The folate-binding protein of rat kidney. Purification, properties, and cellular distribution.

Folate-binding protein (FBP) from rat kidney was isolated, and its properties and location in the kidney were determined. The particulate fraction of rat kidney homogenate was freed of its bound folate, solubilized with Triton X-100, and the FBP was purified using a combination of DEAE-cellulose and affinity chromatography. The purified protein migrated as a single band on sodium dodecyl sulfate-disc gel electrophoresis, has an isoelectric point of 5.7, contains 21.7% carbohydrate, and has an Mr of 28,500-30,000. The purified protein retained its affinities for different folate derivatives and its sensitivity to inorganic anions. Inorganic anions enhanced the binding of 5-methyltetrahydrofolate; chloride ion was the most effective, followed by Br- greater than I- greater than SO2-4. Chloride ion was also found to lower the dissociation constant of the folic acid-FBP complex at 50 degrees C by about 10-fold. This effect is thought to derive from the formation of a ternary FBP-folic acid-Cl- complex which is more stable than the binary FBP-folic acid complex. An antiserum raised against the purified protein in rabbits was used to determine the location of FBP in the kidney by immunofluorescence. Intense fluorescence staining for FBP was localized at the apices (brush border) of proximal tubules. The choroid plexus, an organ previously shown to contain FBP, also exhibited intense fluorescent staining.