Concentration–Response Evidence for Antioxidant, α-Amylase Inhibitory, Anti-Denaturation and Antibacterial Activities of Four Himalayan Medicinal Plants: An Endpoint-Specific Comparison

Comparative screening of medicinal plants often ranks species by a single potency value, although different assays measure different properties and half-maximal estimates depend on how they are derived. This study compares extracts of four Himalayan medicinal plants—Bergenia ligulata (underground organ), Thalictrum foliolosum (leaf), Selinum tenuifolium and Wulfenia amherstiana (underground organs)—across phenolic and flavonoid responses, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging, α-amylase inhibition, inhibition of heat-induced protein denaturation and agar-diffusion antibacterial screening, each tested at 50–250 µg/ml. Each endpoint is interpreted separately, and each reported half-maximal concentration (IC₅₀) is checked against the concentration interval in which the measured curve crosses 50%. S. tenuifolium and W. amherstiana gave the strongest DPPH responses (90.6% and 91.2% at 250 µg/ml), whereas B. ligulata did not reach 50%. All four extracts inhibited α-amylase (67–71% at 250 µg/ml) and protein denaturation (72–83%) more strongly than the reference compounds tested in the same series. Of 15 reported IC₅₀ values, 8 lay within the observed crossing interval, 4 were extrapolated beyond the tested range and 3 contradicted the measured data. Normalised phenolic and flavonoid totals rose 1.8- to 3.4-fold with assay concentration, indicating a calculation or working-range problem. The results identify preparations that merit confirmation but do not establish therapeutic efficacy, and they show that potency rankings should be reported with the evidence that supports them.