The Research Center for Plants and Human Health, Institute of Urban Agriculture, Chinese Academy of Agricultural Sciences, identified the composition of polyphenols in pomegranate peels by LC-MS/MS method, established a new extraction and purification process of pomegranate peel-derived punicalagin, and demonstrated its α-glucosidase inhibitory mechanism by enzyme inhibition kinetics and molecular docking. This research has been published in Food chemistry, and provides a new understanding of the mechanism of α-glucosidase inhibition of pomegranate punicalagin, contributing to the application of punicalagin as α-glucosidase inhibitors in the prevention and treatment of diabetes mellitus.

Controlling the dietary carbohydrates is one of effective strategies in the management of type 2 diabetes. Of all available drugs, α-glucosidase inhibitors appear to be one of the most effective drugs in hindering the digestion of carbohydrates. So far, many natural polyphenols have been found with α-glucosidase inhibitory activities without causing adverse effects. The pomegranate peel, a major processing by-product of pomegranate fruits, is an inexpensive and abundant source of polyphenols. Similar to other polyphenols, punicalagin from the pomegranate peel exhibits plenty of bioactivities. In recent years, the extraction, purification, and bioactivities of punicalagin have attracted a lot of attention. However, there are few mature technologies available to obtain punicalagin from the pomegranate peel. It is speculated that ultrasonic-assisted extraction (UAE) combining with macroporous resins and prep-HPLC can be effective to extract and purify punicalagin from the pomegranate peel, and the α-glucosidase inhibitory mechanism of punicalagin may be associated with the direct binding between punicalagin and α-glucosidase.

To test this hypothesis, the main compositions of polyphenols in pomegranate peels were identified by LC-MS/MS, among which the content of punicalagin was the highest. Then a new UAE process of punicalagin was optimized, and the yield of punicalagin reached 505.89 ± 1.73 mg/g DW. Punicalagin was further purified by D101macroporous resin and preparative HPLC successively, and its purity was 92.15%. Finally, the inhibitory activity of purified punicalagin on α-glucosidase was evaluated by enzyme inhibition kinetics and molecular docking, and its inhibitory type and mechanism were further discussed. The inhibitory activity of the purified punicalagin against α-glucosidase (IC50 = 82 ± 0.02 μg/mL) was similar to that of the punicalagin standard (IC50 = 58 ± 0.014 μg/mL), both exhibiting a mixed inhibitory mechanism. Molecular docking results showed that a steric hindrance was formed between punicalagin and α-glucosidase with an intermolecular energy of −7.99 kcal/mol. The results of this study provide scientific basis for the development of punicalagin as a potential inhibitor of α-glucosidase, and help accelerate the wide development and application of pomegranate peel-derived punicalagin.

The research was supported by the Local Financial Funds of National Agricultural Science and Technology Center, Chengdu (No. NASC2020KR02) and the Central Public-interest Scientific Institution Basal Research Fund (No. Y2020XK05).

Fig. 1. the chromatogram of polyphenols detected in pomegranate peel powder

Fig. 2. the molecular docking result between punicalagin and α-glucosidase

By Gan Renyou (ganrenyou@caas.cn)