21. Cathepsin S is expressed by some epithelial cells.22. M cells express the protease cathepsin E, similar to other antigen presenting cells. 23. Cathepsin D is an aspartic endo-protease that is ubiquitously distributed in lysosomes.24. While pH optima of many lysosomal proteases are acidic, cathepsin S is an exception. 25. In contrast, cathepsin S remains stable and has a physiological role outside the lysosome. 26. In vitro, cathepsin S retains some enzyme activity in the presence of 3M urea. 27. The expression of cathepsin S can be triggered by proinflammatory factors secreted by tumor cells. 28. Cathepsin K antibodies are marketed for research into expression of this enyzme by various cells.29. Nonetheless, these structures contain endocytic markers even small lysosomal proteins such as cathepsin D. 30. Cathepsin D is involved in CLN10.
