pH in the endosome. Measurements during pinocytosis and receptor-mediated endocytosis.

By fluorescence spectroscopy, the average pH within endocytic compartments was determined during endocytosis of fluorescein conjugates by macrophages and hepatocytes. In mouse macrophages and hepatocytes fluorescein conjugates taken up either in the fluid phase or by binding to cell surface receptors were rapidly transferred to an acidic compartment (pH 5-5.5). The half-time for this process was generally less than 4 min. The pH within yeast-containing phagosomes was also rapidly reduced to similar levels, following a unique and transient increase. In each case, the acid endosomal compartments involved probably do not contain lysosomal enzymes. When fluorescein conjugates of asialoglycoproteins were internalised by hepatocytes at 20 degrees C, no proteolysis occurred within the acidic endosome until the temperature was raised. Fluorescein conjugates of concanavalin A (conA) and polylysine were relatively more slowly internalised by macrophages. The half-times for uptake, estimated by fluorescence change, were comparable with the turnover time for bulk plasma membrane. The relatively high average pH experienced by these conjugates indicated that a small proportion of these non-specific cell-surface labels was always in contact with the extracellular medium.