Naturally occurring folic acid in feedstuffs is conjugated with varying numbers of glutamic acid molecules. Polyglutamate forms of folic acid, usually containing three to seven glutamyl residues linked by peptide bonds, are the most abundant coenzyme form in living tissues (Wagner, 1984). Synthetic folic acid is the monoglutamate form. Folic acid has the largest number of biologically active forms of any vitamin. Over 100 active forms have been identified, varying according to the reduction state of the pteridine nucleus (di- or tetrahydrofolate), the presence or absence of methyl or other single-carbon groups on the N5 and N10 atoms, and the number of glutamic acid residues present (Girard, 1998a).
Folic acid is a yellow-orange crystalline powder, tasteless and odorless, and insoluble in alcohol, ether and other organic solvents. The acid form is slightly soluble in hot water, but folate salts are quite soluble. Folic acid is fairly stable to oxygen and heat in neutral and alkaline solution, but unstable in acid solution. From 70% to 100% folic acid activity is destroyed on autoclaving at pH 1 (O’Dell and Hogan, 1943). Light and ultraviolet radiation readily degrade folic acid. Cooking reduces folic acid activity.
Polyglutamate forms of folic acid are hydrolyzed to pteroylmonoglutamate in the small intestine prior to absorption. The enzyme responsible for the hydrolysis of pteroylpolyglutamate is a carboxypeptidase known as folate conjugase (Baugh and Krumdieck, 1971). Most likely, several conjugase enzymes are responsible for hydrolysis of the long-chain folate polyglutumates to the monoglutamates, which then enter the mucosal cell (Rosenberg and Newmann, 1974). Pteroylmonoglutamate is absorbed predominantly in the duodenum and jejunum, apparently by an active process involving sodium. Kesavan and Noronha (1983) suggest, based on data from rats, that luminal conjugase is a pancreatic secretion and that the hydrolysis of polyglutamate forms of folic acid occur in the lumen rather than at the mucosal surface or within the mucosal cells.
After hydrolysis and absorption from the intestine, folates are transported in plasma as monoglutamate derivatives, predominantly as 5-methyl-tetrahydrofolate. These monoglutamate derivatives are absorbed from circulation by specific tissue transport systems. Then the pteroylpolyglutamates, the major folic acid form in cells, are synthesized in stepwise fashion by the enzyme folate polyglutamate synthetase.
Specific folate-binding proteins (FBP) have been identified in the liver, kidney, small intestinal brush border membranes, leukocytes, reticulocytes, blood serum and milk (Tani and Iwai, 1984). The physiological roles of these FBPs are unknown, although it has been suggested they play a role in folic acid transport analogous to the intrinsic factor in the absorption of vitamin B12.
Studies have shown that 79% to 88% of labeled folic acid is absorbed, and that absorption is rapid, with serum concentrations usually peaking about two hours after ingestion. The mean availability of folic acid in seven separate foods was close to 50%, varying from 37% to 72% (Babu and Skrikantia, 1976). Folic acid is widely distributed in tissues, largely in the conjugated polyglutamate forms. Urinary excretion of folic acid represents a small fraction of total excretion. However, fecal folic acid concentrations are quite high, often higher than intake, representing not only undigested folic acid, but more importantly, considerable bacterial synthesis in the large intestine. Bile contains high levels of folic acid due to enterohepatic circulation, with most biliary folic acid reabsorbed from the small intestine. This suggests that a physiologic mechanism exists to regulate folate re-uptake based on tissue requirements.
