A Stilbe rubra leaf extract: phytochemical composition and effect on digestive enzyme activity in vitro

Astilbe rubra Hook. f. & Thomson, a perennial herbaceous plant of the Saxifragaceae family. The range of this species is in East Asia. In Russia, natural populations grow in the south of the Far East. A. rubra is of interest not only as an ornamental plant, but also as a medicinal, food, and honey plant. In East Asian countries, the rhizome is used as a traditional remedy for various diseases, and the leaves and stems are used for food purposes. Unlike the roots, phenolic compounds of leaves of A. rubra and their biological activity have not been sufficiently studied. Polyphenols are known to exert positive effects on human health through multiple mechanisms including mitigation of oxidative stress, protection of integrity of key macromolecules such as low-density lipoprotein cholesterol and supercoiled DNA. This bioactive group is also able to interact with proteins, which makes them have inhibitory effects on metabolic enzymes associated with type 2 diabetes (e.g., α-amylase, α-glucosidase) and obesity (e.g., pancreatic lipase). Inhibitory potential for aqueous and methanol extract of this species on &alpha-amylase and lipase has been previously shown, suggesting a complex effect on key enzymes dysfunctional in a number of human metabolic disorders associated with obesity and diabetes. This work provides the first detailed study of the composition of phenolic compounds of the leaves. In this regard, the aim of the study was to investigate the composition and content of phenolic compounds in A. rubra leaves extract, as well as their effect its effect on digestive enzymes. The studied A. rubra plants were introduced and grew on the territory of the Botanical Garden of All-Russian Scienti c Research Institute of Medicinal and Aromatic Plants (VILAR). An ultra-efficient liquid chromatographic system with a photodiode detector and a triple quadrupole mass spectrometer was used to determine the composition. The obtained MS data were analyzed using the DataAnalysis 4.0 program (Bruker Daltonik GmbH, Germany). For identification, we applied the analysis of UV and MS spectral data, comparing the obtained results with published data. The total content of phenolic compounds was determined by the Folin-Ciocalteu method, and the content of flavonoids by the method based on the complexation reaction of flavonoids with aluminum chloride. The effect of the extract on the activity of pancreatic amylase and lipase was measured by a colorimetric method. The significance of differences was analyzed using one-way analysis of variance followed by Tukey's multiple comparison test for group means. The performed analysis revealed 27 phenolic compounds were identified in the extract of leaves of A. rubra : 10 oligomers of procyanidin types A and B with a degree of polymerization of up to 4, 12 flavonoids (derivatives of quercetin, kaempferol, chrysoeriol, catechin and epicatechin) and 5 phenolic acids (isomers of caffeoylthreonic and caffeoylquinic acids, coumaroylquinic acid). The total content of phenolic compounds in the leaf extract was 276.8±9.2 mg of gallic acid equivalent/g of dry weight, and flavonoids were 46.7±6.08 mg of quercetin equivalent/g of dry weight. The studied extract did not have an inhibitory effect on pancreatic α-amylase in vitro but moderately inhibited the activity of pancreatic lipase. The obtained results expand knowledge in the field of biologically active substances of A. rubra leaves and their application, showing the potential possibility of using not only rhizomes but also leaves.