Organ-Specific Phenolic Profiles, Antioxidant Capacity, and Enzyme-Inhibitory Activities of the Turkish Endemic Origanum bilgeri

Background/Objectives: Origanum bilgeri P.H. Davis is a Turkish endemic Lamiaceae species whose non-volatile phenolic composition and organ-specific bioactivities remain unexplored. This study investigated the phenolic composition of flower, leaf, stem, and root extracts and evaluated their antioxidant and enzyme-inhibitory activities. Methods: Ultrasound-assisted ethanolic extracts of the four organs were characterized by liquid chromatography–tandem mass spectrometry (LC–MS/MS) targeting 28 phenolic compounds. Total phenolic and flavonoid contents, outcomes of six antioxidant assays, and inhibitory activities against acetylcholinesterase, butyrylcholinesterase, tyrosinase, α-amylase, and α-glucosidase were determined. Measurements were performed in triplicate as technical replicates, and organ-level differences were interpreted descriptively. The relative antioxidant capacity index (RACI) was calculated from the standardized outcomes of the six direct antioxidant assays, with total phenolic content (TPC) and total flavonoid content (TFC) excluded from the index. Pearson correlation analysis and principal component analysis (PCA) were used for exploratory evaluation of phytochemical and bioactivity patterns. Results: Twenty phenolic compounds were detected in at least one organ. Rosmarinic acid predominated in all organs and was most abundant in the leaf (3236 µg/g extract) and stem (3093 µg/g extract), whereas the root was markedly enriched in verbascoside (2775 µg/g extract) and chlorogenic acid (1461 µg/g extract) and exhibited the highest total phenolic content, reaching 160.87 mg gallic acid equivalents (GAE)/g extract. The assay-based RACI ranked the root highest (0.54), followed by the stem (0.02), leaf (−0.27), and flower (−0.30). The root also showed the strongest 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity (half-maximal inhibitory concentration, IC50 = 0.28 mg/mL) and cupric reducing antioxidant capacity (CUPRAC) (half-maximal effective concentration, EC50 = 0.22 mg/mL) among the investigated organ extracts. Under the present assay conditions, all organ extracts yielded lower α-glucosidase IC50 values (0.97–1.10 mg/mL) than acarbose (1.16 mg/mL). Exploratory Pearson correlation analysis indicated strong organ-level covariation among several antioxidant parameters, phenolic constituents, and α-glucosidase inhibitory activity. PCA based on the Pearson correlation matrix showed that principal components 1 and 2 (PC1 and PC2) accounted for 50.73% and 38.49% of the total variance, respectively (cumulative variance: 89.22%), providing an exploratory visualization of the multivariate organization of phytochemical and bioactivity variables. Conclusions: O. bilgeri exhibited distinct organ-specific phytochemical and bioactivity patterns under the conditions investigated. The root was particularly characterized by high verbascoside and chlorogenic acid contents, elevated total phenolic content, strong antioxidant performance across several assays, and α-glucosidase inhibitory activity. These findings support further investigation of O. bilgeri, particularly its root, using independent biological replicates and complementary phytochemical and biological approaches.