Endopeptidase So (, E. coli cytoplasmic proteinase, proteinase So, Escherichia coli serine proteinase So) is an enzyme. This enzyme catalyses the following chemical reaction
Hydrolysis of proteins, but not Bz-Tyr-OEt, Ac-Phe-beta-naphthylester, or Bz-Arg-OEt