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digestion procedure (steps 1 and 2 of the above-mentioned steps) (Garrett et al., 1999), carotenoids
are transferred from the food to bile salt micelles, could be useful to assess the bioavailability
of carotenoids from different types of food matrices
in vitro
. These i rst two steps of carotenoid
absorption have been mimicked using Caco-2 cells cultured on plastic (Garrett et al., 2000).
17.2.2 K
INETICS
OF
b
-C T
RANSPORT
THROUGH
I
NTESTINAL
C
ELLS
Only a few studies have been done on the kinetics of carotenoid absorption. Based on earlier rat
studies (El-Gorab et al., 1975; Hollander and Ruble, 1978), the intestinal absorption of carotenoids
was thought to be a passive diffusion process determined by the concentration gradient of the car-
otenoid across the intestinal cell membrane. The kinetics of b-C transport through Caco-2 cell
monolayers, characterized for both steps (cellular uptake and secretion in CM), showed curvilinear,
time-dependent (Figure 17.3a), and saturable, concentration-dependent (apparent Km of 7-10 mM;
Figure 17.3b) processes (During et al., 2002). Thus, these data suggest that the intestinal transport of
carotenoids might be facilitated by the participation of a specii c epithelial transporter; a hypothesis
that contrasts with previous investigations. The contrast between studies could be due to the use of
different models, human cells and rats, respectively, and possibly due to the use of different b-C
concentration ranges, 0.5-23 mM (During et al., 2002) and 0.5-11 mM (Hollander and Ruble, 1978)
or 6-60 mM (El-Gorab et al., 1975), respectively.
The saturation of b-C transport through Caco-2 cell monolayers occurred at b-C concentrations
of 15-20 mM (equivalent to a daily b-C intake of 100 mg or more); a concentration range far higher
than the “physiological” concentration range. It was estimated that the b-C concentration of 1 mM at
the apical side of cells (or 400 pmol b-C/cm
2
of Caco-2 cell monolayer) was close to the physiologi-
cal level of b-C found in the gut (200 pmol b-C/cm
2
of surface of absorption) after ingestion of a
daily b-C dose of 5 mg (During et al., 2002).
Under linear concentration conditions (for a b-C concentration range of 0.12-6 mM) at 16 h
incubation and under cell culture conditions mimicking the
in vivo
postprandial state, the extent
of absorption of all-
trans
b-C through Caco-2 cell monolayers was 11
; a value similar to that
reported from different human studies. In humans, the bioavailability of a single dose of b-C
%
in cells
in basolateral medium
in cells
in basolateral medium
Km=10 μM
4000
Vm = 6500 pmol β-C/16 h
800
3000
600
2000
400
Km=7 μM
Vm = 3500 pmol β-C/16 h
1000
200
0
0
0
5
10
15
0
5
10
15
20
25
(a)
Incubation time (h)
(b)
Initial β-C concentration (μM)
FIGURE 17.3
Kinetics of b-C transport through Caco-2 cell monolayers as a function of (a) the incubation
time at a i xed b-C concentration (1 mM) and (b) the initial b-C concentration for 16 h incubation. (Modii ed
from During, A. et al.,
J. Lipid Res
., 43, 1086, 2002.)
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