Folate

In 1931, a research group led by Lucy Wills showed
that an autolysed yeast preparation (Marmite),
which was therapeutically ineffective against the
pernicious anaemia caused by vitamin B12 defi ciency,
was effective against nutritional megaloblastic anaemia
in pregnant women. These researchers induced
a similar anaemia in monkeys which then responded
to crude liver extracts. Other substances that cured
specifi c defi ciency anaemias in monkeys and chicks
were isolated from yeast by different research groups
and assigned the names ‘vitamin M’ and ‘vitamin Bc’.
Another substance isolated from liver was shown to
be essential to the growth of Lactobacillus casei and
therefore called the ‘L. casei factor’. In 1941, Mitchell
and co-workers processed four tons of spinach leaves
to obtain a purifi ed substance with acidic properties
which was an active growth factor for rats and L. casei.
They named the factor ‘folic acid’ (from folium, the
Latin word for leaf). Eventually, all of the above substances
proved to be the same when Angier’s group in
1946 accomplished the synthesis and chemical structure
of folic acid.

The term ‘folate’ is used as the generic descriptor
for all derivatives of pteroic acid that demonstrate
vitamin activity in humans. The structure of the
parent folate compound, folic acid, comprises a bicyclic
pterin moiety joined by a methylene bridge to
p-aminobenzoic acid, which in turn is coupled via an
α-peptide bond to a single molecule of L-glutamic
acid (Fig. 17.1, top).
(Note: In the present context, the term ‘folic acid’
refers specifi cally to pteroylmonoglutamic acid
which, with reference to the pteroic acid and glutamate
moieties, can be abbreviated to PteGlu. ‘Folate’ is
a non-specifi c term referring to any folate compound
with vitamin activity. ‘Folacin’ is a non-approved term
synonymous with ‘folate’.)
Folic acid is not a common natural physiological
form of the vitamin. In most natural foods, the pteridine
ring is reduced to give either the 7,8-dihydrofolate
(DHF) or 5,6,7,8-tetrahydrofolate (THF) (see Fig. 17.1).
These reduced forms can be substituted with a covalently
bonded one-carbon adduct attached to nitrogen positions 5 or 10 or bridged across both positions. The
following substituted forms of THF are important
intermediates in folate metabolism: 10-formyl-THF, 5-
methyl-THF, 5-formimino-THF, 5,10-methylene-THF
and 5,10-methenyl-THF (see Fig. 17.1).
An important structural aspect of the 5,6,7,8-tetrahydrofolates
is the stereochemical orientation at
the C-6 asymmetric carbon of the pteridine ring. Of
the two stereoisomers, 6S and 6R (formerly called 6l
and 6d), only the 6S is biologically active and occurs in
nature. Methods of chemical synthesis of tetrahydrofolates,
whether by catalytic hydrogenation or chemical
reduction, yield a racemic product (i.e. a mixture
of both stereoisomers).
All folate compounds exist predominantly as polyglutamates,
containing typically from fi ve to seven
glutamate residues in γ-peptide linkage. The γ-peptide
bond is unique in mammalian biochemistry.
Folate conjugates are abbreviated to PteGlun derivatives,
where n is the number of glutamate residues; for
example, 5-CH3-H4PteGlu3 refers to triglutamyl-5-
methyltetrahydrofolic acid.
Methotrexate (4-amino-10-methylfolic acid; Fig.
17.2) is a folate antagonist which is used as an anticancer
drug.

Dietary sources
Polyglutamyl folate is an essential biochemical constituent
of living cells, and most foods contribute some
folate. The folates generally exist in nature bound to
proteins (Baugh & Krumdieck, 1971) and they are
also bound to storage polysaccharides (various types
of starch and glycogen) in foods (Cerná & Káš, 1983).
In the United States, dried beans, eggs, greens, orange
juice, sweet corn, peas and peanut products are good
sources of folate that are inexpensive and available all
the year round.

Humans and other mammals cannot synthesize
folate in their tissues and thus they must obtain the
vitamin from exogenous sources via intestinal absorption.
The intestine is exposed to two sources of
folate: (1) dietary folate and (2) folate synthesized
by bacteria in the large intestine. The latter source is
available to the host tissues through direct absorption
in the colon.
It is fundamental in folate metabolism that folate
monoglutamates are the circulatory and membrane-
transportable forms of the vitamin, whereas
polyglutamates are the intracellular biochemical and
storage forms.
17.4.1 Folate transport proteins
Cellular uptake of folate involves two functionally different
membrane transport proteins: (1) the reduced
folate carrier, which is an organic anion exchange
protein present in the plasma membrane of a wide
variety of cells, and (2) the less ubiquitous folate receptor,
which internalizes folate by a receptor-mediated
process. The affi nities of these proteins for folates
and antifolates differ signifi cantly (Table 17.2). After
internalization, folates are retained in the cytoplasm
by polyglutamylation.
In certain specifi c cell types, such as human placental
trophoblast cells, a functional coordination between
the two transport proteins has been proposed.
Enterocytes and hepatocytes lack the folate receptor
and so folate transport in the human intestine and
liver is mediated solely by the reduced folate carrier.