Characteristics of Folic Acid-binding Protein in Folate-deficient Serum

Folate-deficient serum contains a specific folic acid-binding protein (FABP) that has a rapid association and slow dissociation rate for the binding of ³H-pteroylglutamic acid (³H-PGA). FABP is also present in normal serum but in lesser amounts. The molecular weight of FABP is less than 100,000 and may represent more than one protein. FABP elutes as a beta globulin and is recovered in the transferrin band region in polyacrylamide gel electrophoresis. FABP binds oxidized folyl-, mono-, and polyglutamates in preference to reduced folates. FABP retards the delivery and uptake of ³H-PGA into HeLa cell monolayer cultures. The characteristics of FABP suggest it to be a membrane-derived intracellular folate storage protein and perhaps an important regulator of folate uptake into the cell and a storage site for folyl polyglutamates. The characteristics of serum FABP suggest it to be similar to beta lactoglobulin (the folate-binding protein isolated from cow’s milk).