Ultrasound-Assisted Acidic Natural Deep Eutectic Solvent as a New Strategy for Extracting Seabuckthorn Leaf Phenolics: Process Optimization, Compositional Identification, and Metabolic Enzyme Inhibition Capacity

An efficient acidic natural deep eutectic solvent (choline chloride-acetic acid, Chcl-AA) was screened out and subsequently combined with ultrasound treatment to recover seabuckthorn leaf phenolics (SLP), and the Plackett–Burman and Box-Behnken designs were used to optimize the process parameters. The optimal parameters were water content of NADES 30%, solvent to solid ratio 32 mL/g, extraction time 14 min, extraction temperature 40°C, ultrasonic power 400 W, and ultrasonic duty cycle 75%. The values of total phenolic content (TPC) and total flavonoid content (TFC) under these conditions were 99.65 ± 2.49 milligram gallic acid equivalent per gram of dry weight (mg GAE/g DW) and 52.02 ± 1.21 milligram rutin equivalent per gram of dry weight (mg RE/g DW), respectively. Twenty-five phenolics were initially identified from SLP by ultra-performance liquid chromatography coupled with quadrupole-time of flight mass spectrometry (UPLC-Q-TOF/MS), among which hydrolyzable tannins, isorhamnetin derivatives, and quercetin derivatives are the main constituents. The SLP extract was further evaluated as to its ability to inhibit metabolic enzymes, i.e., α-glucosidase, α-amylase, pancreatic lipase, cholesterol esterase, xanthine oxidase, and angiotensin-converting enzyme. SLP exhibited significant inhibition of α-glucosidase (IC₅₀ = 3.31 μg/mL), pancreatic lipase (IC₅₀ = 57.62 μg/mL), and angiotensin-converting enzyme (IC₅₀ = 63.13 μg/mL). Additionally, the compounds extracted using Chcl-AA exhibited superior bioactivity when compared to those using conventional green solvents, i.e., ethanol and water. These results provide a valuable technical buttress for the efficient extraction of SLP and their further application and research in functional foods.