Purification, properties, and immunological characterization of folate-binding proteins from human leukemia cells

A new matrix for affinity chromatography using pteroylglutamic acid coupled to an epoxy-activated matrix via hexanediamine resulted in negligible ligand leakage and permitted the purification of soluble and membrane-associated folate-binding proteins from human leukemia cells contained in a human spleen. Two species of membrane-associated folate-binding proteins were purified from the solubilized membrane fraction of the tissue using 2 M guanidine-HCl to elute the proteins from the affinity matrix. The higher molecular weight binding protein had an Mr of approximately 310,000 and the smaller species had an Mr of approximately 28,000 by gel filtration. By SDS-polyacrylamide gel electrophoresis the smaller species of membrane-associated protein had a molecular weight of 35,500, but the molecular weight of the larger membrane-associated species could not be determined by this method because of the high concentration of residual Triton X-100 in the sample which interfered with the silver staining of the gel. Two folate-binding proteins, which by SDS-polyacrylamide gel electrophoresis had molecular weights of 34,500 and 32,000, were purified from the 44,000 X g supernatant fraction of the tissue homogenate by acid elution from the affinity matrix. Despite the different cell components from which the soluble and membrane-associated folate-binding proteins were purified, the amino acid compositions were similar, especially with respect to the apolar amino acids. All these forms of folate-binding proteins had higher affinity for oxidized than for reduced folates, and very low affinity for 5-formyltetrahydrofolate and methotrexate. Although these proteins cross-react with one antiserum raised previously to a folate-binding protein from other human leukemia cells, they do not cross-react with the folate-binding proteins purified from two other sources of human leukemia cells, from human placenta, or from the human KB cell line.