However free radicals have also been

However, free radicals have also been suggested to cause DM [11]. Although free radicals typically originate from the surrounding environment, several physiological and biochemical processes in the human body also produce reactive oxygen species, such as the superoxide radical, hydroxyl radicals, and peroxyl radicals, as by-products [12]. Therefore, the search for antidiabetic and antioxidant drug constituents from plants has been attracting increasing attention in recent years. Quercus phillyraeoides A. Gray (Q. phillyraeoides) of the family Fagaceae is an evergreen tree that is distributed in the limestone mountains and ampa bed rocks of East Asia (Korea, China and Japan). The leaves of Quercus species have been used in Korean folk medicine for dysentery, diarrhea, hemorrhage, dermatitis, and the exclusion of extravasated blood [13]. Previous phytochemical studies on Q. phillyraeoides led to the identification of several tannins from the leaves of Q. phillyraeoides[14]. However, the bioactivities of Q. phillyraeoides have not yet been examined. The objective of the present study was to identify the active constituents from the leaves of Q. phillyraeoides that are responsible for inhibitory activity against α-glucosidase and subsequently evaluate the kinetics responsible for this enzyme inhibitory activity. An in vitro assay of α-glucosidase inhibitory activity was conducted using an α-glucosidase enzyme obtained from Saccharomyces cerevisiae (S. cerevisiae) yeast. The antioxidant properties of isolated constituents were also evaluated using several assays as an assessment to alleviate oxidative stress related to DM. These assays may be used for preliminary observations in the evaluation of pharmacological activities and also to verify the medicinal effects of these active constituents isolated from plants.
Materials and methods
Results
Discussion Compound 1 was isolated as a minor component in the methanol soluble fraction using gradient n-hexane, ethyl acetate, and methanol as solvents. The further acid hydrolysis of compound 1 resulted in β-sitosterol and d-glucose, as confirmed by TLC and compared with standard samples. The resulted β-sitosterol was also further confirmed using gas chromatography. Based on these results, compound 1 was identified as β-sitosterol-d-glucoside. Fractions 2–6 were major components in the methanol soluble fraction and were identified as condensed tannins with estimated molecular weights in the range of 3400–5700g/mol. Using a thiolysis reaction, the terminal units of fractions 2–6 were released as catechin and epicatechin, which was confirmed by a GC–MS analysis. The present study was conducted in order to evaluate the antidiabetic and antioxidant potentials of constituents isolated from Q. phillyraeoides. Antidiabetic properties were assessed in terms of the ability to inhibit an intestinal carbohydrate-digesting enzyme, namely α-glucosidase. α-Glucosidase inhibitors delay the activity of this enzyme and also the formation of glucose from the hydrolysis of carbohydrates, thereby lowering the amount of glucose available for absorption in the intestine; therefore, they play a significant role as chemotherapeutic agents for non-insulin-dependent DM. Acarbose, miglitol, and voglibose are some examples of α-glucosidase inhibitors presently being used in the primary treatment of type 2 DM [25]. However, since they lack specificity in their action and cause several side effects, safer anti-hyperglycemic agents are needed. Effective and safe α-glucosidase inhibitors from nature have been sought in the development of physiological functional food or constituents for antidiabetic therapy [26]. We investigated the inhibitory activities of constituents isolated from the leaves of Q. phillyraeoides against α-glucosidase from S. cerevisiae. p-NPG was used as the substrate and the yellow color of the enzyme\'s degradation product, p-nitrophenol, was produced and measured using a spectrophotometer. Quercetin was used as a positive control based on a previous study in which phenolic compounds exhibited stronger inhibitory effects on α-glucosidase than acarbose [27].