Purification, Composition, and Anti-Inflammatory Activity of Polyphenols from Sweet Potato Stems and Leaves

Sweet potato stems and leaves (SPSL) are rich in bioactive polyphenols, yet their utilization remains underexplored. This study established an efficient method for SPSL polyphenol enrichment using macroporous resins, with UHPLC-QE-MS/MS characterization of the purified polyphenols (PP) and subsequent evaluation of anti-inflammatory activity. The results showed that NKA-II resin demonstrated the best purification effect on SPSL polyphenols among the six tested resins. The optimal enrichment procedure of NKA-II resin was as follows: loading sample pH 3.0, 4.48 mg CAE/mL concentration, and 80% ethanol (v/v) eluent. A total of 19 major compounds were characterized in PP, including 12 phenolic acids and seven flavonoids, with a polyphenol purity of 75.70%. PP pretreatment (100 and 500 μg/mL) significantly inhibited LPS-induced release of NO (by 40.62% and 68.61%), IL-1β (by 40.07% and 68.34%), IL-6 (by 40.63% and 52.41%), and TNF-α (by 52.29% and 73.76%) compared to the LPS group (p < 0.05), demonstrating potent anti-inflammatory effects. Western blot analysis revealed that PP exerted anti-inflammatory effects by inhibiting the NF-κB (via suppression of IκBα phosphorylation/degradation and blockade of p65 nuclear translocation) and MAPK (via inhibition of p38, ERK, and JNK phosphorylation) signaling pathways. These findings support the utilization of this agricultural by-product in functional food development, particularly as a source of natural anti-inflammatory compounds for dietary supplements or fortified beverages.