Maintaining low intracellular sodium concentrations is vital for almost all organisms. Na(+) efflux is generally governed by P-type ATPases, Na(+)/K(+)-ATPase in animals and Na(+)-ATPase, called ENA, in fungi and plants. Trypanosoma cruzi, which parasitizes mammalian cells, must undergo drastic adaptations to high Na(+) concentrations outside and low Na(+) concentrations inside host cells. However, T. cruzi Na(+) efflux pumps have not been identified. We report here the cloning and characterization of the gene encoding Na(+)-ATPase in T cruzi, which resembled fungal and plant ENAs, termed TcENA. TcENA was a plasma membrane protein expressed throughout the parasite life cycle. The transcription level of TcENA was higher in insect stage epimastigotes and blood stream trypomastigotes than in intracellular amastigotes, probably reflecting the high Na(+) concentration outside the host cells. Biochemical analysis of TcENA expressed heterologously in mammalian cells demonstrated, for the fist time, that the ATPase activity of TcENA is stimulated by both Na(+) and K(+) and is insensitive to ouabain, a specific inhibitor of Na(+)/K(+)-ATPases. Furthermore, epimastigotes overproducing TcENA showed increased tolerance to high Na(+) stress. Our findings suggest that TcENA acts as a sodium pump and provide insights into the regulation of ion homeostasis in the parasitic protist.

We present the case of a 68-year-old female patient who was diagnosed with cancer of the descending colon in July 1994 and underwent partial resection of the colon (type 2, moderately to well differentiated adenocarcinoma, se, ly1, v1, n(–)). In April 1996, she was admitted to a nearby hospital for symptoms of ileus, which improved at the hospital. However, she was referred to our hospital for melena. In blood test, Hb was 8.7 g/dl, showing anemia, and carcinoembryonic antigen level was elevated to 50.7 ng/ml. Abdominal CT and small bowel series showed only mild expansion of the small bowel, suggesting no obvious occlusion. Abdominal surgery was performed in May 1995 for repeated development of ileus symptoms and suspicion of bleeding from the small bowel. Since the findings of the abdominal surgery showed a circular tumor in the lower ileum, partial resection of the small bowel was performed. Histopathological examination showed type 3, moderately to well differentiated adnocarcinoma, se, ly2, v0, n = 1/13. The principal tumor was located within the subserosa and grew up exclusively through the muscularis propria and the submucosa, into the mucous layer. The mucosa remained slightly on the surface layer. Based on these findings, the patient was diagnosed with metastasis of descending colon cancer to the small bowel. Her prognosis was good, and neither metastasis nor redevelopment of the cancer have been confirmed to date, 11 years and 7 months since the surgery.

A high deoxycholic acid concentration and deoxycholic acid to cholic acid ratio may be indicators of colorectal cancer. Further study is needed to improve sensitivity and specificity, perhaps by combining fecal bile acid measurements with other tests, and a large prospective trial may be warranted to determine whether these measurements have value in screening for this common cancer.

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