A HIGH-AFFINITY SOLUBLE FOLATE RECEPTOR IN FLUIDS OF NONNEOPLASTIC 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 characte ristics typical of high-affinity folate-binding proteins. These includ ed positive cooperativity, a tendency to increased binding affinity wi th decreasing receptor concentration, a slow ligand dissociation at pH 7.4 and inhibition by folate analogues. The folate receptor was proba bly synthesized in the lining epithelial cells of the cysts which show ed positive immunostaining with antibodies against human milk folate-b inding protein. The gel filtration profile of cystic fluid contained t wo radioligand-bound peaks, 25 and 100 kDa, whereas a single band of 7 0 kDa was seen on SDS-PAGE immunoblotting. Treatment with the enzyme p hosphatidylinositol-specific phospholipase C resulted in a partial con version of the 100 kDa peak to the 25 kDa peak. This suggests that ins ertion of a hydrophobic glycosylphosphatidylinositol tail into Triton X-100 micelles could give rise to large molecular size forms of the re ceptor on gel filtration.