Phosphorin, A Phosphate-Binding Hydrophobic Protein Isolated from Renal Brush Border Membranes

Abstract

Reabsorption of inorganic phosphate (Pi) by the renal proximal tubule occurs from the tubular fluid across the brush border membrane into the cell interior by a Na⁺-dependent co-transport system. A number of the features of this Na⁺-Pi symport system have been documented by earlier workers^(1,2). These include a description of the kinetic parameters and of the alterations produced by in vivo manipulation of dietary phosphate⁽³⁾ or vitamin D⁽⁴⁾ or by parathyroid hormone status⁽⁵⁾. However, the molecular constituent or constituents of this Na⁺-Pi symporter have not been identified. We summarize here some features of a putative constituent of the Na⁺-Pi symporter, a proteolipid that can be extracted from renal brush borders. We have called this proteolipid “phosphorin” because it binds Pi with selectivity and high affinity and because it is located in brush borders that conduct Na⁺-Pi symport. Although our working hypothesis is that phosphorin might be a constituent of the Na⁺-Pi symporter, firm evidence for such a role for phosphorin remains to be advanced.