Leupeptin hemisulfate

A total of 20 mM leupeptin hemisulfate (catalog number T6564; TargetMol) in deionized water was diluted to 200 μM to 0.06 μM with DMEM containing 1% FBS. Vero cells cultured overnight in 96-well plates were infected by virus at a multiplicity of infection (MOI) of 0.01 for 2 h. The medium was removed, and fresh drug-containing medium was then added to the cells. After 48 h, the cells were lysed in lysis buffer. The viral RNA in 100 μl of the cell supernatant was quantified by reverse transcription-PCR (RT-PCR). Seventy-two hours later, the changes of cytopathic effect were also observed by microscopy. Experiments were performed in triplicate. The experimental results were processed using MS Excel and GraphPad Prism 8.0.

A total of 20 mM leupeptin hemisulfate (catalog number T6564; TargetMol) in deionized water was diluted to 2 mM to 31.25 μM with 25 mM Tris buffer (pH 8.0). A 30-μl inhibitor solution with a series of concentrations in 25 mM Tris buffer (pH 8.0) was first mixed with 10 μl 100 μM peptide substrate (Dabcyl-TSAVLQ↓SGFRKMK-Edans; GenScript). Next, 10 μl of a final concentration of 200 nM M^pro was added to the plate. The relative fluorescence unit (RFU) value was measured with an excitation wavelength of 360 nm and an emission wavelength of 490 nm at 37°C for 1 h by using a SpectraMax Paradigm multimode detection platform (Molecular Devices, USA). Experiments were performed in triplicate. The enzyme activity reaction rate and inhibition rate were calculated by using MS Excel. The inhibition curve was plotted by using GraphPad Prism 8.0.