A combination of cation exchange and ligand-affinity chromatography for purification of two molecular species of the folate binding protein in human milk, one equipped with a hydrophobic glycosyl phosphatidylinositol tail: characterization of hydrophobicity and electrical charge.

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Cation exchange chromatography combined with ligand (methotrexate) affinity chromatography on a column desorbed with a pH-gradient was used for separation and large scale purification of two folate binding proteins in human milk. One of the proteins, which had a molecular size of 27 kDa on gel filtration and eluted from the affinity column at pH 5-6 was a cleavage product of a 100 kDa protein eluted at pH 3-4 as evidenced by identical N-terminal amino acid sequences and a reduction in the molecular size of the latter protein to 27 kDa after cleavage of its hydrophobic glycosylphosphatidyl-inositol tail that inserts into Triton X-100 micelles. Chromatofocusing showed that both proteins possessed multiple isoelectric points within the pH range 7-9. The 100 kDa protein exhibited a high affinity to hydrophobic interaction chromatographic gels, whereas this was only the case with unliganded forms of the 27 kDa protein indicative of a decrease in the hydrophobicity of the protein after ligand binding.