TABLE 2.
Folate transport and receptor proteins1
| Protein | Expression | Substrate affinity |
|---|---|---|
| Reduced folate carrier (RFC1) | All normal adult tissues, tissue culture cells, some tumor cells, some fetal tissues | Reduced folates: Kt = 3 μM; folic acid: Kt = 150–200 μM; methotrexate: similar to reduced folates |
| Proton coupled folate transporter (PCFT) | Small intestine (jejunum, duodenum), kidney, liver, placenta, brain | Reduced folates: Kt = 0.5–0.8 μM; folic acid: Kt = 0.5–0.8 μM; methotrexate: poor affinity |
| Folate receptor α (FR-α) | Kidney proximal tubules, choroid plexus, placenta, erythropoietic cells, vas deferens, ovaries, fallopian tubes, uterine epididymis, lung alveolar, acinar cells of the breast, submandibular salivary and bronchial cells, retinal pigment epithelial cells, breast milk, fetal and adult cells, some tumors | Reduced folates: Kd = 1–10 nM; folic acid: Kd = 0.1 nM; methotrexate: poor affinity |
| Folate receptor β (FR-β) | Placenta, spleen, thymus, thymus, some monocytes, thyroid gland (low), gut mucosal cells (very low), during myelopoiesis, acute myelogenous leukemia blasts, fetal and adult cells | Reduced folates: Kd = 1–10 nM; folic acid: Kd = 0.1 nM; methotrexate: poor affinity |