A high-affinity soluble folate receptor in fluids of non-neoplastic ovarian cysts: Radioligand binding, molecular size, hydrophobic residue, and immunological properties

The presence of a soluble folate receptor in fluids of non-neoplastic ovarian cysts was demonstrated. Radioligand binding exhibited characteristics typical of high-affinity folate-binding proteins. These included positive cooperativity, a tendency to increased binding affinity with decreasing receptor concentration, a slow ligand dissociation at pH 7.4 and inhibition by folate analogues. The folate receptor was probably synthesized in the lining epithelial cells of the cysts which showed positive immunostaining with antibodies against human milk folate-binding protein. The gel filtration profile of cystic fluid contained two radioligand-bound peaks, 25 and 100 kDa, whereas a single band of 70 kDa was seen on SDS-PAGE immunoblotting. Treatment with the enzyme phosphatidylinositol-specific phospholipase C resulted in a partial conversion of the 100 kDa peak to the 25 kDa peak. This suggests that insertion of a hydrophobic glycosylphosphatidylinositol tail into Triton X-100 micelles could give rise to large molecular size forms of the receptor on gel filtration.