Identification of folate binding proteins in rat liver.

Intraperitoneal injection of tritiated folic acid (PteGlu) into rats has revealed the presence of three separate protein fractions in the cytosol fraction of the liver and one in the mitochondria which bind folate derivatives. The proteins in the cytosol (cytosol I, II and III) have approximate molecular weights of 350,000, 150,000, and 25,000 and the protein in the mitochondria has an approximate molecular weight of 90,000 as estimated by gel filtration. The bound folate derivatives are primarily polyglutamate forms while cytosol II contains primarily bound 5-methyltetrahydrofolate polyglutamate derivatives. Little binding of radioactively labeled folic acid or 5-methyltetrahydrofolate to these fractions was observed when binding was carried out in vitro. Significant binding in vitro was observed, however, when a mixture of biosynthetically labeled natural folate derivatives was used. These proteins have not been purified, but cytosol III partially consists of the enzyme, tetrahydrofolate dehydrogenase (EC 1.5.1.3). Studies on the time course of folic acid incorporation into the liver showed that soon after injection nonmetabolized folic acid was bound to the plasma membrane fraction of the liver cell. It is suggested that at least one of the binding proteins in the cytosol may be involved in storage of the vitamin while the binding of nonmetabolized folic acid to the plasma membrane may reflect the existence of a carrier for folic acid transport into the cell.