The folacin present in feeds and foods exists largely as polyglutamates. In plants, folacin exists as a polygluta-mate conjugate containing a g-linked polypeptide chain of (primarily) seven glutamic acid residues. The normal gut proteases do not cleave the glutamate residues from this compound. Instead, a group of intestinal enzymes known as conjugases (folyl polyglutamate hydrolases) removes all but the last glutamate residue. Only the monoglutamyl form is thought to be absorbed into the enterocyte. Most of the folic acid taken up by the brush border is reduced to tetrahydrofolate (FH4) and then methylated to N5-methyl-FH4, the predominant form of folate in blood plasma. The majority of the N5-methyl-FH4 in plasma is bound to protein.

Like thiamin, folic acid has a free amino group (on the pteridine ring), and this makes it very sensitive to losses in activity due to heat treatment, particularly if heat is applied to foods or feeds containing reducing sugars such as lactose or glucose. Whether the free amino group of folacin (or thiamin) can bind to the free aldehyde moiety of pyridoxal or pyridoxalphosphate is not known. Intestinal conjugase inhibitors may be present in certain beans and pulses, and these may impede folacin absorption.[5] Only 38% retention of folacin activity occurs when folic acid is stored for 3 wk at 45°C in a mineral-free premix; even greater losses occur when minerals are included in the premix.[2]

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