Characterization of a high-affinity folate receptor in normal and malignant human testicular tissue

We have characterized the folate receptor in normal and malignant tissue fr om male gonads. Radioligand binding displayed characteristics typical of ot her folate receptors. Those included a high-affinity type of binding (K = 1 0(10 M-1)), apparent positive cooperativity changing into non-cooperativity at low receptor concentrations, a tendency to increased binding affinity w ith decreasing receptor concentrations, a slow dissociation at pH 7.3 becom ing rapid at pH 3.5 and inhibition by Folates, in particular oxidized forms . The gel filtration profile of Triton X-100 solubilized tissue contained a 25 and 100 kDa peak of radioligand-receptor. The latter peak could represe nt receptor equipped with a hydrophobic membrane anchor that inserts into T riton X-100 micelles. The concentration of radiolabelled receptor ranged fr om 0.41 nmol/g protein to 1.68 nmol/g protein in specimens of normal testic ular tissue from patients with prostatic carcinomas and from 1.54 nmol/g pr otein to 3.82 nmol/g protein in testicular tissue from young individuals. C ompared to normal testicular tissue the concentration of receptor in semino ma tissue was low (0.38-1.27 nmol/ g protein) but showed a higher degree of immunoreactivity in the presence of antibodies against human milk folate b inding protein as evidenced by ELISA and immunohistochemistry data. Hence a folate receptor isoform homologous to human milk folate binding protein is apparently expressed in seminomas where the total expression of receptor, however, seems to be lower than in normal testicles.