After four washes, the bound protein was detected using hACE2- or hDPP4-specific goat antibody (0.5 μg/ml, R&D system) for 2 h at 37 °C, followed by incubation with HRP-conjugated anti-goat IgG antibody (1:5,000, Thermo Fisher Scientific) for 1 h at 37 °C.