Biological Significance and Activity Control of Cathepsin E Compared with Cathepsin D
Protein degradation in mammalian cells is thought to occur via two major pathways: a lysosomal and a non-lysosomal pathway. The former may participate in degradation of the majority of cellular proteins nonspecifically and the latter may preferentially degrade abnormal and short-lived proteins.1 Recently, a number of non-lysosomal proteinases, such as cytosolic proteinases and plasma membrane-associated proteinases, have been identified in mammalian tissues. Most of the non-lysosomal enzymes have been shown to exhibit optimal activity at neutral pH. The aspartic proteinases are one of the four known main classes of proteinases and catalyze the hydrolysis of a variety of protein substrates below pH 5. The aspartic proteinases in mammalian cells are tentatively classified into two groups. One is a secretory group consisting of enzymes that function in extracellular spaces (pepsin, gastricsin etc.). The other is a non-secretory group consisting enzymes that function primarily within the cell. Cathepsins D and E are the two main non-secretory aspartic proteinases. Cathepsin D is a typical and well characterized lysosomal enzyme that is identified in almost all the mammalian cells. The wide distribution of cathepsin D throughout most tissues suggests its general role in proteolysis of cellular proteins. Besides its lysosomal role, cathepsin D has been suggested to be involved in a variety of physiological and pathological processes, for example in the proteolytic processing of lysosomal enzymes2–4 and inflammatory and neoplastic disease states.5–7 By contrast, cathepsin E is a relatively poorly characterized enzyme. Recent immunochemical studies have demonstrated that cathepsin E is a non-lysosomal protein, a part of which is present in the cytosol.8–10 However, the endogenous Substrates for cathepsin E are not known and the cellular function of this enzyme is therefore unclear.
KeywordsAspartic Proteinase Erythroid Differentiation Human Erythrocyte Membrane Mature Erythrocyte Uninduced Cell
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- 5.A. R. Poole, in: “Dynamics of Connective Tissue Macromolecules” P. M. C. Burleigh and A. R. Poole, eds., North-Holland Publishing Company, Amsterdam (1975).Google Scholar
- 24.T. Saku, H. Sakai, Y. Shibata, Y. Kato and K. Yamamoto, J. Biochem., in press.Google Scholar
- 29.G. L. Dale, S. L. Norenberg, T. Suzuki and L. Forman, in: “The Red Cell,” G. J. Brewer, ed., Alan R. Liss, Inc., New York (1989).Google Scholar