A high-affinity folate binding protein in normal human leukocytes: ligand binding characteristics, ionic charge and molecular size.

High-affinity binding of [3H]folate to supernatant from homogenized human leukocytes containing large amounts of binding protein displayed apparent positive cooperativity. The DEAE-Sepharose CL-6B chromatographic profile of the supernatant at pH 6.3 contained a major peak of folate binding (Mr approx. 25 000) in the front effluent and a smaller more acidic peak (Mr approx. 25 000) that emerged after a rise in NaCl from 30 mmol/l to 1 mol/l. Triton X-100 solubilized cell sediment from the leukocyte homogenate contained some high-affinity folate binding activity (Mr approx 25 000), typically 5-10% of the total binding activity.