Cathepsin L

Cathepsin L is a lysosomal cysteine protease expressed in most eukaryotic cells. Cathepsin L is known to hydrolyze a number of proteins, including the proform of urokinasetype plasminogen activator, which is activated by Cathepsin L cleavage. Cathepsin L has also been shown to proteolytically inactivate α1antitrypsin and secretory leucoprotease inhibitor, two major protease inhibitors of the respiratory tract. These observations, combined with the demonstration of increased Cathepsin L activity in the epithelial lining fluid of the lungs of emphysema patients, have led to the suggestion that the enzyme may be involved in the progression of this disease. Cathepsin L has also been identified as a major excreted protein of transformed fibroblasts, indicating the enzyme could be involved in malignant tumor growth. In Cathepsin L deficient mice, it appears to play a critical role in cardiac morphology and function, epidermal homeostasis, regulation of the hair cycle, and MHC class I-mediated antigen presentation in cortical epithelial cells of the thymus. Mouse Cathepsin L is synthesized as a 334 amino acid precursor with a signal peptide (residues 117), a pro region (residues 18-113), and a mature chain (residues 114-334) .