

Water-Mediated Hydrogen Bonds

(Figure 10)

Hydrophobic Interactions

(Figure 11)

Charged Interactions

(Figure 11)

OAF and OD1 of Asp231

3.0 Å

OAB and OD2 of Asp472

2.9 Å, 2.7 Å

CAW and CE3 of Trp327 (π)

3.7 Å

SAY and OD2 of Asp472

3.6 Å

OAH and OD2 of Asp231

2.8 Å

OAD and OD1 of Asp571

2.8 Å, 2.7 Å

CAR and CZ of Phe604 (π)

3.6 Å

OAG and NE2 of His629

3.1 Å

OAB and OD1 of Asp355

2.6 Å

 

 

 

OAG and OD2 of Asp355

2.3 Å

 

 

 

OAC and NZ of Lys509

2.9 Å

 

 

 

OAE and NH1 of Arg555

2.6 Å

 

 

 

OAE and OD2 of Asp571

2.6 Å

 

 

 

 

Figure 9. Direct hydrogen bonds between ntSI and kotalanol. Asp231, Asp355, Lys509, Arg555, and Asp571 interact with kotalanol via seven direct hydrogen bonds. (A) Base view of ntSI complexed with kotalanol. (B) With a 90°-rotation counterclockwise on y-axis. (C) Close-up view of the active site.

 

Figure 10. Water-mediated hydrogen bonds between ntSI and kotalanol. Residues Asp472 and Asp571 interact with kotalanol via two water-mediated hydrogen bonds. (A) Base view of ntSI complexed with kotalanol. (B) With a 180°-rotation on y-axis. (C) Close-up view of the active site.

 

 

Figure 11. Hydrophobic and charge-charge interactions between ntSI and kotalanol. Residues Trp327 and Phe604 interact with kotalanol via two hydrophobic interactions. Residues Asp472 and His629 interact with kotalanol via two charge-charge interactions. (A) Base view of ntSI complexed with kotalanol. (B) With a 180°-rotation on y-axis. (C) With a 45°-rotation counterclockwise on z-axis. (D) Close-up view of the active site.

 

Comparison Between Apo-ntSI and Kotalanol-Bound ntSI

Figure 12 displays an overlay of the apo-ntSI protein and kotalanol-bound ntSI protein (rmsd=0.4Å for 870 α-carbon atomic pairs). There is no evidence of induced fit.

 

Figure 12. Overlay of apo-ntSI and kotalanol-bound ntSI. Light blue denotes apo-ntSI, yellow denotes the kotalanol-bound ntSI, and orange denotes kotalanol. (A) Base view of the overlay. (B) With a 90°-rotation counterclockwise on y-axis. (C) With a 90°-rotation counterclockwise on z-axis. (D) Close-up view of the active site.

 

ntMGAM Complex with Kotalanol

The structure of the ntMGAM-kotalanol complex was determined at 1.9Å using the same method as that for ntSI-kotalanol complex with kotalanol (Sim et al., 2010a).

Kotalanol binds to the active site cleft of ntMGAM, where maltose and amylose bind, as shown in Figure 13. When kotalanol occupies the cleft, maltose and amylose are unable to bind. Table 2 enumerates observed intermolecular interactions between ntMGAM and kotalanol.

Figure 13. Comparison of the solvent-accessible molecular surfaces of apo-ntMGAM and kotalanol-bound ntMGAM. Blue denotes basic residues, red: acidic residues, and white: hydrophobic residues. (A) Base view of ntMGAM complexed with kotalanol. (B) With a 180°-rotation on y-axis. (C) With a 45°-rotation counterclockwise on z-axis. (D) Focused close-up view of the empty cleft of ntMGAM. (E) Close-up view of the active site.

 

Table 2. Interactions Between ntMGAM and Kotalanol

Direct Hydrogen Bonds

(Figure 14)

Water-Mediated Hydrogen Bonds

(Figure 15)

Hydrophobic Interactions

(Figure 16)

Charged Interactions

(Figure 16)

OAH and OD2 of Asp203

2.7 Å

OAF and OG1 of Thr205

2.9 Å, 2.8 Å

CAW and CD1 of Tyr299 (π)

3.9 Å

SAY and OD2 of Trp406

3.3 Å

OAC and OD1 of Asp203

2.9 Å

OAK and OH of Tyr299

2.7 Å, 2.8 Å

CAR and CZ of Phe575 (π)

3.9 Å

 

OAB and OD1 of Asp327

2.7 Å

OAB and NE1 of Trp406

2.6 Å, 2.9 Å

 

 

OAG and OD2 of Asp327

2.4 Å

OAB and OD2 of Asp443

2.6 Å, 2.8 Å

 

 

OAE and NH1 of Arg526

2.7 Å

OAD and NE1 of Trp539

2.8 Å, 2.9 Å

 

 

OAE and OD2 of Asp542

2.4 Å

OAD and OD1 of Asp542

2.8 Å, 2.6 Å

 

 

OAG and NE2 of His600

3.0 Å

 

 

 

 

Figure 14. Direct hydrogen bonds between ntMGAM and kotalanol. Residues Asp203, Asp327, Arg526, Asp542, and His600 interact with kotalanol via seven direct hydrogen bonds. 
(A) Base view of ntMGAM complexed with kotalanol. (B) With a 90°-rotation counterclockwise on y-axis. (C) Close-up view of the active site.

 

Figure 15. Water-mediated hydrogen bonds between ntMGAM and kotalanol. Residues Thr205, Tyr299, Trp406, Asp443, Trp539, and Asp542 interact with kotalanol via six water-mediated hydrogen bonds. (A) Base view of ntMGAM complexed with kotalanol. (B) With a 180°-rotation on y-axis. (C) Close-up view of the active site.

 

Figure 16. Hydrophobic and charge-charge interactions between ntMGAM and kotalanol. Residues Tyr299 and Phe575 interact with kotalanol via two hydrophobic interactions. Residue Trp406 interacts with kotalanol via one charged interaction. (A) Base view of ntMGAM complexed with kotalanol. (B) With a 180°-rotation on y-axis. (C) Close-up view of the active site.

  

Comparison Between Apo-ntMGAM and Kotalanol-Bound ntMGAM

Figure 17 displays an overlay of the structures of apo-ntMGAM kotalanol-bound ntMGAM protein (rmsd=0.3Å for 861 α-carbon atomic pairs). There is no evidence of induced fit.

Figure 17. Overlay of apo-ntMGAM and kotalanol-bound ntMGAM. Light brown denotes apo-ntMGAM, blue denotes the kotalanol-bound ntMGAM, and light blue denotes kotalanol. (A) Base view of the overlay. (B) With a 90°-rotation counterclockwise on y-axis. (C) With a 90°-rotation counterclockwise on z-axis. (D) Close-up view of the active sites.

 

Comparison Between Kotalanol-Bound ntSI and Kotalanol-Bound ntMGAM

Figure 18 displays an overlay of the kotalanol-bound ntSI protein and the kotalanol-bound ntMGAM protein (rmsd=0.6Å for 821 α-carbon atomic pairs). Thus, ntSI and ntMGAM share the same mode of kotalanol binding.

 

Figure 18. Overlay of kotalanol-bound ntSI and kotalanol-bound ntMGAM. Yellow denotes the kotalanol-bound ntSI, light brown is the kotalanol bound to ntSI, blue is the kotalanol-bound ntMGAM, and light blue is the kotalanol bound to ntMGAM. (A) Base view of the overlay. (B) With a 90°-rotation clockwise on z-axis. (C) With a 90°-rotation counterclockwise on y-axis. (D) Close-up view of the active sites.


VI. Kotalanol Administration for Patients

Kotalanol has not been mass-produced as a drug. Capsules containing S. oblonga extracts are, however, available in the form of over-the-counter oral supplements (typically dosed at 500 or 1000 mg once a day). Kotalanol is not currently approved by the United States Food and Drug Administration. Prices for 500 mg capsule range from 30-57 cents in the US, from suppliers such as Vitacost and Swanson Superior Herbs.


Kotalanol, like acarbose, remains primarily within the gastrointestinal tract, and is only minimally absorbed into the bloodstream and where it undergoes renal excretion. The half-life and oral bioavailability of kotalanol have not been reported.


VII. Kotalanol Drug-Drug Interactions and Side-Effects

Since kotalanol is an α-glucosidase inhibitor, it prevents a normal rise in blood glucose levels. However, when taken with other α-glucosidase inhibitors, such as acarbose, a severe drop in blood sugar levels may occur.


Side effects associated with kotalanol include diarrhea, belching, nausea, and abdominal (Williams et al., 2007). Such side effects typically last for less than 24 hours, and are thought to be due to incomplete digestion of carbohydrates passing through the small intestine. Instead of undergoing cleavage by either SI or MAGM in the small intestine, large carbohydrates are either excreted or digested by gut bacteria.


VIII. Kotalanol Efficacy in Patients with Type 2 Diabetes

The efficacy of kotalanol in treating type 2 diabetes was evaluated in 2007 during the course of a randomized, double-blinded crossover clinical study, designed to determined the effect of S. oblonga extracts on postprandial (post-feeding) glycemia and insulinemia following a high-carbohydrate meal ingested by patients with type 2 diabetes (Williams et al., 2007). Results are listed in Table 3.

 

Table 3. Effects of Kotalanol in Type-2 Diabetes Patients (Williams et al., 2007)

 

Control

240 mg

480 mg

Overall Glucose Level

-

14% Decrease

20% Decrease

Peak Glucose Response

-

19% Decrease

27% Decrease

Overall Insulin Level

-

14% Decrease

19% Decrease

Peak Insulin Response

-

9% Decrease

12% Decrease


IX. Comparison Between Kotalanol and Other α-Glucosidase Inhibitors

The structure of kotalanol is similar to that of salacinol (C9H18O9S2) (Mohan et al., 2010), another natural product found in the Salacia genus of plants (Sim et al., 2010a). As shown in Figure 19A, they both possess a 1,4-anhydro-4-thio-D-arabinitol core and polyhydroxylated acyclic chain (Rossi et al., 2006). However, kotalanol is more potent than salacinol in terms of its ability to inhibit intestinal α-glucosidases (Yoshikawa et al., 1998), suggesting that direct and water-mediated intermolecular interactions observed using X-ray crystallography between the hydroxyl group-rich tail of kotalanol and SI and MGAM play important roles in enzyme inhibition.

Figure 19. Comparison between kotalanol and other α-glucosidase inhibitors. The yellow- and green-highlighted regions of the structures denote the non-identical portions of the compounds relative to each other. (A) Comparison between kotalanol and salacinol. (B) Comparison between kotalanol and acarbose.


Kotalanol is a more potent inhibitor of SI than acarbose (C25H43NO18), which is the most widely used non-insulin anti-diabetic drug targeting α-glucosidase enzymatic activity (Yoshikawa et al., 1998). Figure 19B displays a comparison between the structures of the two compounds. Acarbose strongly inhibits enzymatic activities of  the C-terminal domains of SI and MGAM. However, unlike kotalanol, it only weakly inhibits the enzymatic activities of the N-terminal domains of SI and MGAM (Sim et al., 2010b).


X. Conclusion

Kotalanol is a natural product capable of reducing post-prandial glucose levels via α-glucosidase inhibition. Though not FDA-approved, it may confer benefits on individuals with type 2 diabetes. Further clinical studies will be needed to determine the safety and efficacy of long-term kotalanol treatment. Should kotalanol demonstrate an adequate safety profile, eventual US FDA approval will likely depend on demonstrating adequate hemoglobin A1c lowering together with beneficial impact on cardiovascular disease.


 

Note:

PDB IDs used Title UniProt Residue Range
3lpo Crystal structure of the N-terminal domain of sucrase-isomaltase. Residues 29 to 898 (PDB) :: residues 62 to 931 (UniProt)
3lpp Crystal complex of N-terminal sucrase-isomaltase with kotalanol. Residues 29 to 898 (PDB) :: residues 62 to 931 (UniProt)
2qly Crystal Structure of the N-terminal subunit of human maltase-glucoamylase. Residues 1 to 868 (PDB) :: residues 87 to 954 (UniProt)
3l4v Crystal complex of N-terminal human maltase-glucoamylase with kotalanol. Residues 1 to 868 (PDB) :: residues 87 to 954 (UniProt)

 

 

 

Deposition References: 
  • Mohan, S. and Pinto, B.M. Towards the elusive structure of kotalanol, a naturally occurring glucosidase inhibitor. Nat. Prod. Rep. 2010, 27, 481-488.
  • Mohan, S., Sim, L., Rose, D.R., and Pinto B.M. Probing the active-site requirements of human intestinal N-terminal maltase-glucoamylase: Synthesis and enzyme inhibitory activities of a six-membered ring nitrogen analogue of kotalanol and its de-O-sulfonated derivative. Elsevier. 2010,  18, 7794-7798.
  • Quezada-Calvillo, R., Sim, L., Ao, Z., Hamaker, B.R., Quaroni, A., Brayer, G.D., Sterchi, E.E., Robayo-Torres, C.C., Rose, D.R., and Nichols, B.L. Luminal Starch Substrate “Brake” on Maltase-Glucoamylase Activity Is Located within the Glucoamylase Subunit. J. Nutr. 2008, 138, 685-692.
  • Rossi, E.J., Sim, L., Kuntz, D.A., Hahn, D., Johnston, B.D., Ghavami, A., Szczepina, M.G., Kumar, N.S., Sterchi, E.E., Nichols, B.L., Pinto, B.M., and Rose, D.R. Inhibition of recombinant human maltase glucoamylase by salacinol and derivates. FEBS J. 2006, 273, 2673-2683.
  • Sim, L., Jayakanthan, K., Mohan, S., Nasi, R., Jonston, B.D., Pinto, B.M., and Rose, D.R. New glucosidase inhibitors from an ayurvedic herbal treatment for type 2 diabetes: structures and inhibition of human intestinal maltase-glucoamylase with compounds from Salacia reticulata. Biochemistry. 2010a, 49, 443-451.
  • Sim, L., Willemsma, C., Mohan, S., Naim, H.Y., Pinto, B.M., and Rose, D.R. Structural basis for substrate selectivity in human maltase-glucoamylase and sucrase-isomaltase N-terminal domains. J. Biol. Chem. 2010b, 285, 17763-17770.
  • Williams, J.A., Choe, Y.S., Noss, M.J., Baumgartner, C.J., and Mustad, V.A. Extract of Salacia oblonga lowers acute glycemia in patients with type 2 diabetes. Am. J. Clin. Nutr. 2007, 86, 124-130.
  • Yoshikawa, M., Murakami, T., Yashiro, K., and Matsuda, H. Kotalanol, a potent α-glucosidase inhibitor with thiosugar sulfonium sulfate structure, from antidiabetic ayurvedic medicine Salacia reticulata. Chem. Pharm. Bull. 1998, 46, 1339-1340.