Preparation, structural analysis, antioxidant and digestive enzymes inhibitory activities of polysaccharides from Thymus quinquecostatus Celak. leaves

In this study, polysaccharides (DJPs) were obtained from Thymus quinquecostatus Celak. leaves, which were DJP (the crude polysaccharide extracted by hot water), DJP30, DJP50, DJP70 (three polysaccharide fractions obtained by graded precipitation with final ethanol concentrations of 30%, 50%, and 70%), and DJP70-1 (a purified polysaccharide isolated from DJP70 by DEAE-52 cellulose column). DJPs were all high-molecular-weight heteropolysaccharides, with different contents of total carbohydrate, protein, uronic acid, and phenols. They were mainly composed of arabinose, galactose, glucose, galacturonic acid, and rhamnose, and DJP70-1 also surprisingly contained a higher proportion of mannose (Man). Structural characteristics were determined using infrared spectrum (IR), methylation-gas chromatography-mass spectrometry (GC-MS), and 1D/2D nuclear magnetic resonance (NMR) analysis, with the existence of 14 kinds of glycosyl linkages of DJP70-1. Morphological features of DJP70-1 determined by Congo red and scanning electron microscopy (SEM) were also significantly different from other DJPs. Moreover, these polysaccharides exhibited strong scavenging effects on 2, 2′-diphenyl-1-picrylhydrazyl (DPPH) and 2, 2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radicals, though the relatively weak scavenging effect on hydroxyl free radicals in vitro. Furthermore, the crude polysaccharide DJP exerted a protective effect against 2, 2′-azo-bis-(2-methylpropylimid)-dihydrochloride (AAPH)-induced oxidative stress in the zebrafish model by directly scavenging excessive reactive oxygen species (ROS) and activating the kelch-like ECH-associated protein 1 (KEAP1) - nuclear factor erythroid 2-related factor 2 (NRF2)/antioxidant response element (ARE) signaling pathway in vivo. α-amylase and α-glucosidase inhibitory activities evaluation results showed that DJPs were also good potential hypoglycemic agents.