Table 2.

Overview and expression profiling and transport kinetic features of folate transporters

Cellular (anti) folate uptake systems
	PCFT (proton-coupled folate transporter)	RFC (reduced folate carrier)	FR (folate receptor α,β,γ isoform)
Membrane orientation	Transmembrane	Transmembrane	GPI - anchored
Localization	Enterocytes	Immune cells Tumor cells	Kidney (FRα) Tumor cells (FRα) Myeloid cells/activated Macrophages (FRβ) Hematopoietic cells (FRγ, soluble, secreted form)
pH optimum	5.0–5.5	7.2–8.0	7.4–8.0
Affinity folic acid	Km 1–5 μM	Km 200–400 μM	Kd 0.1–1 nM
Affinity 5-methyl-THF	Km 2–10 μM	Km 1–5 μM	Kd 5–10 nM
Affinity MTX	Km 2–10 μM	Km 2–10 μM	Kd 50–100 nM