Constitutive Secretion of Acid Hydrolases in Tetrahymena thermophila

THE role of lysosomal enzymes in intracellular digestion is now well established 11]. Most often we think of lysosomal hydrolases in catabolism of endogenous or foreign material taken up by endocytosis. There is however, a number of reports dealing with the release of acid hydrolases into the extracellular fluid in a variety of eukaryote cells. These cells range from Saccharomyces cerevisiae 15], Dictyostelium discoideum 10], Leishmania donovani 20], Acanthamoeba castellani 22], Entamoeba histolytica 12, 31], and species of Tetrahymena 1–3, 6] to mammalian cells in culture 49]. Concerning the latter, fibroblasts and hepatocytes in culture release acid hydrolases to the extracellular medium, but only if the synthesis of a specific recognition marker is impaired in the cells. This marker (man-nose-6-phosphate) is used for receptor mediated segregation of lysosomal enzymes into the lysosomal compartments. If the receptor or the marker are lacking, the hydrolases fail to enter the lysosomal compartment, and are secreted in immature form together with molecules belonging to the constitutive secretory pathway of the cells 8, 49]. Such a release of acid hydrolases seems to occur spontaneously from mammalian osteoclasts 4]. Macrophages, on the other hand, need a specific stimulation for their release process 40]. In lower eukaryotes the release may