On the Proteolytic Enzymes of Animal Tissues

It has long been known that aqueous extracts of various animal tissues (spleen, kidney, liver, etc.) exhibit proteinase activity, which was assigned to an intracellular enzyme named cathepsin (1). The finding of peptide derivatives which serve as substrates for well defined proteinases (pepsin, trypsin, chymotrypsin) led to the use of these synthetic compounds for the study of the proteolytic activity of animal tissues (2), and to the recognition that a single tissue may contain separate proteinases (endopeptidases) in addition to several exopeptidases. Accordingly, in 1941, a classification of the then known intracellular proteolytic enzymes was proposed (3) ; the designations assigned to these enzymes (cf. Table I) appear, however, to have outworn their usefulness and it is desirable, therefore, to consider a revision in the nomenclature. In what follows, the term “cathepsin” will be applied only to those intracellular enzymes, derived from animal tissues, which act on proteins, or on synthetic substrates for well defined proteinases. As will be noted from Table I, work with synthetic substrates has permitted the identification of at least three cathepsins; it may be expected that further studies will lead to the characterization of other intracellular proteinases. The present communication deals with the purification and properties of the cathepsin C of beef spleen. This enzyme, which has a specificity similar to that of pancreatic chymotrypsin, was first identified in extracts of swine kidney (7).