(A) Degradative effects of nattokinase in dose-dependent manner. Serial diluted nattokinase (32 µg/mL, 8 µg/mL, 2 µg/mL, 500 ng/mL, 125 ng/mL, 31.25 ng/mL, and 7.8125 ng/mL) were mixed with S protein expression cell lysate and incubated. Full length of S protein (S1 and S2 subunits) and S2 subunit were detected as upper and lower bands, respectively. Ratio of total S was indicated as the relative quantity of S protein (S protein + S2 protein). (B) Degradative effects of nattokinase in time-dependent manner. S protein expression cell lysate was incubated with 1 µg/mL nattokinase for 0, 10, 30, 60, 120, and 180 min. (C) Effects of heating treatment or protease inhibitors. Lane 1: HEK293 lysate; lane 2: HEK293 lysate (S protein); lane 3: HEK293 (S protein) + nattokinase (5 µg/mL); lane 4: HEK293 (S protein) + nattokinase (5 µg/mL) + Protease inhibitor I; lane 5: HEK293 (S protein) + nattokinase (5 µg/mL) + Protease inhibitor III; lane 6: HEK293 (S protein) + heat-treated nattokinase (5 µg/mL). (D) Degradative effect on RBD of S protein and ACE2. RBD of S protein and ACE2 coding plasmids were transfected with HEK293 cells, respectively. Cell lysates were incubated with nattokinase (7.5 µg/mL) and heat-treated nattokinase (7.5 µg/mL) and Western blotting was performed.