Cofactor in serum for high-affinity folate binding in milk. 
Affinity-chromatographic purification of the folate-binding protein from whey of cow's milk results in the removal of a heat-resistant cofactor that is important in high-affinity folate binding (Biochim, Biophys.  Acta 579: 479, 1979).  Thus a shift from negative to positive cooperativity occurred in the presence of cofactor (Biochim, Biophys.  Acta 579: 479, 1979).  We demonstrate the presence of a similar cofactor in sera from men, pregnant women, and umbilical cords.  Even highly diluted (10 000-fold) serum samples were active, and the cofactor was also present in erythrocyte hemolysates (10 000-fold diluted).  The identity of the cofactor, which may act as an overall modulator of high-affinity folate binding, is still obscure.