Organ-Specific LC–MS/MS Phenolic Profiling and Multifunctional Antioxidant and Enzyme Inhibitory Activities of Onosma sintenisii

<i>Onosma sintenisii</i> Hausskn. ex Bornm. is an endemic species of Türkiye whose phytochemical composition and biological activities remain insufficiently characterized at the organ level. The present study aimed to investigate organ-specific phenolic profiles and associated antioxidant and enzyme inhibitory activities of <i>O. sintenisii</i>. Ultrasonic-assisted methanolic extracts obtained from flowers, leaves, stems, and roots were analyzed using validated LC-ESI-MS/MS, and their biological potential was evaluated through multiple in vitro antioxidant assays (DPPH, ABTS, CUPRAC, FRAP, phosphomolybdenum, and metal chelation) as well as enzyme inhibition tests against acetylcholinesterase, butyrylcholinesterase, tyrosinase, α-amylase, and α-glucosidase. The results revealed pronounced organ-dependent variation in both phenolic composition and bioactivity. Rosmarinic acid was identified as the major phenolic compound in all organs, with the highest concentration detected in root extracts, which also exhibited the strongest antioxidant capacity and the most potent α-glucosidase inhibition. Flavonoid glycosides were predominantly accumulated in aerial parts. Correlation analysis demonstrated that hydroxycinnamic acids, particularly rosmarinic acid, are the main contributors to antioxidant and enzyme inhibitory effects. These findings indicate that <i>O. sintenisii</i>, especially its roots, represents a promising natural source of multifunctional phenolic compounds with potential pharmaceutical and nutraceutical applications.