Folic Acid Binding by Serum and Milk

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Abstract
The binding of folic acid by serum and milk has been studied by incubating serum and milk samples with tritium-labeled folic aicd (3H-pteroylglutamic acid, 3HPGA), followed by removal of unbound 3HPGA. The serum bond was weak, and the bound 3HPGA could be removed from serum by coated charcoal. This differed from the firm binding of PGA by milk; PGA bound to milk could not be removed by a concentration of coated charcoal which removed it from serum. The DEAE cellulose chromatography showed an 3HPGA binder of milk in the phosphate buffer climate, which was not present in serum. The presence of a binder of this nature in an apocrine secretion such as milk, but not in serum, may provide an explanation for the very rapid removal of PGA injected intravenously, should other tissues prove to have similar avid binders for PGA. Since the milk PGA binder is specific for PGA, it was used no develop a coated charcoal assay for PGA. The binder will not attach natural folates and thus cannot be used at present for assay of folate activity in natural materials, such as serum.