Folate receptor in malignant effusions of ovarian carcinoma.

Binding of 3H-folate in human ovarian adenocarcinoma tissue was of a high-affinity type (K approximately 10(10) M-1) and displayed apparent positive cooperatively. A high-affinity folate receptor was also present in ascitic fluid and pleural effusion. Radioligand dissociation was slow at pH 7.4, but rapid at pH 3.5. The folate analogues methotrexate and in particular 5-formyltetrahydrofolate acted as inhibitors of 3H-folate binding in ascitic fluid. Ovarian adenocarcinoma tissue showed immunostaining with rabbit antibodies against human milk folate-binding protein. The gel filtration diagram contained two peaks of radiolabelled folate (at 25 and 100 kDa). The 25 kDa peak was predominant in ascitic fluid and pleural effusion. A single band of 70 kDa was seen on SDS-PAGE immunoblotting of tissue and malignant effusions. The concentration of folate receptor in tissue and fluid specimens could be determined by an immunochemical method (ELISA) utilizing antibodies against human milk folate-binding protein.