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*
I.N.R.A., Unité de Nutrition Lipidique, 21034 Dijon Cédex, France;
Département de Nutrition, ENSBANA, EP CNRS n°1777 CESG-Dijon et Département Génie Biologique, IUT de Dijon, Université de Bourgogne
Laboratoire de Physiologie de la Nutrition, Université de Paris-Sud, 91405 Orsay, France
1To whom correspondence should be addressed.
 |
ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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KEY WORDS: • conjugated linoleic acids • pancreatic lipase • absorption • triacylglycerol • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, Belury 1995
,
Doyle 1998
, Fitch Haumann 1996
,
Fritsche and Steinhart 1998
, Ip et al. 1994
, Parodi 1997
). Surprisingly, little is
known about the intestinal absorption of CLA. However, the intestine is
the first step of nutrient delivery to tissues and as such may modulate
the bioavailability of the ingested fatty acids, and further their
biological effects. In that respect, not all the CLA isomers
accumulated similarly in body tissues. In feeding experiments using a
commercial mixture of CLA isomers given as free fatty acids, the
9c,11t-isomer was usually recovered in the
adipose tissue to a larger extent than the
10t,12c homolog (Kramer et al. 1998
, Sugano et al. 1997
), suggesting a
different intestinal absorption rate between both isomers. Only one
study examined the intestinal transport of dietary CLA through the
lymphatic pathway in rats (Sugano et al. 1997
). The
total lymphatic recovery of CLA was <70% that of linoleic acid.
Unfortunately, the analytical conditions used were likely to produce
inaccurate values regarding the CLA isomer profile recovered in lymph,
as evidenced by the unusual high level of tt-isomer found
(Banni and Martin 1998
, Kramer et al. 1997
, Werner et al. 1992
). On the other hand, in
an older study, the free fatty acid and the triacylglycerol form have
been reported to differentially modify the rate and possibly the extent
of the blood CLA recovery over 48 h (Reiser 1950
).
In that instance, CLA in the triacylglycerol form featured a more rapid
apparent absorption than the free form. The CLA total recovery and
isomers absorption profile have not been addressed in that latter study
(Reiser 1950
). A mixture of CLA isomers acylated in triacylglycerols and blended with a trilinolein carrier was administrated to lymph-canulated rats. Their quantitative and qualitative recoveries were determined over 24 h. The activity of the pancreatic lipase, the main lipolytic enzyme in the duodenum, and the hydrolytic products’ composition were also measured in vitro to determine whether the lymphatic apparent recovery of CLA was correlated with the initial step of hydrolysis in the lumen.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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All chemicals and linoleic acid (99% pure) were supplied by
Sigma-Aldrich-Supelco (Saint Quentin Fallavier, France).
[1-14C]-linoleic acid (2 GBq/mmol) acid was purchased
from Amersham (Amersham, Courtaboeuf, France). Solvents were from SDS
(Peypin, France). Hexane, chloroform, methanol, acetone and
acetonitrile were distilled before use. The other solvents were of HPLC
grade. CLA was a generous gift of Natural Lipids (Hovdebygda, Norway),
and the purity was increased to 95.3% after an additional preparative
HPLC enrichment step (Table 1
). Silica gel plate (20 x 20 cm, 2.5 mm thickness) as well as
silicic acid for column chromatography (Silica Si, particle size
70–200 mesh) were from SDS (Peypin, France). Nylon tubing (7.5 cm
o.d.) was from Walter Coles, Co. (London, United Kingdom).
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Triacyl-sn-glycerols were synthesized with 1 mol of
glycerol esterified with 3 mol of free fatty acids solubilized in
CH2Cl2, and 3.3 mol of 4-dimethylaminopyridine
in the presence of 1,1'-diclyclocarbodiimide as described elsewhere
(Kodali et al. 1987
, Martin et al. 1997
).
Triesters of CLA, trilinolein and tri-[1-14C]linolein (40
GBq/mmol) were prepared therefrom. The test oil administered to rats
was made up of a mixture of both trilinolein and triesters of CLA (9:1,
wt/wt), and 90 to 150 kBq of tri-[1-14C]linolein.
Animals, surgery and lymph collection.
Official French regulations (n°87848) for the care and use of
laboratory animals were followed (n°03056). Male Wistar rats weighing
180–250 g were obtained from Center d’élevage Dépré
(Saint Doulchard, France). They were housed for 1 wk before the study
in a controlled environment, with constant temperature and humidity and
a dark period from 2000 to 0800 h. They were fed the standard
nonpurified diet AO4 containing 3.5 g/100 g lipid (UAR Villemoisson sur
Orge, Epinay sur Orge, France) and had free access to tap water. A
laparotomy was performed under slight ethyl-ether anesthesia on fed
rats. Mesenteric lymph duct cannulation was performed using a
heparinized polyethylene catheter (n°1, i.d. 0.3., o.d. 0.7; Biotrol,
Paris, France) (Caselli et al. 1979
, Martin et al. 1997
). Immediately after the surgery, rats were placed in
restraining cages in an air-conditioned room (25°C). They did not
receive solid food but had free access to a water solution containing
sodium chloride (7 g/L) and potassium chloride (2 g/L).
After 16 h of food deprivation, basal lymph was collected for 1 h in tubes placed inside Dewar flasks containing ice, immediately before lipid administration. Then 250 mg of one of the experimental oils was administered intragastrically. The lymph was collected during two time periods (0–6 and 6–24 h) from conscious rats. Only rats with a basal lymph flow over 0.8 mL/h were considered. Rats in which the basal lymph flow did not increase two to three times during lipid absorption or whose lymph flow was perturbed by clotting were discarded. The samples were immediately treated for both morphological study of lipoproteins and lipid extraction.
Pancreatic lipase hydrolysis.
The standard assay medium and conditions used have been already
reported and validated (Martin et al. 1997
). Briefly, 10
mg of triacylglycerol from each of the experimental oils (triester of
CLA, trilinolein or the mixture of trilinolein/triester of CLA, 9:1,
wt/wt), was digested in the presence of 0.1 g/L sodium taurocholate and
1.8 g/L CaCl2 at 37°C with 20 µg (2,000 U) of
pancreatic lipase (E. 3.1.1.3, type VI-S) and 4 µg of colipase. The
reaction was stopped at selected time points (2, 5, 10, 20 and 40 min)
by acidification with diluted HCl. One portion (2:5) of the extracted
lipids was further fractionated by TLC to obtain the free fatty acids
and the 2-monoacyl-sn-glycerol released (Martin et al. 1997
). The other portion was used for gas chromatography
analysis of the total lipid profile. The velocity of the
triacylglycerol substrate hydrolysis by the lipase was calculated from
v = k.[S]. The first order constant
k was determined graphically from kt
= -2.3 x log
([TG]t/[TG]t0), where
[TG]t denotes the amount of triacylglycerol substrate
remaining at time t, and
[TG]to denotes the initial amount of
triacylglycerol in the assay medium (36 µmol/L). The initial velocity
was determined for [S] = [TG]t0. The
disappearance of substrate triacylglycerols was monitored by gas
chromatography (see below). Each assay was duplicated.
Lipid analysis.
Lymph lipids were extracted with methylal/methanol (4:1, v/v)
(Delsal 1944
). These lipids and the
2-monoacyl-sn-glycerol lipolytic recovered from the
lipolytic products were transmethylated according to the method of
Carreau and Dubacq (1978)
as used elsewhere
(Sébédio et al. 1997
). The free fatty acids
arising from the pancreatic lipase hydrolysis were methylated for 30
min at room temperature with 140 g/L boron trifluoride in methanol
(Werner et al. 1992
). After methylation, samples
containing CLA were further fractionated by reversed-phase HPLC,
and both linoleic acid and the CLA isomers contained in the diene
fraction were quantified by polar capillary column-gas
chromatography as described (Lavillonnière et al. 1998
).
The analysis of the total hydrolytic products arising from the in vitro
lipolysis (i.e., free fatty acid, monoacyl-sn-glycerol,
diacylglycerol, remaining triacylglycerol) was carried out on short
nonpolar capillary column-gas chromatography (Kuksis et al. 1993
, Martin et al. 1997
) but without any
further derivatization. Quantification was made after correction of the
detector response using real standards, and the results were expressed
in mol/100 mol.
Lymph particles size determination.
An aliquot of each lymph fraction was prepared (Caselli et al. 1979
) for the transmission electron microscope observation of
lipoproteins.
Simultaneously, a second part of the two lymph fractions collected was diluted (1:2) in a saline solution (9 g/L) to determine the size partition of lipoprotein particles using a laser granulometer (Malvern Instruments, Orsay, France).
Fatty acid percentage recovery.
The percentage of apparent fractional recovery of CLA into lymph was
calculated from the relative recovery of 18:2(n-6) according to the
formula:
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 |
 |
 |
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 |
where the subscript D refers to diet, the subscript L refers to lymph, [14C] refers to the radioactivity in lymph lipids (in dpm/mg or dpm in total lipid, as indicated) and the fractional recovery determined by the radioactivity recovered during the selected time fractions (i.e., 0–6 and 6–24 h).
Statistics.
Results were computed and statistically analyzed by the SigmaStat®
software (Jandel Scientific, San Rafael, CA). For the lymph recovery
experiment (n = 5), comparisons were made using the
Student’s t test or the one-way ANOVA (or the ANOVA
on Ranks when the normality test failed), as indicated. One-way
repeated measures of variances (or one-way repeated measures of
variances on ranks when the normality test failed) compared the in
vitro hydrolytic features among the test oils according to
time-treatment (n = 6 time-treatments per
test oil). Student-Newman-Keuls test was used as soon as heterogeneity
among groups was demonstrated by the ANOVA. The level of significance
was set at P 
0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Accordingly, the granulometric analysis detected
particles of various size >140 nm (88% between 140 and 190 nm) with
only few of very large diameter (5% with a size over 280 nm) (Fig. 1C
).
|
) (65% of the lipoproteins particles are
over 100 nm). Of the remaining particles (size inferior to 100 nm),
35% were thus considered to be VLDL (Fig. 1D
). In vitro hydrolysis.
Significant differences between the hydrolytic features of the test
oils were not observed (Fig. 2
). Nonetheless, slightly less monoacylglycerol was produced during the
hydrolysis of the triester of CLA (Fig. 2B
), whereas
dilinolein was released at a faster rate from the trilinolein substrate
during the early times of digestion (Fig. 2C
). On the other
hand, the initial velocity of lipase (calculated under conditions
equivalent to a constant substrate saturation of the enzyme) was
significantly higher when the triester of CLA was added to trilinolein,
compared to trilinolein alone (v = 49.1 µmol
triacylglycerol/(min · mg) and v = 41.2 µmol
triacylglycerol/(min · mg), respectively, P = 0.05).
An intermediary value was found with triester of CLA (v = 45.7
µmol triacylglycerol/(min · mg), P > 0.05). The
presence of 18:2(n-6) in the mixture oil caused a faster release of the
9c,11t-isomer over the
10t,12c-isomer, both as free fatty acids and
2-monoacyl-sn-glycerol (Fig. 3
). Conversely, in the test oil composed of the triester of CLA, both the
9c,11t- and 10t,12c-isomers
were similarly released into the 2-monoacyl-sn-glycerol and
free fatty acids moieties (Fig. 3)
. In addition, when present together
in the test oil, the breakdown of the triester of CLA was less
efficient than that of trilinolein, as shown by the significant
enrichment of the 2-monoacyl-sn-glycerol and free fatty
acids moieties with linoleate (P = 0.05) (Fig. 4
). The enrichment was even greater in 2-monoacyl-sn-glycerol.
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The lymph lipids at the baseline were high in both palmitic and
linoleic acids, with no CLA dectected (Table 2
). As expected, the administered fatty acids (linoleic acid and CLA)
increased over time, peaking in the 0–6-h time fraction. The fatty
acid composition of the 6–24-h fraction featured intermediary values
between those of the baseline and 0–6-h fractions. This was due to the
termination of dietary lipid absorption and to a greater proportion of
the endogenous pool contributing to the lymph lipid composition. As
expected, CLA apparent recovery per hour was greater during the first
6 h than during the 6–24-h period (Table 3
). This phase corresponded to the peak of dietary fat absorption
(Carlier et al. 1991
, Tso 1994
) as
evidenced by the presence of large-size chylomicrons which were
much more abundant during the first 6 h of the test oil digestion
(Fig. 1)
. The CLA fractional and total apparent recoveries were not
different from those of linoleic acid over the 24 h of lymph
collection.
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). Especially, similar proportions of the 9c,11t-
and 10t,12c-isomers were found in lymph,
irrespective of the time fraction considered. The only noticeable
differences between the test oil and the lymph lipid were the lower
all-trans isomers of CLA found in the 6–24-h lymph lipid
fraction (P = 0.05).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The lymphatic absorption of conjugated linoleate was as high as that of
linoleate during both phases of lipid absorption considered in our
study (Table 3)
. More than half of the CLA in lymph was transported
during the first 6 h after force-feeding (Table 3)
, which
corresponds to the peak of lipid digestion and absorption by the
intestine (Carlier et al. 1991
, Tso 1994
). This value is higher than that reported by Sugano et al. (1997)
in lymph cannulated rats (<70% relative to
linoleic acid), but with CLA given as free acids. It should be noted
that another methodology for the determination of fatty acid recovery
was used, which makes a direct comparison with our study difficult.
However, consistent with this, Reiser et al. (1950)
found many years ago that CLA accumulation was higher in rat tissue and
plasma lipids when these fatty acids were given as triacylglycerols
rather than as free acids in the diet. We propose that the
triacylglycerol form could be more advantageous than the free form to
improve the lymphatic absorption of CLA.
Unexpectedly, the in vitro release of CLA by the lipase into the free
form and 2-monoacyl-sn-glycerol form was consistently less
than for linoleic acid (Fig. 4)
. In addition, the release of the
9c,11t-isomer of CLA by the lipase was faster
than that of the 10t,12c-isomer (Fig. 3)
. These
findings are not consistent with the above-reported similar
recovery of CLA and linoleate (Table 3)
, nor with the similar CLA
isomer repartition determined in lymph lipids (Fig. 5)
. Similarly,
Yang et al. (1990)
measured a sixfold difference between
the rate of the most released and the least released fatty acid from
fish oil triacylglycerol while using an identical in vitro assay
(1000–5000 U of porcine pancreatic lipase, 15–25 mg of substrate).
These authors mentioned that this disparity noticed in vitro only
occurs under conditions of a low hydrolysis rate. They concluded that
such disparity is not sufficient to account for a selective fatty acid
absorption, since in vivo, the excess pancreatic lipase would alleviate
this apparent resistance and allow a nondiscriminating hydrolysis of
the fatty acyl ester bonds. Compared to the in vitro situation, the
participation of the other digestive lipases in vivo (i.e., gastric and
carboxylester lipases) and/or the quality of the emulsion in
the duodenum (Armand 1998
, Bernbäck et al. 1989
, Carey et al. 1983
, Chen et al. 1989
and 1994
, Iverson et al. 1991
) might
additionally help to overcome this apparent lower efficiency of the
pancreatic lipase toward glycerol esters of CLA.
In contrast to our findings, Sugano et al. (1997)
found
that not all of the individual CLA isomers were equally absorbed when
administered as free fatty acids, and that the tt-isomers became
the main isomers present in lymph. The discrepancy between this study
and ours is likely to arise from the analytic procedure used to
quantify CLA by gas chromatography, which required derivatization into
methyl esters. The derivatization method applied by these authors
generates large amounts of tt-isomers from the
ct- and tc-precursors (Banni and Martin 1998
, Kramer et al. 1997
, Shantha et al. 1993
, Werner et al. 1992
).
An intriguing observation in the in vitro assay was the significantly higher initial velocity of pancreatic lipase when triesters of CLA were present in the test oils. This deserves a more careful in vivo examination, especially in cases of low pancreatic lipase, since it suggests that the triester of CLA would enhance the efficiency of overall fat digestion in the lumen.
In conclusion, when given as a triacylglycerol, the two main CLA
isomers (9c,11t- and
10t,12c-isomers) were equally transported through
the lymphatic pathway, which is the main route of long-chain fatty
acid delivery to the interior milieu (Besnard et al. 1996
, Carey et al. 1983
). Therefore, based on
our results, one cannot ascribe a distinct cellular accumulation or a
difference in the biological activity between CLA isomers due to
selective absorbability. Also, the lymphatic recovery of CLA given in
the triacylglycerol form was not different from that of linoleate
[96% of that of 18:2(n-6)]. Our results also point out that in the
mixture oil, the lower efficiency of pancreatic lipase toward ester
bonds of CLA observed in vitro, and especially from those of the
10t,12c-isomer, can be overcome by the
physiologic conditions prevailing in the intestinal tract.
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FOOTNOTES |
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Manuscript received June 9, 1999. Initial review completed July 29, 1999. Revision accepted December 21, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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