This study reports on the distribution of bicarbonate-stimulated ATPase in rat intestinal epithelial cells. Brush-border membranes and basolateral membranes were separated from each other and from mitochondrial and other intracellular membranes by differential and density gradient centrifugation. Bicarbonate-sensitive ATPase activity followed the mitochondrial marker succinic dehydrogenase closely throughout all the centrifugation steps. The low HCO3--ATPase activity in purified brush-border and basolateral plasma membranes could be accounted for quantitatively by the small mitochondrial contamination. Consequently, there are no grounds for postulating that this enzyme has a direct role in H+ or HCO3- transport across the rat small intestine.

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