Tuesday, July 3, 2007

Digestion and absorption of dietary vitamin B6

Vitamin B6 is present in foods mainly as the PN, PLP
and PMP vitamers. In many fruits and vegetables,
30% or more of the total vitamin B6 is present as PNglucoside.
The binding of PLP to protein through aldimine
(Schiff base) and substituted aldamine linkages
is reversibly dependent on pH, the vitamin–protein
complexes being readily dissociated under normal
gastric acid (low pH) conditions. The release of PLP
from its association with protein is an important step
in the subsequent absorption of vitamin B6, as binding
to protein inhibits the next step, hydrolysis of PLP by
alkaline phosphatase (Middleton, 1986). It would appear,
therefore, that the widespread practice of raising
the post-prandial gastric and upper small intestinal
pH by the use of pharmaceutical antacids may impair
vitamin B6 absorption.
Physiological amounts of PLP and PMP are largely
hydrolysed by alkaline phosphatase in the intestinal
lumen before absorption of free PL and PM (Hamm
et al., 1979; Mehanso et al., 1979). When present in
the lumen at non-physiological levels which saturate
the hydrolytic enzymes, substantial amounts of PLP
and PMP are absorbed intact, but at a slower rate than
their non-phosphorylated forms.
The absorption of PN, PL and PM takes place
mainly in the jejunum and is a dynamic process involving
several interrelated events. The vitamers cross
the brush-border membrane by simple diffusion as
shown, for example, in everted intestinal sacs (Tsuji et al., 1973), brush-border membrane vesicles (Yoshida
et al., 1981) and isolated intestinal loops (Middleton,
1979). In humans, PM is absorbed more slowly or
metabolized differently, or both, than either PL or
PN (Wozenski et al., 1980). Middleton (1983) noted a
signifi cant positive correlation between PLP luminal
disappearance and both alkaline phosphatase activity
and net water absorption in perfused segments of rat
jejunum. It is conjectural that increased water absorption
results in a greater concentration of PL within the
lumen, allowing absorption to proceed more rapidly.
Within the enterocyte PN, PL and PM are converted
to their corresponding phosphates by the catalytic action
of cytoplasmic pyridoxal kinase, and transaminases
interconvert PLP and PMP. The conversion of
a particular vitamer to other forms by intracellular
metabolism creates a concentration gradient across
the brush border for that vitamer, thus enhancing
its uptake by diffusion (Middleton, 1985). The phosphorylated
vitamers formed in the cell are largely
dephosphorylated by non-specific phosphatases, thus
permitting easy diffusion of vitamin B6 compounds
across the basolateral membrane. The major form
of vitamin B6 released to the portal circulation is the
non-phosphorylated form of the vitamer predominant
in the intestinal lumen.
Absorption capacity in rats was not affected directly
by dietary vitamin B6 supply (Roth-Maier et al., 1982)
and so it is supposed that homeostatic regulation of
vitamin B6 is not due to a variation of absorption.

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