ABSTRACT
A new isoflavonoid glycoside, iridin A (9), along with eight known isoflavonoids: irilone 4'-methyl ether (1), irilone (2), irisolidone (3), irigenin S (4), irigenin (5), irilone 4'-O-β-D-glucopyranoside (6), iridin S (7), and iridin (8) were separated from Iris × germanica L., Iridaceae, rhizomes. The structural elucidation of these flavonoids was achieved with the aid of extensive spectroscopic techniques and comparing with the published data. They were estimated for their α-amylase and 1,1-diphenyl-2-picrylhydrazyl inhibitory capacities. Compounds 3, 5, and 9 showed α-amylase inhibitory activities with % inhibition 70.8, 67.5, and 70.5, respectively compared to acarbose (a reference α-amylase inhibitor). Moreover, 9 exhibited moderate antioxidant activity with IC50 8.91 µM.
Keywords:
Iridin A; Iris germanica; Isoflavonoid; Iridaceae; α-Amylase inhibitors; Antioxidant
Introduction
Diabetes is a serious disease growing at alarming rates around the world. It constitutes the 6th leading cause of global mortality (WHO, 2016WHO, 2016. Global Report on Diabetes. World Health Organization, Geneva, pp. 1–88.). Type-II diabetes mellitus (T2D) is a widespread disorder of fat and glucose metabolism, which is strongly linked with diets (Garg et al., 1994Garg, A., Bantle, J.P., Henry, R.R., Coulston, A.M., Griver, K.A., Raatz, S.K., Brinkley, L., Chen, T.D., Grundy, S.M., Huet, B.A., 1994. Effects of varying carbohydrate content of diet in patients with non-insulin dependent diabetes mellitus. JAMA 271, 1421-1428.). Postprandial hyperglycemia (PPHG) is an independent risk factor for the development of diabetic complications (Kwon et al., 2005Kwon, Y.-I., Vattem, D.A., Shetty, K., 2005. Evaluation of clonal herbs of Lamiaceae species for management of diabetes and hypertension. Asia Pac. J. Clin. Nutr. 15, 107-118.). It was reported that the best therapeutic path for type-II diabetes is to lower PPHG through retardation of the intake of glucose by repression of α-amylases and α-glucosidases, which are responsible for the oligo- and disaccharides breakdown into glucose (Kim et al., 2005Kim, Y.M., Jeong, Y.K., Wang, M.H., Lee, W.Y., Rhee, H.I., 2005. Inhibitory effect of pine extract on α-glucosidase activity and postprandial hyperglycemia. Nutrition 21, 756-761.; Hanhineva et al., 2010Hanhineva, K., Torronen, R., Bondia-Pons, I., Pekkinen, J., Kolehmainen, M., Mykkanen, H., Poutanen, K., 2010. Impact of dietary polyphenols on carbohydrate metabolism. Int. J. Mol. Sci. 11, 1365-1402.). Polyphenols and flavonoids function as chemo-preventive agents against reactive oxygen species (ROS) induced oxidative damage and inhibit carbohydrate hydrolyzing enzymes, contributing to the lowering of PPHG (Mai et al., 2007Mai, T.T., Thu, N.N., Tien, P.G., Van Chuyen, N., 2007. α-Glucosidase inhibitory and antioxidant activities of Vietnamese edible plants and their relationships with polyphenol contents. J. Nutr. Sci. Vitaminol. 53, 267-276.; Ibrahim et al., 2015Ibrahim, S.R.M., Mohamed, G.A., Abdel-Latif, M.M.M., El-Messery, S.M., Al-Musayeib, N.M., Shehata, I.A., 2015. Minutaside A, new α-amylase inhibitor flavonol glucoside from Tagetes minuta: antidiabetic, antioxidant, and molecular modeling studies. Starch/Stärke 67, 976-984.). They are commonly found in nuts, vegetables, fruits, and tea (Arranz et al., 2010Arranz, S., Silvan, J.M., Saura-Calixto, F., 2010. Non extractable polyphenols, usually ignored, are the major part of dietary polyphenols: a study on the Spanish diet. Mol. Nutr. Food Res. 54, 1646-1658.). Strong efforts have been made to seek for more safe and efficient antioxidants and α-amylase inhibitors from natural source to unveil functional foods for the prevention and control of diabetes (Tundis et al., 2010Tundis, R., Loizzo, M.R., Menichini, F., 2010. Natural products as αamylase and α-glucosidase inhibitors and their hypoglycaemic potential in the treatment of diabetes: an update. Mini Rev. Med. Chem. 10, 315-331.; Tarling et al., 2008Tarling, C.A., Woods, K., Zhang, R., Brastianos, H.C., Brayer, G.D., Andersen, R.J., Withers, S.G., 2008. The search for novel human pancreatic α-amylase inhibitors: high-throughput screening of terrestrial and marine natural product extracts. ChemBioChem 9, 433-438.). Iris × germanica L., Iridaceae, leaves are rich in vitamins and ascorbic acid (Ibrahim et al., 2012Ibrahim, S.R.M., Mohamed, G.A., Al-Musayeib, N.M., 2012. New constituents from the Egyptian Iris germanica L. rhizomes. Molecules 17, 2587-2598.). Its extract stimulates respiration, reduces the activity of the smooth muscle, possesses serotonin inhibitory activity, and induces a transient hypotension (Asghar et al., 2010Asghar, S.F., Rehman, H.U., Rahman, A.U., 2010. Phytochemical investigation of Iris germanica. Nat. Prod. Res. 24, 131-139.; Kukula-Koch et al., 2015Kukula-Koch, W., Sieniawska, E., Widelski, J., Urjin, O., Głowniak, P., Skalicka-Woźniak, K., 2015. Major secondary metabolites of Iris spp.. Phytochem. Rev. 14, 51-80.). Its root decoction is used as stimulants, antispasmodic, anti-inflammatory, aperients, diuretic, violently cathartic, and emmengogue (Rahman et al., 2003aRahman, A.U., Nasim, S., Baig, I., Orhan, I., Sener, B., Ayanoglu, F., Choudhary, I.M., 2003a. Isoflavonoid glycosides from the rhizomes of Iris germanica. Helv. Chim. Acta 86, 3354-3362.,bRahman, A.U., Nasim, S., Baig, I., Jalil, S., Orhan, I., Sener, B., Choudhary, M.I., 2003b. Anti-inflammatory isoflavonoids from the rhizomes of Iris germanica. J. Ethnopharmacol. 86, 177-180.; Mohamed et al., 2013Mohamed, G.A., Ibrahim, S.R.M., Ross, S.A., 2013. New ceramides and isoflavone from the Egyptian Iris germanica L. rhizomes. Phytochem. Lett. 6, 340-344.). Previous phytochemical studies of I. germanica revealed the isolation of different constituents, including flavonoids (Asghar et al., 2010Asghar, S.F., Rehman, H.U., Rahman, A.U., 2010. Phytochemical investigation of Iris germanica. Nat. Prod. Res. 24, 131-139.; Ibrahim et al., 2012Ibrahim, S.R.M., Mohamed, G.A., Al-Musayeib, N.M., 2012. New constituents from the Egyptian Iris germanica L. rhizomes. Molecules 17, 2587-2598.; Mohamed et al., 2013Mohamed, G.A., Ibrahim, S.R.M., Ross, S.A., 2013. New ceramides and isoflavone from the Egyptian Iris germanica L. rhizomes. Phytochem. Lett. 6, 340-344.), triterpenes (Ito et al., 1995Ito, H., Miyake, Y., Yoshida, T., 1995. New piscicidal triterpenes from Iris germanica. Chem. Pharm. Bull. 43, 1260-1262.; Orhan et al., 2002Orhan, I., Sener, B., Hashimoto, T., Asakawa, Y., Ozgüven, M., Ayanogìu, F., 2002. Iristectorone K, a novel monocyclic triterpene ester from Iris germanica rhizomes growing in Turkey. Fitoterapia 73, 316-319.), sterols, phenolics (Ibrahim et al., 2012Ibrahim, S.R.M., Mohamed, G.A., Al-Musayeib, N.M., 2012. New constituents from the Egyptian Iris germanica L. rhizomes. Molecules 17, 2587-2598.), ceramides (Mohamed et al., 2013Mohamed, G.A., Ibrahim, S.R.M., Ross, S.A., 2013. New ceramides and isoflavone from the Egyptian Iris germanica L. rhizomes. Phytochem. Lett. 6, 340-344.), and benzoquinones (Asghar et al., 2010Asghar, S.F., Rehman, H.U., Rahman, A.U., 2010. Phytochemical investigation of Iris germanica. Nat. Prod. Res. 24, 131-139.). As part of an ongoing search for isolation of antioxidants and α-amylases inhibitors from plant source, further investigation of the rhizomes of I. germanica identified a new isoflavonoid glycoside: iridin A (9) and eight known compounds (1–8). This study gives an account on the structural assignment of the isolated flavonoids using different spectroscopic analyses. Furthermore, the antioxidant and antidiabetic potentials of the isolated compounds were estimated using DPPH and α-amylase inhibitory tests, respectively.
Materials and methods
General experimental procedures
Electrothermal 9100 Digital Melting Point apparatus was used for measuring the melting points (Electrothermal Engineering Ltd., Essex, England). Ultraviolet spectra were assessed by a Hitachi 300 spectrophotometer. A LCQ DECA mass spectrometer was utilized to get the ESIMS (ThermoFinnigan, Bremen, Germany). HRESIMS was achieved using a LTQ Orbitrap (ThermoFinnigan, Bremen, Germany). Bruker Avance DRX 500 MHz spectrometer was used for measuring the NMR spectra. Compounds isolation was achieved using silica gel (0.063–0.200 mm), sephadex LH-20 (0.25–0.1 mm), and RP18 (0.04–0.063 mm) (Merck, Darmstadt, Germany). Silica gel 60 F254 pre-coated plates (0.2 mm, Merck, Darmstadt, Germany) were utilized for thin-layer chromatography. Six ml standard LiChrolut extraction tube (RP18, 40–63 µm, Merck, Darmstadt, Germany) was used for compounds purification. The spots on TLC were visualized by UV at λmax 255 and 366 nm, then exposed to NH3 vapor after that spraying with p-anisaldehyde:H2SO4 reagent and heating at 110 ºC for 1–2 min. All chemicals materials were secured by Sigma Chemical Aldrich (St. Louis, MO, USA).
Plant material
The rhizomes of Iris × germanica L., Iridaceae, were collected in May 2014 from the botanical garden at the Faculty of Agriculture, Assiut University. The plant was kindly authenticated by A. A. Fayed, Professor of Plant Taxonomy, Faculty of Science, Assiut University. A specimen under the registration number IG-5-2014 was kept in the Department of Natural Products herbarium, Faculty of Pharmacy, King Abdulaziz University, Saudi Arabia.
Extraction and isolation
The powdered rhizomes (270 g) were extracted with MeOH (4 × 2.5 l). The MeOH extract was evaporated to yield a brown residue (12.9 g). The later was applied on a vacuum liquid chromatography (VLC) using hexane, CHCl3, and EtOAc to afford hexane (IG-1, 2.9 g), CHCl3 (IG-2, 2.4 g), EtOAc (IG-3, 1.1 g), and aqueous (IG-4, 4.8 g) fractions. Fraction IG-2 (2.4 g) was subjected to VLC using hexane:EtOAc as eluent to get seven subfractions: IG-2A to IG-2G. Subfraction IG-2D (240 mg) was separated on SiO2 CC (40 g, 50 × 3 cm) using CHCl3:MeOH (98:2–90:10) to get compounds 1 (18 mg) and 2 (11.5 mg). Subfraction IG-2E (410 mg) was subjected to SiO2 CC (70 g, 50 × 3 cm) eluting with CHCl3:MeOH (94:6–90:10) to yield 3 (12 mg) and 4 (14 mg). SiO2 CC of subfraction IG-2F (218 mg) (70 g, 50 × 2 cm) using CHCl3:MeOH (93:7–85:15) as an eluent gave compound 5 (5.2 mg). Sephadex LH-20 CC (150 g, 100 × 3 cm) of IG-3 (1.1 g) using MeOH:CHCl3 (9:1) afforded five subfractions: IG-3A to IG-3E. RP-18 CC of subfraction IG-3A (118 mg) (100 g, 50 × 2 cm) eluting with H2O:MeOH gradient gave 7 (17 mg) and 9 (6.9 mg). Subfraction IG-3B (130 mg) was applied on SiO2 CC (20 g, 50 × 2 cm) using a CHCl3:MeOH (90:10–80:20) to give 8 (14.1 mg). SiO2 CC of sub-fraction IG-3D (115.4 mg) (50 g, 50 × 2 cm), eluting with CHCl3:MeOH gave impure 6, which was purified on LiChrolut EN/RP-18 solid phase extraction tube using H2O:acetonitrile gradient to yield 6 (6.7 mg).
Spectral data
Iridin A (9): Yellow amorphous powder (6.9 mg); mp 211–213 ºC (dec.); UV (MeOH) λmax (log ɛ): 207, 267, 330 nm; IR (KBr) γmax: 3397, 2956, 1694, 1605, 1509 cm-1; NMR data (DMSO-d6, 500 and 125 MHz) see Table 1; HRESIMS m/z 539.1391 (calcd for C24H27O14, [M+H]+ 539.1395).
α-Amylase inhibitory activity
The assay was carried out using α-amylase by EnzCheck® Ultra Amylase Assay Kit (E33651) as previously outlined (Mohamed, 2008Mohamed, G.A., 2008. Alliuocide G, a new flavonoid with potent α-amylase inhibitory activity from Allium cepa L.. ARKIVOC Xi, 202-209.; Sayed et al., 2008Sayed, H.M., Mohamed, M.H., Farag, S.F., Gamal, M.A., Omobuwajo, O.R., Proksch, P., 2008. Fructose-amino acid conjugate and other constituents from Cyperus rotundus L.. Nat. Prod. Res. 22, 1487-1497.; Ibrahim et al., 2015Ibrahim, S.R.M., Mohamed, G.A., Abdel-Latif, M.M.M., El-Messery, S.M., Al-Musayeib, N.M., Shehata, I.A., 2015. Minutaside A, new α-amylase inhibitor flavonol glucoside from Tagetes minuta: antidiabetic, antioxidant, and molecular modeling studies. Starch/Stärke 67, 976-984.).
DPPH assay
The free radical scavenging capacities (FRS) of compounds 1–9 (Conc. 5, 10, 20, 40, 80, and 100 µM in HPLC MeOH) was evaluated using DPPH assay as outlined previously (Mohamed, 2014Mohamed, G.A., 2014. New cytotoxic cycloartane triterpene from Cassia italica aerial parts. Nat. Prod. Res. 28, 976-983., 2016Mohamed, G.A., 2016. Tagenols A and B: new lipoxygenase inhibitor flavonols from Tagetes minuta. Phytochem. Lett. 16, 141-145.; Mohamed et al., 2014Mohamed, G.A., Ibrahim, S.R.M., Al-Musayeib, N.M., Ross, S.A., 2014. New anti-inflammatory flavonoids from Cadaba glandulosa Forssk.. Arch. Pharm. Res. 37, 459-466.).
Results and discussion
The MeOH extract of I. germanica rhizomes was fractionated on VLC using n-hexane, CHCl3, and EtOAc. The CHCl3 and EtOAc fractions were subjected to sephadex LH-20, silica gel, and RP-18 CC to afford one new (9) and eight known compounds (1–8).
Compound 9 was isolated as yellow amorphous powder and gave positive flavonoids tests (Mabry et al., 1970Mabry, T.J., Markham, K.R., Thomas, M.B., 1970. The Systematic Identification of Flavonoids. Springer, New York, NY.). It showed a pseudomolecular ion peak at m/z 539.1391 [M+H]+ (calcd for C24H27O14, 539.1395) in HRESIMS spectrum, compatible with the molecular formula C24H26O14. The ESIMS spectrum showed a significant fragment ion peak at m/z 377 [M+H-Glu]+. Its UV absorption bands at 207, 267, and 330 nm characterized the isoflavone nature of 9 (Mabry et al., 1970Mabry, T.J., Markham, K.R., Thomas, M.B., 1970. The Systematic Identification of Flavonoids. Springer, New York, NY.; Harborne, 1994Harborne, J.B., 1994. The Flavonoids: Advances in Research Since 1986. Chapman and Hall, London.). The IR spectrum showed absorption bands for hydroxyl, α,β-unsaturated carbonyl, aromatic functionalities at 3397, 1694, and 1605 and 1509 cm-1, respectively (Silverstein and Webster, 1998Silverstein, R.M., Webster, F.X., 1998. Spectrometric Identification of Organic Compounds, 6th ed. John Wiley, New York, NY.). The 13C, DEPT, and HSQC NMR spectra revealed twenty four carbon signals. The 1H NMR spectrum of 9 showed four singlet signals at δH 8.49, 9.30, 9.21, and 12.95, which were assigned to H-2 of isoflavone, 8-OH, 3'-OH, and 5-chelated OH group (Harborne, 1988Harborne, J.B., 1988. The Flavonoids. Advances in Research Since 1980. Chapman & Hall, New York.; El-Shanawany et al., 2013El-Shanawany, M.A., Sayed, H.M., Ibrahim, S.R.M., Fayed, M.A.A., Radwan, M.M., Ross, S.A., 2013. A new isoflavone from Blepharis ciliaris of an Egyptian origin. Med. Chem. Res. 22, 2346-2350.). The two meta-coupled aromatic protons δH 6.89 (d, J = 1.5 Hz, H-2') and 6.74 (d, J = 1.5 Hz, H-6') indicated the presence of a tetra-substituted ring B (Table 1). They correlated to the carbons, resonating at δC 110.8 and 104.7, respectively in the HSQC spectrum. Moreover, three methoxy group signals at δH 3.82 (6-OCH3)/δC 58.7 (6-OCH3), 3.72 (4'-OCH3)/60.1 (4'-OCH3), and 3.84 (5'-OCH3)/55.9 (5'-OCH3) were observed. Their attachment at C-6, C-4', and C-5' was confirmed by observed HMBC cross peaks of 6-OCH3 to C-6 (δC 135.1), 4'-OCH3 to C-4' (δC 136.9), and 5'-OCH3 to C-5' (δC 153.8) (Fig. 1). Furthermore, the 1H NMR spectrum displayed anomeric proton signal at δH 5.08 (d, J = 7.9 Hz), indicating the presence of β-configured glucose moiety (Mohamed et al., 2013Mohamed, G.A., Ibrahim, S.R.M., Ross, S.A., 2013. New ceramides and isoflavone from the Egyptian Iris germanica L. rhizomes. Phytochem. Lett. 6, 340-344., 2015Mohamed, G.A., Ibrahim, S.R.M., Elkhayat, E.S., Ross, S.A., Sayed, H.M., El-Moghazy, S.A.M., El-Shanawany, M.A., 2015. Blepharisides A and B, new flavonol glycosides from Blepharis ciliaris growing in Saudi Arabia. Phytochem. Lett. 11, 177-182.). Its placement at C-7 was secured by the HMBC correlation of H-1″ to C-7 (δC 150.2). Thus, 9 was assigned as 5,8,3'-trihydroxy-6,4',5'-trimethoxy-7-O-β-D-glucopyranosyl isoflavone and named iridin A.
The known compounds were identified as irilone 4'-methyl ether (1) (Bilia et al., 1993Bilia, A.R., Flammini, F., Flamini, G., Morelli, I., Marsili, A., 1993. Flavonoids and a saponin from Spartium junceum. Phytochemistry 34, 847-852.), irilone (2) (Eckhard et al., 2003Eckhard, W., Frederik, S.J., Karin, K., Jutta, K., Norbert, F., Clarissa, G., 2003. Cancer chemoprotective in vitro activities of isoflavones isolated from Iris germanica. Planta Med. 69, 15-20.), irisolidone (3) (Kang et al., 2008Kang, K.A., Zhang, R., Piao, M.J., Ko, D.O., Wang, Z.H., Kim, B.J., Park, J.W., Kim, H.S., Kime, D.H., Hyun, J.W., 2008. Protective effect of irisolidone, a metabolite of kakkalide, against hydrogen peroxide induced cell damage via antioxidant effect. Bioorg. Med. Chem. 16, 1133-1141.), irigenin S (4) (Ibrahim et al., 2012Ibrahim, S.R.M., Mohamed, G.A., Al-Musayeib, N.M., 2012. New constituents from the Egyptian Iris germanica L. rhizomes. Molecules 17, 2587-2598.), irigenin (5) (Akashi et al., 2005Akashi, T., Ishizaki, M., Aoki, T., Ayabe, S., 2005. Isoflavonoid production by adventitious-root culture of Iris germanica (Iridaceace). Plant Biotechnol. 22, 207-215.), irilone 4'-O-β-D-glucopyranoside (6) (Mohamed et al., 2014Mohamed, G.A., Ibrahim, S.R.M., Al-Musayeib, N.M., Ross, S.A., 2014. New anti-inflammatory flavonoids from Cadaba glandulosa Forssk.. Arch. Pharm. Res. 37, 459-466.), iridin S (7) (Mohamed et al., 2013Mohamed, G.A., Ibrahim, S.R.M., Ross, S.A., 2013. New ceramides and isoflavone from the Egyptian Iris germanica L. rhizomes. Phytochem. Lett. 6, 340-344.), and iridin (8) (Arisawa and Morita, 1976Arisawa, M., Morita, N., 1976. Studies on constituents of genus Iris. VII. The constituents of Iris unguicularis Poir. (1). Chem. Pharm. Bull. 24, 815-817.) by inspection of the spectroscopic data and comparing of these data with literature as well as co-TLC with authentic samples.
Isoflavonids are phenols with low molecular weight, which can give hydrogen atoms from their phenolic hydroxyl group(s) to the deleterious oxy radicals, forming the less-reactive phenoxyl radicals. Also, they have ability to chelate metal ions and exhibit potential antioxidant activity (Tikkanen et al., 1998Tikkanen, M.J., Wahala, K., Ojala, S., Vihma, V., Adlercreutz, H., 1998. Effect of soybean phytoestrogen intake on low density lipoprotein oxidation resistance. Proc. Natl. Acad. Sci. U. S. A. 95, 3106-3110.). The antioxidant activity of 1–9 was assessed using DPPH assay. This assay is a widely used process for estimating the antioxidants capacity to scavenge the stable free radical produced from DPPH, which results changing in color from purple to yellow upon gaining a hydrogen radical (H•) to form the stable DPPH-H molecule (Lim et al., 2007Lim, Y.Y., Lim, T.T., Tee, J.J., 2007. Antioxidant properties of several tropical fruits: a comparative study. Food Chem. 103, 1003-1008.). It is noteworthy that the best activity was exhibited by compound 9 (IC50 8.91 µM) compared to propyl gallate (IC50 6.72 µM). Other compounds showed moderate activities with IC50 ranging from 11.74 to 17.03 µM (Table 2). Additionally, the results of the α-amylase inhibitory potential of the tested compounds revealed that 3, 5, and 9 displayed the highest activity with % inhibition 70.8, 67.5, and 70.5, respectively compared to acarbose (100%, reference α-amylase inhibitor) (Table 2). The rest of compounds exhibited moderate to weak activities.
Li et al. (2007)Li, M., Wei, D., Dingn, W., Baruah, B., Crans, C.C., 2007. Antidiabetic effects of Cesium aqua (N,N'-ethylene (salicylideneiminato)-5-sulfonato) oxovanadium (IV) dihydrate in streptozotocin-induced diabetic rats. Biol. Trace Elem. Res. 121, 226-232. stated that the isoflavonoids antioxidant effect increased cell membranes stability and protected them from damage which participates in increasing insulin sensitivity and inhibited free radical generation metabolic disorders in diabetes. Also, Mosihuzzman et al. (2013)Mosihuzzman, M., Naheed, S., Hareem, S., Talib, S., Abbas, G., Khan, S.N., Choudhary, M.I., Sener, B., Tareen, R.B., Israr, M., 2013. Studies on α-glucosidase inhibition and anti-glycation potential of Iris loczyi and Iris unguicularis. Life Sci. 92, 187-192. reported that the isoflavonoids of I. loczyi and I. unguicularis inhibit α-glucosidase and might have considerable potency to treat diabetes and its late complications (Mosihuzzman et al., 2013Mosihuzzman, M., Naheed, S., Hareem, S., Talib, S., Abbas, G., Khan, S.N., Choudhary, M.I., Sener, B., Tareen, R.B., Israr, M., 2013. Studies on α-glucosidase inhibition and anti-glycation potential of Iris loczyi and Iris unguicularis. Life Sci. 92, 187-192.). Previous study by Prieto-Hontoria et al. (2009)Prieto-Hontoria, P.L., Pérez-Matute, P., Fernández-Galilea, M., Barber, A., Martínez, J.A., Moreno-Aliaga, M.J., 2009. Lipoic acid prevents body weight gain induced by a high fat diet in rats: effects on intestinal sugar transport. J. Physiol. Biochem. 65, 43-50. indicated that isoflavonoids possessed hypoglycemic effect by inhibiting the intestinal α-amylase activity and glucose absorption rate. Moreover, they activated hepatic glycogen synthesis and accelerated glucose utilization in peripheral tissues (Hamden et al., 2011Hamden, K., Jaouadi, B., Carreau, S., Aouidet, A., Elfeki, A., 2011. Therapeutic effects of soy isoflavones on a-amylase activity, insulin deficiency, liver-kidney function and metabolic disorders in diabetic rats. Nat. Prod. Res. 25, 244-255.). At conclusion, the isolated isoflavonoids from I. germanica showed a capacity as inhibitors of DPPH and α-amylase. The results clearly showed the effects of the tested isoflavonoids on suppression of the α-amylase enzyme, which is responsible for the high glucose level in the blood. Also, they may reduce ROS production in T2D. Hence, the present work proposes fundamentals for the probable use of I. germanica as functional food. Consequently, an in vivo anti-diabetic research should be undertaken to recognize the other reasonable mechanism of action to control the hyperglycemia.
Structure–activity relationship
Comparing the antioxidant and α-amylase inhibitory capacities of these groups of isoflavonoids, we were able to postulate the structure activity relationship of these compounds, which is in congruence with other reported researches (Arora et al., 1998Arora, A., Nair, M.G., Strasburg, G.M., 1998. Antioxidant activities of isoflavones and their biological metabolites in a liposomal system. Arch. Biochem. Biophys. 356, 133-141.; Xiao et al., 2011Xiao, J., Kai, G., Ni, X., Yang, F., Chen, X., 2011. Interaction of natural polyphenols with α-amylase in vitro: molecular property-affinity relationship aspect. Mol. Biosyst. 7, 1883-1890.). The position and number of hydroxyls were found to be a significant determinant of the antioxidant efficacy. The C-4' hydroxyl is highly significant toward the antioxidant potentials and its substitution by methoxy and/or its loss reduces the abilities of these compounds. The introduction of a hydroxyl group at C-8 significantly increases the activity. The loss of the C-5 hydroxyl group diminishes the activity of isoflavonoids. Blocking the C-7 hydroxyl by glucose had no influence on the antioxidant activities, that way indicating that the existence of a C-7 hydroxyl has insignificant effect on the activities of these compounds. For α-amylase inhibitory activity, hydroxylation at C-5 increased the inhibitory activities for α-amylase. The glycosylation of isoflavonoids lowered their activities. Increase number of hydroxyls in rings A and B increases activity.
Conclusion
One new isoflavonoid glycoside, iridin A (9) and eight known isoflavonoids were separated from I. germanica rhizomes. Their structural elucidation was achieved with the aid of extensive spectroscopic techniques. Compounds 3, 5, and 9 showed α-amylase inhibitory activities. Moreover, 9 exhibited moderate antioxidant activity.
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Ethical disclosuresProtection of human and animal subjects. The authors declare that no experiments were performed on humans or animals for this study.Confidentiality of data. The authors declare that no patient data appear in this article.Right to privacy and informed consent. The authors declare that no patient data appear in this article.
References
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Publication Dates
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Publication in this collection
Mar-Apr 2017
History
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Received
25 May 2016 -
Accepted
20 Oct 2016