A high-affinity folate binding protein in human amniotic fluid. Radioligand binding characteristics, immunological properties and molecular size

The presence of a folate binding protein of high-affinity type (affinity constant 5 · 10⁹M⁻¹, maximum folate binding 3 nM) in human amniotic fluid was demonstrated in equilibrium dialysis experiments (37°C, pH 7.4) with the radioligand³H-folate. Dissociation of³H-folate from the binding protein was slow at pH 7.4 but rapid at pH 3.5. By use of rabbit antibodies against low molecular weight folate binding protein from human milk we determined the concentration of folate binding protein in 5 amniotic fluids (range 1.5–2.3 nM) in an Enzyme-Linked Immunosorbent Assay (ELISA). ultrogel AcA 44 chromatography of amniotic fluid showed that immunoreactive and radioligand bound folate binding protein coeluted in two peaks: a major one (M_r~25 000) and a minor one (M_r~100 000).