Diabetes mellitus (DM) is a symptom of hyperglycemia caused by insufficient insulin secretion or declining insulin sensitivity, and has become one of the most challenging health problem in the 21st century (Schmidt & Hickey, 2009). There are over 422 million people with DM all over the world by 2018, of which China has about 114 million. If comprehensive treatment is not available, DM can cause multiple complications, such as chronic kidney failure, cardiovascular diseases, nonketotic hyperosmolar coma and diabetic ketoacidosis (Campbell, 2011). As we all know, controlling blood glucose level is a key factor in treatmenting the DM. Moreover, α-amylase and α-glucosidase are typical postprandial digestive enzymes, which play important roles in the degradation of carbohydrates then causing blood glucose level elevation (Joshi et al., 2015). Acarbose is an effective α-glucosidase inhibitor in preventing postprandial hyperglycemia. However, the use of acarbose is inevitable causing some side effects (Deng, Lin-Shiau, Shyur, & Lin, 2015). Therefore, the searching for α-glucosidase and α-amylase inhibitors, derived from natural sources without or few side effects, has become a research hotspot in treatmenting the DM (Kwon, 2006). Accumulating studies have shown that plant polysaccharides, a key bioactive component of Chinese herbs, make significant anti-diabetic effects almost without side effects or adverse drug reaction (Ren et al., 2015).
Toona sinensis Roemer (T. sinensis) belongs to the Meliaceae genus of Toona. It has been used for the treatment of enteritis, emesis, vomitting, dysentery, pruritus, carminative and heliosis in Chinese folk medicine (Wang et al., 2020). In modern clinical applications, T. sinensis is mainly used for the treatment of hypoglycemic, anti-oxidation, dyslipidemia, antitumor and anti-inflammatory (Hsieh et al., 2012). T. sinensis has been subjected to phytochemical studies, in which polyphenols are the main active compounds in T. sinensis (Zhao et al., 2021). So far, most of the main reported studies are to study the biological activity of T. sinensis extract. However, there are no available reports about the extraction of total polyphenols from T. sinensis (TSTP) and their anti-diabetic activity. Consequently, it is of great importance to develop a high efficiency method for extracting polyphenols constituents from T. sinensis.
There are various methods to extract polyphenols compounds from natural products, such as soxhlet, heating, boling, refluxing, ultrasonic-assisted and maceration extraction (Feng, García-Martín, Broncano Lavado, López‐Barrera, & Álvarez‐Mateos, 2020; Nishad, Saha, Dubey, Varghese, & Kaur, 2019; Xi & Luo, 2015). Among them, the ultrasonic-assisted extraction (UAE) has gained particular attention due to it's efficient and easy to use, low solvent consumption, and saving money and time (Dai et al., 2021; Li, Chen, & Yao, 2005). UAE utilizes the energy of ultrasonic wave to promote the collision of the extracts, leading to the effective ingredients are quickly and fully dissolved in the solvent (Adjé et al., 2010). UAE has been recently reported as more efficient method for extraction of polyphenols from India Moringa oleifera L., Diospyros kaki and Lobelia nicotianifolia than conventional solvent extraction (Duan, Zhao, Zhang, Liu, & Wang, 2013; Lin, Wu, Wang, Yao, & Wang, 2021; Zimare, Mankar, & Barmukh, 2021). To the best of our knowledge, there are no data on UAE for isolation of TSTP. Therefore, in this study, ultrasonic-assisted ethanol extraction method was used to prepare polyphenols-containing extract from T. sinensis.
Response surface methodology (RSM), as an effective statistical method, is widely used for the optimization of complex process, extraction technology, and so on. Since it can depict the complete effects of variables, evaluate the interactions between multiple parameters, reduce the number of experimental trials and shorten process time. Moreover,it is more precise and effective than many approaches (Hosseinpour, Vossoughi, & Alemzadeh, 2014; Kumar, 2013).
In the present study, a three-level, three-variable (ultrasonic temperature, ultrasonic time and liquid-solid ratio) Box–Behnken design (BBD) of RSM was employed to further optimize UAE conditions for TSTP. Then, the extracted total polyphenols were purified by AB-8 macroporous resin. Finally, the α-glucosidase and α-amylase inhibitory activities of the isolated anti-diabetic total polyphenols were determined.