Many cathepsins are lysosomal proteases that play an important role in antigen presentation, tissue remodeling and enzyme activation/inactivation. They are also implicated in cancer, Alzheimers disease and rheumatoid arthritis. Structurally, they belong to different protease classes and families. Cathepsins B, C, H, L, S, V, X/Z/P, 1, 3 and 6 are cysteine proteases of the papain family. Cathepsins D and E are aspartic proteases of the pepsin family. Cathepsin A, a multifunctional enzyme, is a member of the serine carboxypeptidase family. Cathepsins are synthesized as inactive proenzymes and processed to become mature and active enzymes. Endogenous protein inhibitors, such as cystatins and serpins, inhibit active enzymes.
Cathepsin F (CTSF) is a lysosomal cysteine protease that participates in protein degradation and has been implicated in tumor progression. Cathepsin F is ubiquitously expressed. The propeptide for Cathepsin F has a cystatin-like N-terminal domain, which may be involved in regulating activity. Nuclear Cathepsin F is believed to be required for activation of hepatic stellate cells during liver fibrosis. In addition, mice deficient in Cathepsin F exhibited lysosomal storage defects and progressive neurodegeneration which was similar to the human condition Neuronal Ceroid Lipofuscinosis (NCL).