The heterogeneity and properties of folate binding proteins from chronic myelogenous leukemia cells

Previous studies have demonstrated that some chronic myelogenous leukemia cells contain a macromolecular binding factor for folic acid. This binder, which previously was believed to be a single factor, has now been resolved into two distinct binding proteins. Separation of each binder was obtained by DEAE chromatography of the partially purified lysate of chronic myelogenous leukemia cells. One binder has a molecular weight of 30;000-35,000, and the second binder has a molecular weight of 40,000-45,000. Both proteins bind the mono-, di-, and triglutamates of folic acid, N10-methyl-folate, dihydro-folate, and N5-methyltetrahydrofolate. Neither binder has determinants for N5-formyltetrahydrofolate or methotrexate. The preferred substrates for both binders appear to be the fully oxidized and partially reduced folates rather than the fully reduced folates. The lower-molecular-weight folate binding protein shows reversible binding with partially and fully reduced folates but irreversible binding with oxidized folates. This property suggests that this binder may have some function in the transport and storage of folate. The higher-molecular-weight folate binding protein, however, has only slight reversibility of binding with the partially and fully reduced folates, and it is therefore more difficult to postulate a physiologic function for this binding factor.