Localization of nucleotidases dependent on Ca2+ was investigated cytochemically in axenically cultivated trophozoites of Entamoeba histolytica, strain HM-1:IMSS, with an electron microscope. Ca2+-dependent ATPase (EC 18.104.22.168) activity was found on the plasma membrane and on the inner surface of the limiting membrane of a few cytoplasmic vacuoles. Ca2+-dependent ADPase, Ca2+-dependent thiamine pyrophosphatase, and acid phosphatase (EC 22.214.171.124) activities were detected on the inner surface of the limiting membrane of most of the cytoplasmic vacuoles but not on the plasma membrane. Cytoplasmic vacuoles with these enzymatic activities seemed similar in morphological characteristics. Moreover, the reaction product formed by Ca2+-dependent ADPase, Ca2+-dependent thiamine pyrophosphatase and acid phosphatase was demonstrable on the inner surface of the limiting membrane of vacuoles containing ingested red blood cells. The reaction product formed by these enzymes was also observed on the periphery of ingested red blood cells. The findings suggest that cytoplasmic vacuoles with these enzymatic activities are lysosomal in nature, probably phagolysosomes; therefore, the enzymes appear to be at least partially associated with primary lysosomes of E. histolytica.
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