https://www.mdpi.com...3049/27/17/5405
"Conclusions
In this study, we demonstrated that nattokinase, a serine protease, degrades the S protein of SARS-CoV-2. To investigate whether nattokinase contained in natto extract could inhibit SARS-CoV-2 infection, we analyzed S protein degradation by mixing the S protein expression cell lysate and nattokinase in a dose- and time-dependent manner. The RBD of the S protein binds to the membrane-distal portion of the ACE2 protein. Natto extract has been reported to inhibit SARS-CoV-2 infection in Vero E6 cells via RBD degradation [16]. We demonstrated that S protein degradation by nattokinase was blocked by heat or protein-inhibitor treatments. Our data suggest that the protease activity of nattokinase plays a crucial role in S protein degradation. Taken together, these findings support the notion that the inhibition of SARS-CoV-2 infection by natto extract was due to S protein degradation by nattokinase. Thus, our data indicated that nattokinase and natto extracts have potential effects on the inhibition of SARS-CoV-2 host cell entry via S protein degradation."