Folate binding by human milk protein.

Human whey was subjected to gel chromatography. Two folate binding protein fractions wer eluted: a major one (Mr congruent to 30,000) and a minor one (Mr greater than 200,000). The former folate binding fraction (FB) could be isolated from whey by means of ion exchange chromatography. Equilibrium dialysis experiments with whey and isolated FB indicated the existence of cooperatively interacting sites in folate binding. The folate analogue methotrexate inhibited folate binding. Inhibition was apparently of a competitive type since methotrexate did not reduce maximum binding capacity. Cooperativity was lost in the presence of methotrexate. Binding affinity was inversely proportional to the concentration of FB suggesting involvement of a polymerizing protein system in binding. None of the other fractions eluted on the ion exchange column could bind folate.