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Departments of Pediatrics and Physiology, Steele Memorial Children's Research Center, University of Arizona Health Sciences Center, Tucson, Arizona 85724
Intestinal and renal absorption of inorganic phosphate (Pi) is critical for phosphate homeostasis in mammals. We have isolated a cDNA that encodes a type III Na-dependent phosphate cotransporter from mouse small intestine (mPit-2). The nucleotide sequence of mPit-2 predicts a protein of 653 amino acids with at least 10 putative transmembrane domains. Kinetic studies, carried out in Xenopus oocytes, showed that mPit-2 cRNA induces significant Na-dependent Pi uptake with an apparent Michaelis constant (Km) for phosphate of 38 µM. The transport of phosphate by mPit-2 is inhibited at high pH. Northern blot analysis demonstrated the presence of mPit-2 mRNA in various tissues, including intestine, kidney, heart, liver, brain, testis, and skin. The highest expression of mPit-2 in the intestine was found in the jejunum. In situ hybridization revealed that mPit-2 mRNA is expressed throughout the vertical crypt-villus axis of the intestinal epithelium. The presence of mPit-2 in the mouse intestine and its unique transport characteristics suggest that multiple Na-dependent cotransporters may contribute to phosphate absorption in the mammalian small intestine.
Na-Pi-III; amphotropic murine retrovirus receptor; Ram-1; sodium-dependent phosphate cotransporter; sodium-phosphate transporter; Pit-2
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