The high-affinity folate receptor of normal and malignant human colonic mucosa

The hypothesis that folate depletion is a risk factor for development of colonic neoplasia prompted us to study the presence of a putative folate receptor in human colon mucosa. Binding of 3H-folate to normal and malignant mucosa studied by equilibrium dialysis was of high-affinity type (K = 10(10) L/mol) and displayed apparent positive cooperativity. Radioligand dissociation was slow at pH 7.4, but rapid at pH 3.5. As compared to methotrexate, 5-formyltetrahydrofolate was a potent inhibitor of binding. Gel filtration revealed a 25 kDa and a 100 kDa peak of folate-binding activity. Immunoreactivity studies were performed with rabbit antibodies against human 25 kDa milk folate-binding protein. Immunoblotting showed a single band at 65 kDa, and tissue sections exhibited immunostaining of mucosal areas. The present folate receptor with characteristics similar to those of other high-affinity folate-binding proteins may serve as a regulator of intracellular folate concentration in colon mucosa.