ENDOCYTOSIS IN ENTAMOEBA HISTOLYTICA Acridine

Endocytosis by mammalian cells leads to the rapid uptake of exogenous solutes within plasma-membrane-derived vesicles. In general, fusion of these vesicles with primary or secondary iysosomes occurs and most biological macromolecules are degraded to small molecular weight components (1). Extensive bidirectional flow of membrane takes place between the plasma membrane and the lysosomal apparatus during endocytosis (7). Recent evidence has suggested that solutes pass through at least one intracellular compartment before reaching the lysosome (26) and representative portions of endocytic markers (2, 4, 6, 8) are returned to the cell surface. We now report the existence of a distinct, intermediate vacuolar compartment in the parasitic amoeba, Entamoeba histolytica. This is the primary pinocytic route in this organism. It is equilibrated with the pH of the ambient milieu and allows for the flow of solutes into and out of the cytoplasm. Only after many hours of intraceilular residence are the solutes detected in a separate, acidified lysosomelike compartment.