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As part of our search for botanical sources of SARS-CoV 3CL^pro inhibitors, we selected Torreya nucifera, which is traditionally used as a medicinal plant in Asia. The ethanol extract of T. nucifera leaves exhibited good SARS-CoV 3CL^pro inhibitory activity (62% at 100μg/mL). Following bioactivity-guided fractionation, eight diterpenoids (1–8) and four biflavonoids (9–12) were isolated and evaluated for SARS-CoV 3CL^pro inhibition using fluorescence resonance energy transfer analysis. Of these compounds, the biflavone amentoflavone (9) (IC₅₀=8.3μM) showed most potent 3CL^pro inhibitory effect. Three additional authentic flavones (apigenin, luteolin and quercetin) were tested to establish the basic structure–activity relationship of biflavones. Apigenin, luteolin, and quercetin inhibited 3CL^pro activity with IC₅₀ values of 280.8, 20.2, and 23.8μM, respectively. Values of binding energy obtained in a molecular …