The major excreted protein (MEP) of transformed mouse cells and cathepsin L have similar protease specificity

The major excreted protein of transformed mouse cells is an acid activable cysteine protease (1). In this paper, oxidized insulin B chain is shown to be a substrate for this protease. By isolation and analysis of the insulin B peptides generated by the protease, the bond specificity of this protease was determined. The bonds preferentially cleaved are glu₁₃-a1a₁₄, leu₁₇-val₁₈, and tyr₂₆-thr₂₇. No obvious preference for a specific amino acid was found in these studies. The bond specificity of this cysteine protease for oxidized insulin B chain has been compared with that of other proteases, and it is the same as that reported for cathepsin L, suggesting that the major excreted protein and cathepsin L may be the same protein.