Title: Cathepsins
Literature References: Intracellular proteinases obtained from animal tissue extracts, the richest sources being liver, kidney and spleen. Located primarily in the lysosomal fraction within the cell. Part of the general enzymic apparatus of animal cells; in most cases they do not specialize in functions characteristic of individual tissues. Review of cathepsins A-C: J. S. Fruton in The Enzymes vol. 4, P. D. Boyer et al., Eds. (Academic Press, New York, 1960) pp 233-241; of cathepsins A-E: M. J. Mycek, Methods Enzymol. 19, 285-315 (1970); of cathepsins B, D and G: several authors, Res. Monogr. Cell Tissue Physiol. 2, 57-89, 181-248 (1977); of cathepsins B, D, G, H, L, N and S: several authors, Ciba Found. Symp. 75, 1-68 (1980).
Derivative Type: Cathepsin C
Additional Names: Dipeptidyl transferase; dipeptidyl amino peptidase I
Literature References: Isoln from beef spleen: Tallan et al., J. Biol. Chem. 194, 793 (1952); de la Haba et al., ibid. 234, 316 (1959). Hydrolyzes dipeptidyl amides or esters bearing a free a-amino (or a-imino) group in the N-terminal position, esp. those containing an aromatic amino acid adjacent to the free a-amino group: Planta, Gruber, Biochim. Biophys. Acta 53, 443 (1961); Wurz et al., Biochemistry 1, 19 (1962). Enhances the proteolysis of prothrombin to thrombin and thus plays an important role in blood clotting: Purcell, Barnhart, Biochim. Biophys. Acta 78, 800 (1963).
Derivative Type: Cathepsin D
Literature References: Isoln from bovine spleen: Press et al., Biochem. J. 74, 501 (1960); Webb, ibid. 76, 538 (1960); 84, 455 (1962). Involved in the catabolism of cartilage and connective tissue: Weston et al., Nature 222, 285 (1969).
Derivative Type: Cathepsin G
Literature References: Isoln from human spleen: G. Starkey et al., Biochem. J. 155, 255 (1976). Chymotrypsin-like enzyme. Catalytic and immunological properties: P. M. Starkey, A. J. Barrett, ibid. 273.
Catnep C-Curarine I C-Curarine III Ceanothic Acid Cecropins

Cathepsin K 1TU6.png
Structure of Cathepsin K
Symbol Cathepsin
Pfam PF00112
Pfam clan CL0125
InterPro IPR000668
SCOP 1aec

Cathepsins (Ancient Greek kata- "down" and hepsein "boil"; abbreviated CTS) are proteases (enzymes that degrades proteins) found in all animals as well as other organisms. There are approximately a dozen members of this family, which are distinguished by their structure, catalytic mechanism, and which proteins they cleave. Most of the members become activated at the low pH found in lysosomes. Thus, the activity of this family lies almost entirely within those organelles. There are, however, exceptions such as cathepsin K, which works extracellularly after secretion by osteoclasts in bone resorption.

Cathepsins have a vital role in mammalian cellular turnover, e.g. bone resorption. They degrade polypeptides and are distinguished by their substrate specificities.