Introduction to Human Nutrition

The Vitamins 171

Tetrahydrofolate can carry one-carbon fragments
attached to N-5 (formyl, formimino, or methyl
groups), N-10 (formyl), or bridging N-5–N-10 (methy-
lene or methenyl groups). 5-Formyl-tetrahydrofolate
is more stable to atmospheric oxidation than is folate,
and is therefore commonly used in pharmaceutical
preparations; it is also known as folinic acid, and the
synthetic (racemic) compound as leucovorin.
The extent to which the different forms of folate
can be absorbed varies; on average only about half of
the folate in the diet is available, compared with more
or less complete availability of the monoglutamate. To
permit calculation of folate intakes, the dietary folate
equivalent has been defi ned as 1 μg mixed food folates
or 0.6 μg free folic acid. On this basis, total dietary
folate equivalents = μg food folate + 1.7 × synthetic
(free) folic acid.

Absorption and metabolism of folate

About 80% of dietary folate is as polyglutamates; a
variable amount may be substituted with various
one-carbon fragments or be present as dihydrofolate
derivatives. Folate conjugates are hydrolyzed in the
small intestine by conjugase (pteroylpolyglutamate
hydrolase), a zinc-dependent enzyme of the pancre-
atic juice, bile, and mucosal brush border; zinc defi -
ciency can impair folate absorption.
Free folate, released by conjugase action, is absorbed
by active transport in the jejunum. The folate in milk
is mainly bound to a specifi c binding protein; the
protein–tetrahydrofolate complex is absorbed intact,
mainly in the ileum, by a mechanism that is distinct
from the active transport system for the absorption
of free folate. The biological availability of folate from
milk, or of folate from diets to which milk has been
added, is considerably greater than that of unbound
folate.
Much of the dietary folate undergoes methylation
and reduction within the intestinal mucosa, so that
what enters the portal bloodstream is largely 5-
methyl-tetrahydrofolate. Other substituted and
unsubstituted folate monoglutamates, and dihydrofo-
late, are also absorbed; they are reduced and methyl-
ated in the liver, then secreted in the bile. The liver
also takes up various folates released by tissues; again,
these are reduced, methylated and secreted in the
bile.
The total daily enterohepatic circulation of folate is
equivalent to about one-third of the dietary intake.

Despite this, there is very little fecal loss of folate;

jejunal absorption of methyl-tetrahydrofolate is a

very effi cient process, and the fecal excretion of some

450 nmol (200 μg) of folates per day represents syn-

thesis by intestinal fl ora and does not refl ect intake to

any signifi cant extent.

Tissue uptake of folate

Methyl-tetrahydrofolate circulates bound to albumin,

and is available for uptake by extrahepatic tissues,

where it is trapped by formation of polyglutamates,

which do not cross cell membranes.

The main circulating folate is methyl-tetrahydro-

folate, which is a poor substrate for polyglutamylation;

demethylation by the action of methionine synthetase

(see below) is required for effective metabolic trap-

ping of folate. In vitamin B 12 defi ciency, when methio-

nine synthetase activity is impaired, there will

therefore be impairment of the uptake of folate into

tissues.

Folate excretion

There is very little urinary loss of folate, only some

5–10 nmol/day. Not only is most folate in plasma

bound to proteins (either folate binding protein

for unsubstituted folate or albumin for methyl-

tetrahydrofolate), and thus protected from glomeru-

lar fi ltration, but the renal brush border has a high

concentration of folate binding protein, which acts to

reabsorb any fi ltered in the urine.

The catabolism of folate is largely by cleavage of the

C-9–N-10 bond, catalyzed by carboxypeptidase G.

The p-aminobenzoic acid moiety is amidated and

excreted in the urine as p-acetamidobenzoate and p-

acetamidobenzoyl-glutamate; pterin is excreted either

unchanged or as a variety of biologically inactive

compounds.

Metabolic functions of folate

The metabolic role of folate is as a carrier of one-

carbon fragments, both in catabolism and in biosyn-

thetic reactions. These may be carried as formyl,

formimino, methyl or methylene residues. The major

sources of these one-carbon fragments and their

major uses, as well as the interconversions of the sub-

stituted folates, are shown in Figure 8.15.

The major point of entry for one-carbon fragments

into substituted folates is methylene-tetrahydrofolate,

which is formed by the catabolism of glycine, serine,