Acidification of phagosomes is initiated before lysosomal enzyme activity is detected

We have measured changes of pH in a protein's microenvironment consequent on its binding to the cell surface and incorporation into pinosomes. Changes of pH were measured from single, living cells and selected regions of cells by the fluorescence ratio technique using a photon-counting microspectrofluorimeter. The chemotactic agent and pinocytosis inducer, ribonuclease, labeled with fluorescein (FTC-RNase), adsorbed to the surface of Amoeba proteus, and was pinocytosed by cells in culture media at pH 7.0. The FTC-RNase entered an apparently acidic microenvironment, pH approximately 6.1, upon binding to the surface of amoebae. Once enclosed within pinosomes, this protein's microenvironment became steadily more acidic, reaching a minimum of pH approximately 5.6 in less than 10 min. FTC-RNase pinocytosed by the giant amoeba, Chaos carolinensis, entered pinosomes whose pH was correlated with their cytoplasmic location during the initial 30-40 min after pinocytosis. The majority of pinosomes containing FTC-RNase clustered in the tail ectoplasm of C. carolinensis during this interval and had a pH of approximately 6.5; those released into endoplasm and carried into the tip of cells had a pH below 5.0. As pinosomes became distributed at random in C. carolinensis (1-2 h after initial pinocytosis), differences in pH between tip and tail pinosomes vanished. We have also measured the pH within single phagosomes of A. proteus. Phagosomal pH dropped steadily to approximately 5.4 within 5 min after particle ingestion in 70% of the cells measured, and reached this level of acidity within 10 min in 90% of the cells measured. By contrast, stain for the lysosomal enzyme, acid phosphatase, was evident within only 20% of 5-min-old phagosomes visualized by light microscopy, and within only 40% of 10-min-old phagosomes. A microfluorimetric assay was used to simultaneously record changes in pH, and the initial deposition of lysosomal esterases, within phagosomes of single, living Amoeba proteus. Near complete acidification of the phagosome was recorded from some cells before phagosomal fusion was evident by this microfluorimetric assay. From other cells, however, continued acidification of phagosomes was recorded after lysosomal fusion was initiated. We conclude that acidification of phagosomes by A. proteus is initiated but not necessarily completed prior to phagosome-lysosome formation, and that the two events are closely linked in time. Initial acidification of endosomes is a property intrinsic to the plasma membrane which envelops particles at the cell surface, rather than the result of lysosomal fusion with phagosomes.