Figure 4-4.
Intestinal absorption of dietary folates (THF-[Glu]n) and folic acid
(F) from fortified cereal products and vitamin supplements. In the duodenum and
upper jejunum, extra glutamate residues are cleaved by conjugases (y-glutamyl
carboxypeptidases). Folate (F) and reduced folate (THF) are absorbed by a
proton-coupled, high affinity folate transporter into the mucosal cell, con-
verted to N5-methyl THF and exported into the portal circulation. N5-methyl
THF is taken up into cells by facilitative diffusion, converted to THF by the
B12-requiring methionine synthase and then converted to a polyglutamate.
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