Unveiling a New Cholinesterase Inhibitor Iridoid From Verbascum uschakense (Murb.) Hub.-Mor.: In Vitro and In Silico Evaluation

Verbascum L., a member of the Scrophulariaceae family, is the second-largest genus in Turkish flora. It is represented by over 250 species, many of which have been used as folk medicine. This study aims to determine the cholinesterase inhibitory potential of secondary metabolites of Verbascum uschakense (Murb.) Hub.-Mor, an endemic species to Türkiye that has not been studied phytochemically before. Gluroside, an iridoid glucoside, was isolated from V. uschakense through acetylcholinesterase (AChE) inhibitory activity-guided fractionation alongside four other iridoid glucosides: ajugol, harpagide, aucubin, and catalpol. Additionally, three phenylethanoid glycosides—verbascoside, martinoside, and forsythoside B—were isolated from the antioxidant fractions evaluated using DPPH radical scavenging activity. Gluroside emerged as the most active compound against butyrylcholinesterase (BChE) with an IC₅₀ of 2.5 ± 0.02 μg/mL, exhibiting selectivity over AChE (37.69% inhibition at 200 μg/mL). Molecular modeling predicted strong electrostatic interactions between the glucosides and the catalytic residues of BChE. This is the first report of the isolation of gluroside from a Verbascum species and its cholinesterase inhibitory activity, underpinning the importance of V. uschakense and its secondary metabolites as a new class of cholinesterase inhibitors.