Structure and in Vitro Activity Characterization of Soluble Dietary Fiber from the Outer Leaves of Chinese Cabbage

The present study aimed to elucidate the structural characteristics and evaluate the in vitro antioxidant, hypoglycemic, and hypolipidemic activities of soluble dietary fiber (SDF) extracted from the outer leaves of Chinese cabbage, with the objective of utilizing Chinese cabbage waste. Crude SDF extract was prepared from the outer leaves of Chinese cabbage via ultrasound-assisted enzymatic extraction. This crude extract was subsequently purified using DEAE-52 cellulose and Sephadex G-100 chromatography columns, resulting in an isolated component designated CSF2-1, whose structure was characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, high-performance liquid chromatography and X-ray diffraction. The indices of DPPH, ABTS⁺, and hydroxyl radical scavenging abilities were assessed to represent antioxidant activity and inhibitory potencies against α-glucosidase and α-amylase were assessed to better understand hypoglycemic activity. Inhibitory potencies against cholesterol esterase and pancreatic lipase were assessed for insight into CSF2-1 hypolipidemic activity. The results indicated that CSF2-1 was an α-configured polysaccharide featuring a pyranose ring and a molecular weight of 56.59 kDa. It was primarily composed of galactose and contained minor amounts of arabinose, rhamnose, galacturonic acid, and glucose. The structure of CSF2-1 exhibited a honeycomb-like porous network with the maximum chain thickness of 6.3 nm, and contained a triple-helix configuration but lacked a crystalline structure. The half-maximal inhibitory concentrations (IC₅₀) of CSF2-1 for DPPH and hydroxyl radicals were 0.3 mg/mL and 0.23 mg/mL, respectively. The IC₅₀ values for cholesterol esterase and α-amylase were 34.78 μg/mL and 0.84 mg/mL, respectively. CSF2-1 demonstrated the strongest inhibitory effect on cholesteryl esterase and the inhibition type of CSF2-1 against cholesterol esterase was identified as a non-competitive and reversible mechanism. Soluble dietary fiber from the outer leaves of Chinese cabbage displayed good in vitro activity and could be used as a raw material for developing auxiliary hypoglycemic and lipid-lowering products.