Didymin Ameliorates Dextran Sulfate Sodium (DSS)-Induced Ulcerative Colitis by Regulating Gut Microbiota and Amino Acid Metabolism in Mice

<b>Background:</b> Didymin is a dietary flavonoid derived from citrus fruits and has been shown to have extensive biological functions, especially anti-inflammatory effects, but its mechanism is unclear. The purpose of this study was to investigate the potential mechanism of didymin that alleviates ulcerative colitis. <b>Methods and Results</b>: Our results indicated that didymin could alleviate the symptoms of ulcerative colitis, as it inhibited the expressions of interleukin-6 (IL-6), interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). Didymin also promoted the expressions of claudin-1 and zona occludens-1(ZO-1), which are closely related with restoring colon barrier function. Didymin also increased the abundance of <i>Firmicutes</i> and <i>Verrucomicobiota</i>, while decreasing the abundance of <i>Bacteroidota</i> and <i>Proteobacteria</i>. Meanwhile, didymin significantly altered the levels of metabolites related to arginine synthesis and metabolism, and lysine degradation in the colitis mice. Utilizing network pharmacology and molecular docking, our results showed that the metabolites L-ornithine and saccharin could interact with signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappa-B (NF-κB). In this in vitro study, L-ornithine could reduce the expressions of transcription factors STAT3 and NF-κB, and it also inhibited the expressions of IL-6 and IL-1β in the lipopolysaccharides (LPS) induced in RAW264.7 cells, while saccharin had the opposite effect. <b>Conclusions</b>: Taken together, didymin can regulate gut microbiota and alter metabolite products, which can modulate STAT3 and NF-κB pathways and inhibit the expressions of inflammatory factors and inflammatory response in the DSS-induced colitis mice.