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    • The glucosidase inhibitory activities of

    2018-11-12

    The α-glucosidase inhibitory activities of fractions 2–6 were higher than that of bupropion hydrochloride 1, and this may have been because they consisted of more hydroxyl groups. The removal of hydroxyls from flavonoids has been shown to decrease α-glucosidase inhibitory activity [28]. In this study, compound 1 showed moderate activity as an α-glucosidase inhibitor in this study using an enzyme from S. cerevisiae. A previous study by Ivorra et al., 1988 [29], on normoglycemic and hyperglycemic rats revealed that oral treatments with β-sitosterol glucoside isolated from the aerial part of Centaurea seridis L. var. maritime Lge increased fasting plasma insulin levels. β-Sitosterol glucoside is considered to act by increasing the insulin circulation levels, which was attributable due to their aglycone, β-sitosterol. The higher α-glucosidase inhibitory activities of fractions 2–6 were also in accordance with condensed tannins extracted from the amaranth grain, finger millet, field bean, sunflower seeds, drumstick, and amaranth leaves [30]. Previous studies reported the ability of tannins to inhibit the activity of α-glucosidase. Tannins isolated from tea extracts were found to possess insulin-enhancing properties and regulate hepatic glucose output [31]. Another study revealed that tannins were effective as inhibitors of α-glucosidase activity, similar to synthetic inhibitors such as acarbose and voglibose, which are already being used in the treatment of non-insulin-dependent DM [32]. The results of the present study showed the potential of Q. phillyraeoides A. Gray as a rich source of natural antidiabetic medicine. These constituents may be employed as lead constituents for potentially new antidiabetic medicines derived from plants. The inhibitory mechanisms of the isolated constituents were analyzed further using Lineweaver–Burk plots (Fig. 4). A substrate (p-NPG) with increasingly higher concentrations was treated with an α-glucosidase enzyme with and without the isolated constituents as inhibitors. The results obtained showed that all of the isolated constituents and quercetin as the standard exhibited a mixed type of inhibition (Table 1). Mixed inhibition is a mode of enzyme inhibition in which the inhibitor binds to the enzyme even if it has already bound to the substrate. Furthermore, in mixed inhibition enzyme kinetics, inhibition occurs as a result of the inhibitor binding to a site other than the active site on the enzyme, known as an allosteric site [33]. However, further studies such as a molecular docking approach are required to confirm the interaction between the enzyme and substrate. DPPH is a relatively stable organic radical and it has been widely used to determine the antioxidant activities of natural compounds in an easy, rapid, and sensitive manner [34]. The DPPH alcohol solution is deep purple with an absorption peak at 517nm, and becomes yellow in the presence of a radical scavenger in the system and when the odd electron of nitrogen in DPPH is paired. In the DPPH scavenging assay, antioxidant activity was indicated by decreasing in absorbance as the DPPH radical received an electron or hydrogen radical from an antioxidant compound making a stable molecule [35]. Fractions 2–6 exhibited significant scavenging activities against DPPH free radicals (Table 2). Some previous studies demonstrated the potentially significant biological effects of tannins such as their antioxidant or radical scavenging activities in vitro[36] and antidiabetic properties [30,31]. The antioxidant activities of tannins have been attributed to their free radical and reactive oxygen species-scavenging properties, as well as the chelation of transition metal ions that initialize the oxidation process [37]. Previous findings by Kunyanga et al., 2011 [30] showed that the highest condensed tannin content present in the acetonic extract of various foods grown in Kenya exhibited the highest DPPH free radical scavenging activity. Another study reported that tannins extracted from plants exhibited strong DPPH scavenging activity [38,39]. Tannins are considered to be superior antioxidants because their oxidation may lead to oligomerization via phenolic coupling and increases in the number of reactive sites [40]. On the other hand, compound 1 exhibited weaker activity against DPPH radicals. The evaluation of antioxidant activity using a free-radical scavenging assay may provide information on the ability of an antioxidant to prevent radical species from attacking proteins, fatty acids, DNA, amino acids, and sugar in biological or food systems.