α-Glucosidase inhibitory activities of constituents from Psidium guajava leaves

Abstract Psidium guajava is a plant of the Myrtaceae with various pharmacological activity. In this study, the water extract and the isolated compounds from guava leaves were evaluated for in vitro α-glucosidase inhibition using spectrophotometric method. Ellagic acid, quercetin, quercetin-3-O-glucuronide, avicularin, isoquercitrin, and quercetin-3-galactoside showed α-glucosidase inhibitory activity, and their IC50 value were 25.0, 41.0, 53.5, 46.9, 60.0 and 72.1 μg/mL, respectively compared with the positive control acarbose (IC50 49.2 μg/mL). This study could provide a theoretical basis for the application of Psidium guajava in the treatment of hyperglycemia. Graphical Abstract


Introduction
α-Glucosidase is a key enzyme in the digestion of carbohydrates and the main enzyme causing postprandial blood glucose elevation (Natsir et al. 2018).The digestion and absorption of glucose can be delayed by α-glucosidase inhibitors to reduce postprandial blood glucose (Hossain et al. 2020).Typically used α-glucosidase inhibitors, such as acarbose and miglitol, cause various side effects such as anorexia, abdominal distension and diarrhea (Grunberger 2017;Lembcke et al. 1990;Ozgen et al. 1998).
The development for new hypoglycemic α-glucosidase inhibitors from natural products has become a hot topic for researchers.
Psidium guajava is a plant belonging to the Myrtaceae.Guava trees have been grown by many countries having tropical and subtropical climates (díaz-de-Cerio et al. 2017).Various of bioactive compounds are contained in guava leaves such as polyphenol, triterpenes, flavonoids, and sesquiterpenes (Gutiérrez et al. 2008;Kumar et al. 2021;Ouyang et al. 2016;Zhou et al. 2023).Previous reports have shown that guava leaves possess hypoglycemic, antibacterial, antifungal and larvicidal activities (Ashraf et al. 2016;Bezerra et al. 2018;Chaturvedi et al. 2021;Mendes et al. 2023;Zhang et al. 2020).Although guava leaves tea was used for the treatment of diabetes in folk, the research of α-glucosidase inhibition of the water extract and the isolated substances from guava leaves was insufficient.In this study, potential α-glucosidase inhibitors were identified from guava leaves.Several flavonoids and polyphenolic compounds from guava leaves extract were identified and the α-glucosidase inhibitory effect of the extract and its active compounds were determined.

Results and discussion
Many plants used for the management of diabetes in traditional medicine showed α-glucosidase inhibition.Guava fruit has some inhibitory activity on α-glucosidase by in vitro method (Nakia et al. 2021).Guava leaves tea was used for the treatment of diabetes in folk, therefore, in this study the inhibitory activity of guava leaves water extract (GLWE) on α-glucosidase was determined to evaluate antidiabetic activity of guava leaves.The research showed that GLWE possessed α-glucosidase inhibitory activity in a concentration-dependent manner.This assay was carried out at different concentrations ranging from 20.0 to 320.0 μg/mL.GLWE showed more potent α-glucosidase inhibitory activity with IC 50 of 23.0 μg/mL than that of the positive control acarbose (IC 50 ,49.2μg/ mL).Therefore, we suggest that the inhibition activity against α-glucosidase may be a possible mechanism for guava leaves to exert antidiabetic activity.
The α-glucosidase inhibitory activity of eight compounds from GLWE was evaluated and acarbose was used as positive control.This assay was undertaken at the concentration ranging from 20.0 to 320.0 μg/mL.The result showed that compounds 1-4, 7 and 8 possess strong inhibitory activity on a-glucosidase with IC 50 values ranging from 25.0 to 72.1 μg/mL, compared with that of acarbose (49.2μg/mL).There have been previous reports that some members of flavonoids inhibited α-glucosidase by high-throughput screening platform and glycosylation of hyroxyl groups on flavonoids might decrease the inhibitory effect (Lu et al. 2018).Our result has evidently demonstrated that compounds 1, 7 and 8 displayed significant inhibitory activity on a-glucosidase at IC 50 of 25.0 μg/mL, 46.8 μg/mL and 41.0 μg/mL, respectively.Therefore, these compounds were parts of the inhibitory activity of GLWE and there might be a synergistic or additive effect.The compounds 1, 7 and 8 might be used as bioactive marker compounds for the quality control of raw guava leaves for its future development as a natural antidiabetic agent.