Chemistry Reference
In-Depth Information
(in oil or in capsule) was 9
%
-17
%
using the lymph-cannulation approach (Goodman et al., 1966),
11
using carotenoid and retinyl ester response in the TG-rich lipoprotein plasma fraction approach
(van Vliet et al., 1995), and 3
%
using the most recent isotopic tracer approaches (Novotny et
al., 1995; Lin et al., 2000). The fact that the extent of b-C absorption obtained with Caco-2 cells falls
within the range observed
in vivo
adds coni dence to the
in vitro
model for studying human intesti-
nal absorption of carotenoids. Finally, of the total b-C secreted by Caco-2 cells, 80
%
-22
%
%
was associated
with CM, 10
with the nonlipoprotein fraction (During et al., 2002), pointing
to the importance of CM assembly for b-C secretion into the lymph
in vivo
.
%
with VLDL, and 10
%
17.2.3 S
ELECTIVE
U
PTAKE
OF
A
LL
-
TRANS
b
-C
VERSUS
I
TS
CIS
I
SOMERS
BY
I
NTESTINAL
C
ELLS
Human studies (Jensen et al., 1987; Gaziano et al., 1995; Stahl et al., 1995; You et al., 1996; Johnson
et al., 1997) have consistently reported a preferential accumulation of all-
trans
b-C in total plasma,
and in the postprandial TG-rich lipoprotein plasma fraction, compared to its 9-
cis
isomer. These
differences in plasma response between the two geometrical isomers suggested either a selective
intestinal transport of all-
trans
b-C versus its 9-
cis
isomer or an intestinal
cis-trans
isomerization
of 9-
cis
b-C into all-
trans
b-C. This later possibility was brought up by a study (You et al., 1996)
showing a signii cant accumulation of [
13
C]-all-
trans
b-C in plasma of subjects who ingested only
[
13
C]-9-
cis
b-C. Starting with an initial concentration (1 mM) for the three geometrical isomers of
b-C applied separately to the
in vitro
system described above, it was demonstrated that both 9-
cis
and 13-
cis
b-C were taken up by Caco-2 cells to only one-i fth of the extent of all-
trans
b-C (During
et al., 2002). The extent of absorption of the two
cis
isomers through Caco-2 cell monolayers was
less than 3.5
for all-
trans
b-C) (Table 17.1), indicating that the discrimination
between b-C isomers occurred at the cellular uptake level of the intestinal absorption process.
The b-C isomer selectivity seems to be tissue-specii c; a preferential uptake of the all-
trans
iso-
mer was shown in hepatic stellate HSC-T6 cells and in cell-free system from rat liver microsomes,
but not in endothelial EAHY cells or U937 monocyte-macrophages (During et al., 2002). When
Caco-2 cells were incubated with only 9-
cis
b-C, all-
trans
b-C did not increase in cells or in the
basolateral medium, indicating that there is no
cis-trans
isomerization occurring in intestinal cells.
Thus, the isomerization of 9-
cis
b-C observed
in vivo
(You et al., 1996) could take place in the
%
(compa red to 11
%
TABLE 17.1
Differential Absorption of Individual
Carotenoids through Caco-2 Cell
Monolayers
Carotenoid Name
Extent of Absorption (
%
)
a
All-
trans
b-carotene
11.2 ± 2.4
13-
cis
b-Carotene
3.1 ± 1.9
b
9-
cis
b-Carotene
1.9 ± 1.5
b
9.6
±
1.5
a-Carotene
Lutein
6.7 ± 1.9
b
Lycopene
2.3 ± 2.0
b
Source:
Modii ed from During, A. et al.,
J. Lipid
Res
., 43, 1086, 2002.
a
Values are means ± SD,
n
= 3 or more indepen-
dent experiments, at 16 h incubation for 1 mM
carotenoid.
b
P
< 0.0005 compared to all-trans b-C value.
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