Endocytosis in Entamoeba histolytica. Evidence for a unique non-acidified compartment.

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Abstract
Studies on endocytosis in E. histolytica trophozoites suggest that there are 2 vacuolar compartments. The 1st compartment consists of large vacuoles (> 2 .mu.m diameter). As measured by the fluid phase markers, fluorescein isothiocyanate (FITC)-dextran and horseradish peroxidase (HRP), this compartment is a rapid equilibrium with the external milieu and is constantly exchanging (1-2 h) its contents with the external medium. The contents of these vacuoles are not acidified. This, together with the absence of degradation of fluid phase markers, clearly differentiates these vacuoles from lysosomes of eukaryotes. Labeling externally disposed peptides on the surface membrane of trophozoites with 125I showed that the surface membrane was rapidly internalized over a 2-h period and then reached a plateau. All major 125I surface proteins, with the exception of a set of peptides in the 40,000 MW range, were interiorized and .apprx. 60% of the total radiolabel were in the internal membrane fraction at any given time. The kinetics of this process were similar to those for the uptake of fluid phase markers and are best explained by cycling of the surface membrane into the vacuolar compartment(s) and then back to the cell surface. The 2nd vacuolar compartment consisted of small vesicles (< 2 .mu.m diameter) with acidified contents as indicated by acridine orange uptake. The endocytic nature of these vesicles was shown by their slow (days) labeling with FITC-dextran; spectral analysis internalized FITC-dextran confirmed that this 2nd compartment is acidified (pH 5.2).