Phytochemicals from Turnera subulata Exhibiting Antioxidant, Immunomodulatory, and Microbiological Activity

Turnera subulata is traditionally used to treat inflammatory and infectious conditions; however; its biological activities remain incompletely characterized. In this study, aqueous (AETS) and hydroethanolic (HETS) extracts obtained from the aerial parts (leaves, stems, and flowers), as used in traditional infusions, were compared regarding physicochemical composition, redox behavior, cytotoxicity, immunomodulatory, and antimicrobial activities. HETS showed significantly higher phenolic content (2555.96 ± 43.55 mg GAE/100 mL) compared to AETS (1269.54 ± 20.60 mg GAE/100 mL) and exhibited stronger DPPH (83.05 ± 0.05%) and ABTS (85.1 ± 1.5%) radical scavenging activity. In contrast, AETS showed greater antioxidant capacity in the TRAP assay from 50 µg/mL (p < 0.0001). Both extracts displayed dose-dependent pro-oxidant behavior in the deoxyribose/Fenton system. In vitro assays demonstrated that both extracts exhibited dose-dependent cytotoxicity in SH-SY5Y cells, with no significant cytotoxic effects observed at concentrations ≤ 50 µg/mL. HETS significantly increased IL-10 levels (p < 0.05), indicating immunomodulatory activity. In antimicrobial assays, HETS showed selective activity against Staphylococcus aureus, with MIC values ranging from 0.625 to 1.25 mg/mL, while no relevant inhibition was observed against Escherichia coli. No synergistic interaction with vancomycin was detected. Overall, the results indicate that the extraction solvent strongly influences the phenolic enrichment and biological activity. The hydroethanol extract showed the most consistent bioactivity, highlighting its potential for applications as a natural antioxidant, immunomodulatory, and anti-staphylococcal agent. Future studies should focus on compound isolation, mechanistic validation, and evaluation in in vivo models to support potential commercial and therapeutic applications.