Biology Reference
In-Depth Information
7.5.1
In vitro
digestion
In general, ETs are quite stable under the physiological conditions of the
stomach. The acidic conditions (HCl, pH 1.8-2.0) and the enzymes
present did not hydrolyse the native ETs that can release free EA, and no
degradation of the ETs was observed. The stomach seems to be a first
important location for the absorption of free EA (
vide infra
), but ETs are
not absorbed. Under the physiological conditions of the small intestine,
however, there is a release of free EA from ETs such as punicalagin.
This hydrolysis seems to be due to the pH conditions (neutral to mild
alkaline,
i.e.
, pH 7.0-7.3) rather than to the effect of pancreatic enzymes
and bile salts (Larrosa
et al.
, 2006a).
7.5.2
In vitro
cell lines to study uptake and metabolism
Several human GI cell lines can be obtained from culture collections,
such as those from the stomach (
e.g.
, KATO-III) or colon (
e.g.
, cancer
cells like Caco-2 or normal colon cells) and can be used to evaluate the
uptake, metabolism, transport and excretion,
inter alia
, of the ETs and
EA. These studies can give evidence of the biochemical changes
underwent by the ETs in the cell culture media, and are useful to follow
their metabolic fate, once they enter the cells and are transformed into
specific metabolites produced by each cell line. These studies may be
carried out with pre-digested ETs (
i.e.
, Caco-2 cells may be treated with
ETs extracts that have been subjected to stomach and intestinal
digestions) to simulate more closely the
in vivo
situation. It has become
evident that ETs are not absorbed into these cells, but that they disappear
from the culture medium. This may be due to precipitation, degradation
or combination with proteins present in the medium. However, EA is
absorbed and rapidly transformed into methyl ethers by the action of
COMT (catechol
O
-methyl transferases). This enzyme introduces one or
two methyl ether groups
on the phenolic hydroxyl functions of the
ortho
-
dihydroxyphenyl or catecholic moieties of the EA molecule, thus
forming EA monomethyl ether and dimethyl ethers. Also, conjugation
with glucuronic acid has been observed in these cells. Whitley
et al.
(2003) reported a high accumulation of EA in Caco-2 cells (uptake
