Purification and immunohistochemical localization of aspartic proteinases in rat epidermis

Investigation of skin cathepsin E (EC 3.4.23.-) has been totally ignored compared to skin cathepsin D (ED 3.4.23.5). In this study both cathepsins E and D were simultaneously purified to homogeneity up to 370 and 640 times, respectively, from 2-day-old rat epidermis. The total aspartic proteinase activity of rat epidermis detected after Q-Sepharose column chromatography was attributed to 27% cathepsin E, 63% cathepsin D, and 10% other enzymes. The purified enzymes showed that cathepsin E (90 kDa) is a dimer of 45 kDa subunits whereas cathepsin D is a monomer of 42 kDa. Other biochemical properties of epidermal cathepsins E and D were similar to those reported from other tissue sources. Immunologically cathepsins E and D were distinct from each other and localization of the two enzymes differed in both rat and human skin by immunohistochemistry. Cathepsin E was distributed diffusely in the cytoplasm of almost all epidermal cells, though its concentration increased above suprabasal cells, whereas cathepsin D appeared in particulate form only in cells of the granular layer. The findings indicate that two aspartic proteinases that have similar enzymatic properties exist in the epidermis. They are, however, differentially distributed in the organ, presumably for different functions during the process of keratinization.