Adaptive regulation of ribofl avin absorption

Feeding rats with a vitamin B2-defi cient diet caused
a signifi cant and specifi c up-regulation in ribofl avin
uptake by rat intestinal brush-border membrane vesicles
compared with controls. Conversely, over-supplementing
rats with ribofl avin caused a down-regulation
in the vitamin’s uptake. Both up- and down-regulation
were mediated through changes in the number of the
functional ribofl avin uptake carriers and/or their activity
(decreased or increased Vmax) with no effect on
the affi nity of the transport system (unchanged Km)
(Said & Mohammadkhani, 1993). Similarly, growing
Caco-2 monolayers in a ribofl avin-defi cient medium
caused signifi cant enhancement of ribofl avin uptake,
whereas an over-supplemented medium suppressed
ribofl avin uptake (Said & Ma, 1994). This adaptive upor
down-regulation of ribofl avin uptake was further
studied by Said et al. (1994) who established a role for
the ‘second messenger’ cyclic AMP (cAMP) in the regulation
mechanism. They found that compounds that
increased intracellular cAMP concentration through
different mechanisms caused a signifi cant and concentration-
dependent inhibition (down-regulation)
in ribofl avin uptake. This inhibition of ribofl avin
uptake was in contrast to other fi ndings of stimulation
of intestinal uptake of D-glucose by these compounds
(Sharp & Debnam, 1994), indicating that the effect of
cAMP-stimulating compounds on ribofl avin intestinal
uptake is not generalized in nature.
12.4.2 Absorption of bacterially
synthesized ribofl avin in the large intestine
The normal microfl ora of the large intestine synthesize
considerable amounts of vitamin B2, a signifi cant
portion of which exists as free ribofl avin. The amount
of vitamin B2 synthesized depends on the diet, being
signifi cantly higher following consumption of a vegetable-
based diet compared with a meat-based diet
(Iiuma, 1955). In a study with human subjects, Sorrell
et al. (1971) showed that ribofl avin instilled directly
into the lumen of the mid-transverse colon was absorbed,
as judged by an increase in plasma ribofl avin
concentrations. Colonic absorption of FMN sodium
has also been demonstrated in the rat (Kasper, 1970).
Said et al. (2000) demonstrated the existence of a
high-affi nity, carrier-mediated transport system in the
large intestine using cultured human colonic epithelial
cells (NCM460 cells). Saturable uptake of ribofl avin by
these cells was energy-dependent and Na+-independent.
Transport was regulated by the Ca2+/calmodulin
cell signalling pathway but not by the protein kinase
C pathway. An adaptive up- and down-regulation of
ribofl avin uptake took place when NCM460 cells were
grown in a ribofl avin-defi cient or over-supplemented
medium, respectively. These fi ndings are similar to
those in the small intestine and suggest that the same
mechanism may be operating in the large intestine to
absorb the bacterially synthesized ribofl avin.
12.4.3 Post-absorptive metabolism
Following absorption, B2 vitamers are carried by the
portal blood to the liver. About 50% of circulating
fl avins is ribofl avin, with somewhat less FAD and less
than 10% FMN. The concentration of ribofl avin in
human plasma is about 0.03 μM on average (McCormick,
1989). A proportion of the circulating fl avins is
bound loosely to albumin and tightly to some immunoglobulins.
The extent to which fl avins are bound
to plasma proteins is not believed to be crucial in
regulating tissue availability of the vitamin (White &
Merrill, 1986). Erythrocytes contain four to fi ve times
more fl avin than plasma. There is a relatively slow
equilibration of free riboflavin between the plasma
and erythrocytes, hence flavin levels in erythrocytes
are less subject to recent dietary intake. For this
reason, the activity of an erythrocyte enzyme, glutathione
reductase, is used as an indicator of vitamin
B2 status.

Uptake of riboflavin by human-derived cultured
liver cells is by means of a carrier-mediated, energydependent,
Na+-independent system which appears
to be regulated by an intracellular Ca2+/calmodulinmediated
transduction pathway and by substrate level
in the growth medium (Said et al., 1998). The liver
is the major storage site of the vitamin and contains
about one-third of the total body fl avins, 70–90% of
which is in the form of FAD. Free ribofl avin constitutes
less than 5% of the stored fl avins. Other storage
sites are the spleen, kidney and cardiac muscle. These
depots maintain signifi cant amounts of the vitamin
even in severe defi ciency states.