Deconjugation of polyglutamyl folate

The folates naturally present in foods exist largely in
protein-bound form, the predominant vitamers being
polyglutamyl forms of THF, 5-methyl-THF and 10-
formyl-THF (Gregory, 1984). Folylpolyglutamates,
being large and strongly electronegative molecules,
are not transportable into cells and, before they can be
absorbed, they must be hydrolysed to monoglutamate
forms. None of the known proteases in saliva, gastric
juice or pancreatic secretions are capable of splitting
the γ-peptide bonds in the polyglutamyl side chain.
Polyglutamyl folate can, however, be hydrolysed by
folate conjugase, which is a trivial name for pteroylpolyglutamate
hydrolase, EC 3.4.12.10 (also known
as folylpoly-γ-glutamyl carboxypeptidase). As much
as 50–75% of dietary polyglutamyl folate can be absorbed
after deconjugation to monoglutamyl folate
(Butterworth et al., 1969). The presence of conjugase
activity in many raw foods of both plant and animal
origin results in a high proportion of the dietary folate
being already monoglutamyl when presented to the
intestinal mucosa (Gregory, 1989).
Two folate conjugases have been found in human
jejunal tissue fractions. One, a brush-border exopeptidase,
has a pH optimum of 6.7–7.0 and is activated by
Zn2+. The other, an intracellular endopeptidase of
mainly lysosomal origin, has a pH optimum of 4.5
and no defi ned metal requirement. The brush-border
conjugase splits off terminal glutamate residues one
at a time and is thought to be the principal enzyme in
the hydrolysis of polyglutamyl folate. Brush-border
conjugases from the jejunum of the human and pig
possess similar enzymatic properties (Gregory et al.,
1987) and thus the porcine enzyme can be used to
study folate bioavailability in humans. Interestingly,
the human and the pig are the only species in which
intestinal brush-border conjugase activity has been
demonstrated. Bhandari & Gregory (1990) showed
that extracts from certain foods (e.g. legumes, tomatoes
and orange juice) can inhibit brush-border conjugase
activity from human and porcine intestine in vitro. Organic acids may be responsible for this inhibition
(Wei & Gregory, 1998). Such inhibition may be
a factor affecting the bioavailability of polyglutamyl
folates in diets containing these foods. The intracellular
conjugase may play no role in the digestion of
dietary folate, being, instead, concerned with folate
metabolism within the enterocyte (Halsted, 1990).
Signifi cant conjugase activity has also been reported
in the pancreatic juice of pigs and humans (Gregory,
1995). Bhandari et al. (1990) found the porcine pancreatic
enzyme to be Zn2+-dependent with maximum
activity at pH 4.0–4.5. Feeding stimulated secretion of
pancreatic juice, including conjugase activity. Chandler
et al. (1991) calculated that conjugase activity
in porcine pancreatic juice was minor relative to the
activity of the jejunal brush-border conjugase.