An activity of Ca2+-dependent nucleotidase was detected in axenically-cultivated trophozoites of Entamoeba histolytica. The enzyme was concentrated by differential and sucrose density gradient centrifugation and catalyzed hydrolysis of nucleoside tri- and diphosphates and also thiamine pyrophosphate. Hydrolysis of nucleoside mono-phosphates was not affected by Ca2+. Among substrates tested, ATP was most active. Addition of Zn2+ or heat treatment almost abolished the enzyme activity. The enzyme exhibited almost the identical activity at acid and neutral pH. Among 6 bands isolated by polyacrylamide gel electrophoresis, 4 were stained with ATP, UTP, CTP and ADP, whereas the other 2 were stained only with ATP, UTP and CTP. The concentrated enzyme preparation, primarily composed of membrane fragments, also had activities of acid phosphatase, acid inorganic pyrophosphatase, 5'-nucleotidase and Mg2+-dependent ATPase. These observations suggest that E. histolytica has 2 Ca2+-dependent nucleotidases, i.e. one Ca2+-dependent ATPase and the other Ca2+-dependent nucleoside diphosphatase or an apyrase-like enzyme, and that these nucleotidases are at least partially associated with the plasma membrane or an organelle of lysosomal nature in this parasite.
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