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-Tocopherol in VLDL Is Decreased by Dietary Protein Insufficiency in Young Growing Rats1
Laboratory of Nutritional Biochemistry, Department of Agricultural Chemistry, National Taiwan University, Taipei, Taiwan 106
3To whom correspondence should be addressed.
 |
ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-tocopherol in plasma and most peripheral
tissues were shown previously to be low in young growing rats fed a low
protein diet. To examine the secretion rates of VLDL -tocopherol and
triglycerides, and lipoprotein lipase activity, weanling rats were fed
a low protein (LP, 8 g/100 g lactalbumin) or a normal protein (NP, 20
g/100 g lactalbumin) diet for 6 wk. The absolute secretion rate of VLDL
triglyceride (µmol/h) of the LP group was not
significantly different from that of the NP group (P
> 0.05), but was significantly higher (P < 0.05) when expressed relative to body weight
[µmol/(h · kg)]. The secretion rates of VLDL
-tocopherol were significantly lower (P < 0.05)
in the LP group than in the NP group. The activities of hepatic lipase,
lipoprotein lipase and total heparin-releasable lipase in plasma of
the LP group were only 50–60% those of the NP group (P
< 0.05). The results demonstrated that the secretion rate of VLDL
-tocopherol and activities of lipases in postheparin plasma were
significantly lower in rats fed a low protein diet. Thus, the
redistribution of -tocopherol from liver to peripheral tissues
appears to have been impaired by dietary protein insufficiency.
KEY WORDS: • vitamin E • low protein diet • VLDL secretion rate • lipoprotein lipase • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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,
Kayden and Traber 1993
). Absorbed dietary vitamin E is
incorporated into chylomicron particles (Bjorneboe et al. 1986
, Traber et al. 1988
) in enterocytes, then
is released into lymphatics and subsequently into blood. Within the
circulation, most of the absorbed dietary vitamin E remains in the
chylomicron remnants (Bjorneboe et al. 1987a
) and enters
the liver when chylomicron remnants are taken up by that organ. In the
liver, the newly absorbed vitamin E is repackaged into the VLDL
particle, which is then secreted into the plasma (Bjorneboe et al. 1987b
, Cohn et al. 1988
, Traber et al. 1988
). Accordingly, the redistribution of -tocopherol
from liver to peripheral tissues depends largely on VLDL secretion from
the liver.
The -tocopherol binding protein
(-TBP)4is a cytosolic protein that binds specifically to
RRR--tocopherol and is found in the liver of rats and
humans (Behrens and Madère 1982
, Catignani and Bieri 1977
, Kaplowitz et al. 1989
,
Kuhlenkamp et al. 1993
, Sato et al. 1991
, Yoshida et al. 1992
). The protein is also
called -tocopherol transfer protein (-TTP) because it enhances
the transfer of -tocopherol between membranes (Sato et al. 1991
). As mentioned above, physiologic studies have indicated
that liver is responsible for the preferential discrimination among the
E vitamers (Burton and Traber 1990
) and for the
incorporation of RRR--tocopherol into nascent VLDL, which
then distributes -tocopherol to the peripheral tissues
(Traber et al. 1990
). This is consistent with the ligand
specificity (Sato et al. 1991
) and tissue distribution
pattern of -TTP (Yoshida et al. 1992
). Taken together
with the -tocopherol transfer activity, -TTP is proposed to be
the mediator of preferential incorporation of
RRR--tocopherol into VLDL (Kayden and Traber, 1993
).
We showed previously that dietary protein insufficiency in rats
significantly lowered -tocopherol concentration in peripheral
tissues but not in liver (Huang and Shaw 1994
). The
possibility that liver -TTP might be impaired by dietary protein
inadequacy was then examined, and the expression of -TTP protein and
mRNA were indeed depressed in the liver of rats fed a low protein diet
(Shaw and Huang 1998
). According to the proposed
physiologic role of -TTP, this could lead to a reduced
redistribution of -tocopherol from liver to peripheral tissues as a
result of a decreased export of -tocopherol through VLDL secretion
from liver. To test the hypothesis the secretion rates of VLDL
triglycerides and -tocopherol were examined in this study in rats
fed a low protein diet. The lipase activity in the postheparin plasma
was also measured because it is involved in the clearance of the
triglyceride-rich lipoprotein.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Male Long-Evans weaning rats purchased from the Laboratory Animal
Center, College of Medicine, National Taiwan University (Taipei,
Taiwan) were housed individually in stainless steel wire cages in a
room maintained at 25 ± 2°C with a controlled 12-h light: dark
cycle. The rats were divided into two groups as follows: the NP group
was fed a control diet containing a normal level of protein (20 g/100 g
lactalbumin); the LP group was fed a low protein diet (8 g/100 g
lactalbumin). Diets and tap water were freely available. Body weights
and food intake were recorded weekly. The compositions of the test
diets are shown in Table 1
. Animal care and handling conformed to NIH guidelines (NRC 1985
).
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After 6 wk of feeding, rats from each group were deprived of food
overnight. The next morning, rats were anesthetized by intraperitoneal
injection of pentobarbital (Nembutal Sodium Solution, Abbot
Laboratories, North Chicago, IL; 62.5 mg/kg body), and a catheter was
implanted into the jugular vein. After blood was collected as the 0-h
sample, a dose of heparin (250 U/kg body) was infused
(Krauss et al. 1973
). The postheparin blood was drawn
after 5 min. Tyloxapol (200 g/L in saline; Sigma Chemical, St. Louis,
MO), which is the same polyethylene glycol as Triton WR-1339, was
infused via the catheter to block VLDL degradation, at a dose of 400
mg/kg of body (Shiomi et al.1987
). Blood samples were
collected 2 and 4 h after Tyloxapol infusion. All blood samples
were collected in heparin-containing tubes and the plasma was
isolated by low speed centrifugation at 1000 x g for 15
min. The VLDL fraction was isolated by ultracentrifugation (Oda et al. 1991
). Aliquots of plasma were added to the
thin-walled polyallomer tubes (13 x 64 mm), and a NaBr
solution (d = 1.006 kg/L) was layered over the
surface. After capping, the samples were centrifuged at 121,000 x g for 16 h (50.4 Ti rotor, XL-90 ultracentrifuge,
Beckman, Fullerton, CA). The VLDL fraction was recovered from the top
layer of each tube.
Plasma and VLDL triglycerides concentrations were determined by
enzymatic analysis using a reagent kit (Randox Laboratory, Crumlin,
UK). The concentration of -tocopherol in plasma and VLDL was
analyzed by HPLC as previously described (Huang and Shaw 1994
).
Calculation of the VLDL secretion rate.
The secretion rate of VLDL triglycerides and -tocopherol were
calculated according to Oda et al. (1991)
using the
following equation: secretion rate of VLDL triglyceride
(µmol/h) or -tocopherol (nmol/h) = (C4 - C0)V/4, where
C0 and C4 are the
concentrations of triglyceride (or -tocopherol) before (0 h) and
4 h after Tyloxapol administration, and V is the
plasma volume derived on the basis of 4 mL plasma/100 g body weight.
Because the body weight of rats fed the low protein diet was
significantly lower than that of the control rats, the secretion rate
obtained was further divided by the body weight of each rat and
expressed as µmol/(h · kg body) for VLDL-
triglyceride secretion or nmol/(h · kg body) for
VLDL--tocopherol secretion.
Lipase assays.
Lipase activity was determined as described by Nilsson-Ehle and Schotz (1976)
with some modifications. The substrate was
freshly prepared as follows: calculated amounts of labeled triolein
(glycerol tri[9,10(n)-3H] oleate, 0.66 TBq/mmol; Amersham
Pharmacia Biotech, Uppsala, Sweden) and unlabeled triolein in
chloroform (Sigma Chemical) were mixed and the solvent evaporated under
a stream of nitrogen, then sonicated after the addition of a gum arabic
solution. The reaction mixture for total lipase activity contained the
following: triolein, 5 mmol/L; gum arabic, 5 g/L; albumin, 100 g/L;
heated serum (normal rat serum preheated for 60 min at 56 ± 1°C), 10 g/100 g; Tris buffer, 0.2 mol/L, pH 8.6; CaCl2,
0.01 mol/L; NaCl, 0.1 mol/L and 0.01 mL of plasma sample in a final
volume of 0.5 mL. The mixture was incubated at 25°C for 30 min and
the reaction was terminated by the addition of 3.25 mL
methanol:chloroform:heptane (1.41:1.25:1, v/v/v) and 0.75 mL of
0.1mol/L potassium carbonate-borate buffer (pH 10.5). After
vigorous mixing for 15 s, the mixture was centrifuged at 800 x
g for 10 min, and the upper phase separated and counted in a
Beckman LS 5000 CE counter for radioactivity. This procedure detects
heparin-releasable lipase activity of both hepatic and nonhepatic
origin.
Hepatic lipase (HL) activity was determined as described above except
that plasma samples were preincubated with 1 mol/L NaCl for 30 min, the
reaction mixture maintained a final NaCl concentration of 1 mol/L and
the heated serum [apolipoprotein (apo) C source] was omitted. It is
well established that lipoprotein lipase (LPL), but not HL, is strongly
inhibited under these conditions (Krauss et al. 1973
).
Thus, the activity of LPL could be calculated by subtracting HL
activity from total lipase activity. Enzyme activity (1 mU)
is defined as the amount of enzyme that catalyzes the release of 1 nmol
of oleic acid per min at 25°C.
Statistical analysis.
Data were expressed as means ± SD. The significance of difference (P < 0.05) between the two groups was analyzed by Student’s t test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). The final body weight of rats fed the LP diet was 
60% that of the
NP group. These data indicated the protein-insufficient status of
the LP group.
|
). The -tocopherol concentration and the ratio of -tocopherol to
triglyceride in plasma were significantly lower in rats of the LP group
than those of the NP group, also consistent with our previous report
(Huang and Shaw 1994
).
|
-tocopherol
concentration in plasma (data not shown) and in VLDL (Fig. 1
) increased linearly up to 4 h. The VLDL triglyceride
concentrations at 2 and 4 h after Tyloxapol administration were
significantly higher (P < 0.05) in the LP group than
in the NP group. Nevertheless, the -tocopherol concentration at
4 h after Tyloxapol administration was significantly lower
(P < 0.05) in the LP group than in the NP group.
Consequently, the difference between the two groups was magnified when
these data were expressed as the ratio of -tocopherol/triglycerides
in VLDL after Tyloxapol treatment (Fig. 1)
. The secretion rates of VLDL
triglyceride and -tocopherol were calculated on the basis of these
data. The absolute secretion rate of VLDL-triglyceride
(µmol/h) did not differ between the two groups.
(Table 4
). However, the LP group had a significantly higher (P
< 0.05) relative VLDL-triglyceride secretion rate
[µmol/(h · kg body]. On the other hand, the absolute
and relative secretion rates of VLDL -tocopherol were significantly
lower in rats fed the LP diet than in those fed the NP diet (Table 4)
.
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54%
that of the NP group (P < 0.05, Table 5
). Both HL and LPL activities also were significantly lower in the LP
group than in the NP group (P < 0.01).
|
|
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The Triton technique was used in this study to measure VLDL secretion
in vivo. Triton WR-1339 blocks the degradation of lipoproteins, either
by coating the plasma lipoproteins (Schotz et al. 1957
)
or by direct inhibition of LPL activity (Borensztajn et al. 1976
). The validity of this technique has been questioned
(Palmer et al. 1978
) and the calculated rates of
secretion may be too low by 5–10% (Otway and Robinson 1967
). However, the results are consistent with those from
other independent methods and describe metabolic responses adequately
(Bird et al. 1984
, Huang and Williams 1980
). We conducted these experiments with anesthetized rats
because our preliminary experiments showed that the
VLDL-triglyceride secretion rate of anesthetized rats was not
different from that of awake animals (data not shown).
The VLDL triglyceride concentrations at 2 and 4 h after Tyloxapol
injection were significantly higher in the LP group than in the NP
group (Fig. 1)
. When the absolute secretion rate (µmol/h)
was calculated by multiplying the rate of the concentration change by
plasma volume, the groups did not differ. This likely was because the
plasma volume was estimated as 4% of the body weight and the body
weight of the LP group was significantly lower than that of the NP
group (Table 4)
. Because rats fed an 8% lactalbumin diet had
significantly lower liver weight (Huang and Fwu 1993
,
Huang and Shaw 1994
), the secretion rate per g liver or
per 100 g body weight, in fact, is higher in the LP group than in
the NP group.
The fatty liver observed in kwashiorkor has been attributed to
decreased export of lipid from liver. Severe protein deficiency (
6
g/100 g casein) in rats resulted in significantly lower apo B and C,
the major apolipoproteins in VLDL. Hence, decreased synthesis of
apolipoprotein in the liver is considered to be a major cause of fatty
liver in severe protein malnutrition such as kwashiorkor
(Gouache et al. 1991
, Meghelli-Bouchenak et al. 1987
). On the basis of these data, it is conceivable that
-tocopherol secretion from liver could be reduced as a result of
lower VLDL secretion in protein deficiency. Should this be the case,
the -tocopherol/triglyceride ratio in plasma should be unaffected.
Results of the present study and our previous report (Huang and Shaw 1994
), however, do not support this speculation. Not only
was the plasma triglyceride level not significantly lower in the LP
group than in controls, but there was also a 50% reduction in the
plasma -tocopherol/triglyceride ratio in the LP group.
It is unclear why the relative VLDL triglyceride secretion rate
[µmol/(h · kg)] in the LP group was significantly
higher than that of the control group. In agreement of this result of
the present study, Portman et al. (1981)
also observed
an increased secretion rate of VLDL triglyceride [mg/(kg · 24 h)]
in rhesus monkeys fed a 4% casein diet for a long time compared with
that of monkeys fed a normal protein diet. It is not clear whether the
increased rate of VLDL triglyceride secretion in protein-inadequate
animals observed in these two studies is related to the high dietary
fat level (
15 g/100 g) used, or to the relatively greater amount of
carbohydrate in the LP diet, which may lead to an increase in hepatic
lipogenesis.
The present study clearly demonstrated that the absolute and relative
secretion rates of VLDL--tocopherol were significantly lower in the
LP group than in the NP control group. The significantly lower ratio of
-tocopherol to triglyceride in plasma of rats fed the LP diet may be
secondary to the higher VLDL triglyceride secretion rate. However, this
may not be the case because the VLDL -tocopherol concentration
4 h after Tyloxapol treatment was significantly lower in the LP
group than in the NP group (Fig. 1)
. In addition, the plasma
-tocopherol concentration of rats of the LP group was only 55% that
of the control rats (Table 3)
. The result indicates that the
redistribution of -tocopherol from liver to peripheral tissues was
impaired in rats fed the LP diet.
Kayden and Traber (1993)
proposed that -TTP is the
major mediator responsible for preferential incorporation of
RRR--tocopherol into nascent VLDL, which are then
secreted from the liver. In support of this view, Arita et al. (1997)
demonstrated that the secretion of -tocopherol was
more efficient in cells expressing -TTP than in matched cells
lacking -TTP. We have found that the -TTP protein level and mRNA
expression were significantly lower in rats fed a low protein diet
(Shaw and Huang 1998
). The result of the present study
that secretion rate of VLDL -tocopherol was lower in rats fed a low
protein diet further supports the proposed physiologic role of -TTP.
LPL is important in the metabolism of triglyceride-rich
lipoprotein, including VLDL and chylomicrons. Consistent with previous
reports (Lamri et al.1995
, Portman et al. 1981
), the activities of total lipase, HL and LPL in the
postheparin plasma were significantly lower in rats fed a low protein
diet. It is not clear why such a paradoxical situation existed in that
the plasma triglyceride concentration was not significantly higher in
protein-deficient rats despite a significantly greater
VLDL-triglyceride secretion rate accompanied by a significantly
lowered plasma heparin-releasable lipase activities. Portman et al. (1981)
also observed similar puzzling results.
Traber et al. (1985)
demonstrated that LPL enhanced the
uptake of -tocopherol by cells in culture. Although the tissue
origin of the LPL in the postheparin plasma cannot be specified, the
lower activity of LPL in rats fed a low protein diet may account in
part for the lower -tocopherol concentration in peripheral tissues
(Huang and Shaw 1994
).
Vitamin E deficiency occurs in human patients with fat malabsorption
and in individuals with various genetic disorders (Sokol 1993
, Traber et al. 1993
). Impaired
digestion, absorption, transportation, tissue uptake and storage of
vitamin E, rather than inadequate dietary intake, seem to underlie most
circumstances of human vitamin E deficiency. Our observations
concerning the importance of protein nutrition in the utilization of
dietary vitamin E may be relevant for humans. Karla et al. (1998)
showed that children with protein-energy
malnutrition were vitamin E deficient, as revealed by a low serum
-tocopherol, which correlated with neurological signs.
In conclusion, we have demonstrated that the secretion rate of VLDL
-tocopherol was decreased significantly in growing rats fed a low
protein diet. The results support the role of liver -tocopherol
binding protein in mediating the incorporation of -tocopherol into
nascent VLDL for secretion.
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FOOTNOTES |
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2 Current address: Department of Health and
Nutrition, Chia-Nan College of Pharmacy and Science, Jen-Ter
Hsiang, Tainan 717, Taiwan.
4 Abbreviations used: apo, apolipoprotein; -TTP
(-TBP), -tocopherol transfer protein (-tocopherol binding
protein); HL, hepatic lipase; LP, low protein; LPL, lipoprotein lipase;
NP, normal protein control.
Manuscript received May 31, 2000. Initial review completed August 3, 2000. Revision accepted August 28, 2000.
|
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