curative properties, it is the treatment of disorders that
remains the focus of attention.
By adsorbing ‘aggressors’ (infectious factors) of the
gastrointestinal mucosa
barrier, these materials can serve as both prophylactic and
therapeutic agents.
11.6.1. INTERACTIONS OF CLAY MINERALS WITH
GASTROINTESTINAL MUCUSAt the surface of the gut, a mucus gel adheres to the epithelial
cells of the mucosa.
This adherent mucus is dynamic, being continuously secreted by
the caliciform cells
and regularly eroded by environmental ‘aggressors’ present in
the gut lumen.
The mucus gel is largely composed of glycoprotein polymers,
lipids, and proteins,
linked together by covalent bonds. As such, it acts as a
physical barrier protecting
the mucosa against penetration by extraneous molecules and
mechanical injury.
By maintaining a pH gradient and competing with the epithelial
surface for microorganisms,
the mucus gel also acts as a chemical barrier.
Thus, a weakening of the mucus gel barrier may be at the origin
of disorders such
as gastritis and colitis (Droy-Lefaix, 1987). Short-term treatment with clay
minerals,
such as smectite (calcium and sodium bentonites)s
(
More´ et al.,
1987) and
attapulgite (More´ et al.,
1992) increases the thickness of the adherent mucus. This may be ascribed to
interactions of mineral particles with mucus components (
Leonard et al.,
1994)
by which the gastrointestinal
glycoproteins are modified, and their polymerization is enhanced
(Droy-Lefaixet al., 1986
. Similarly, aluminium
(hydr)oxides (e.g., boehmite) can reduce mucus degradation (
Bouyssou et al., 1990). The beneficial effects of
minerals are also associated
with improvements in the rheological properties of the mucus
gel, such as
spinability. This reflects the increased extent of
polymerization, and the improvement in quality, of the adheret mucus (
Droy-Lefaix et al.,
1985).
Changes in the
physico-chemical properties of the mucus, induced by the action
of clay minerals,
were confirmed by electron paramagnetic resonance and
fluorescence spectroscopy.
The results indicate that clay mineral ingestion decreases mucus
solubility. At the
same time, the viscosity and hydrophobicity of the mucus
increases, enhancing its
adhesion to epithelial cells.
11.6.2. CLAY MINERALS, MUCOSAL BARRIER, AND
GASTROINTESTINAL ‘AGGRESSORS’By acting directly on the mucus gel, clays and clay minerals
exert a stabilizing
effect on the mucosal barrier (Gwozdzinski et al.,
1997),
providing protection against
different ‘aggressors’ of the gastrointestinal
mucosa.
Pepsin, a substance necessary for digestion, is a typical
‘aggressor’. Experiments
with rats showed that if pepsin secretion at the surface of the
gastric mucosa is
strongly increased (due to pathological disregulation), the
adherent mucus layer is
progressively disrupted. At the same time haemorrhagic mucosal
lesions appear, and
significant bleeding occurs in the lumen as well as localized
ulceration in an otherwise
intact epithelium. By binding to the mucus components, smectite
(calcium and sodium bentonites) can completely
inhibit the damage induced by pepsin (Leonard et al., 1994).Samson et al. (1995)
, for example, showed that
patients with ulcerative colitis
show a six-fold greater mean total faecal proteinase activity
(expressed in mmol
terminal NH2/min/g dry weight of faeces) than the control. smectite (calcium and
sodium bentonites) totally inhibits
this enzyme activity. The effects of smectite (calcium and
sodium bentonites) on mucus proteolysis are assessed using a
model of mucolytic activity, assayed by the release of degraded
colonic mucin
from the adherent mucus gel of freshly prepared pig colonic bags
in vitro. Similarly,
trypsin (2 mg/mL) releases three times more soluble mucin per
bag than the
control. smectite (calcium and sodium bentonites) (100 mg/mL)
inhibits trypsin activity, causing the level of degraded
mucin to fall below the normal value. This is ascribed to the
interaction of smectite (calcium and sodium bentonites)
with the adherent mucus layer, and the binding of trypsin to the
mineral (Samson et al., 1995
Clay minerals can also provide protection against attack by bile
acids that cause
gastrointestinal ulceration. In rats, oral administration of
sodium glycodeoxycholate
or sodium taurocholate induces severe erosion of the jejunal
mucosa. After treatment
with smectite (calcium and sodium bentonites) (which interacts
closely with the mucus glycoproteins) the severity of
surface erosion is greatly diminished
(Fioramonti et al.,
1990),
while the rheological
properties of the adherent mucus gel are maintained within
normal limits (Droy- Lefaix et al., 1985 Because of their strong bioadhesive properties, clay minerals
also afford protection
of the colon against damage from reactive oxygen species.
Oxygenated free
radicals, released by infiltration of white cells into the
colonic mucosa barrier, are
very unstable. Their presence can induce severe erosion of the
colonic mucosa,
leading to mucolysis. By maintaining the solution viscosity of
the colonic mucin, and
inhibiting the hypersecretion of mucus, smectite (calcium and
sodium bentonites) can prevent the onset of mucolysis"...
Pearson et al., 1996; Knight et al.,
1998).