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Free keywords:
Alkaline Phosphatase; Active Transport; Enrichment Factor; Proximal Tubule; Transport Mechanism
Abstract:
In the proximal tubule of the kidney, calcium is reabsorbed by an active transport mechanism. Recently, a Ca2+-activated ATP-phosphohydrolase has been described in plasma membranes from rat kidney cortex, which is different from the Ca2+-ATPase present in mitochondria. To elucidate the role of this enzyme in transepithelial calcium transport we studied its localization within the cell of the proximal tubule. For this purpose, a plasma membrane fraction was subdivided by preparative free flow electrophoresis and the distribution of alkaline phosphatase (brush border microvillus membranes) and Na+−K+-ATPase (basal-lateral plasma membranes) was compared with that of Ca2+-ATPase. The distribution pattern obtained and the corresponding enrichment factors show that a nonmitochondrial Ca2+-ATPase is-in analogy to the Na+−K+-ATPase-located only in the basal-lateral plasma membranes of the proximal tubule. Regarding the different substrate specificity and the insensitivity of the enzyme towards sodium, potassium and ouabain it seems to be possible to differentiate between the two enzymes at a molecular level. It is proposed that the Ca2+-stimulated ATPase is involved in the active transtubular transport of calcium.
