© 1999 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Human Apolipoprotein (Apo) B-48 and ApoB-100 Kinetics With Stable Isotopes
Presented in part at the 70th Scientific Sessions of the American Heart Association, Orlando, Fla, November 11, 1997, and published in abstract form (Circulation. 1997;96[suppl I]:I-143).
From the Lipid Metabolism Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, Mass, and Population Kinetics, Department of Bioengineering, University of Washington, Seattle (P.H.R.B.).
Correspondence to Francine K. Welty, Division of Cardiology, Beth Israel Deaconess Medical Center, One Autumn Street, 5th Floor, Boston, MA 02215. E-mail fwelty{at}caregroup.harvard.edu
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Abstract |
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Abstract—The kinetics of apolipoprotein (apo) B-100 and apoB-48 within triglyceride-rich lipoproteins (TRLs) and of apoB-100 within IDL and LDL were examined with a primed-constant infusion of (5,5,5-2H3) leucine in the fed state (hourly feeding) in 19 subjects after consumption of an average American diet (36% fat). Lipoproteins were isolated by ultracentrifugation and apolipoproteins by SDS gels, and isotope enrichment was assessed by gas chromatography/mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II. The pool sizes (PS) of TRL apoB-48, VLDL apoB-100, and LDL apoB-100 were 17±10, 273±167, and 3325±1146 mg, respectively. There was a trend toward a faster fractional catabolic rate (FCR) for VLDL apoB-100 than for TRL apoB-48 (6.73±3.48 versus 5.02±2.07 pools/d, respectively, P=0.06). The mean FCRs for IDL and LDL apoB-100 were 10.07±7.28 and 0.27±0.08 pools/d, respectively. The mean production rate (PR) of TRL apoB-48 was 6.5% of VLDL apoB-100 (1.3±0.90 versus 20.06±6.53 mg · kg-1 · d-1, P<0.0001). TRL apoB-48 PS was correlated with apoB-48 PR (r=0.780, P<0.0001) but not FCR (r=-0.1810, P=0.458). VLDL apoB-100 PS was correlated with both PR (r=0.713, P=0.0006) and FCR (r=-0.692, P=0.001) of VLDL apoB-100 and by apoB-48 PR (r=0.728, P=0.0004). LDL apoB-100 PS was correlated with FCR (r=-0.549, P=0.015). These data indicate that (1) the FCRs of TRL apoB-48 and VLDL apoB-100 are similar in the fed state, (2) TRL apoB-48 PS is correlated with TRL apoB-48 PR, (3) VLDL apoB-100 PS is correlated with both PR and FCR of VLDL apoB-100 and PR of TRL apoB-48, and (4) LDL apoB-100 PS is correlated with LDL FCR.
Key Words: apolipoprotein B • metabolism • stable isotopes • LDL cholesterol • lipoproteins
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionAppendix 1References |
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The mechanisms regulating the synthesis and secretion of apolipoprotein (apo) B-100 and apoB-48 are incompletely understood but are of importance because elevated levels of apoB, the main protein in LDL, are associated with an increased risk of coronary heart disease.1 ApoB exists in 2 forms in plasma, apoB-100 and apoB-48,2 both of which are products of the same structural gene on chromosome 2.3 ApoB-100 is synthesized by the liver and secreted within VLDLs, which are metabolized in plasma to form LDL. ApoB-100 contains the LDL receptor–binding domain; therefore, VLDL remnants (IDL) and LDL are removed from the circulation by binding to hepatic LDL receptors.2
Synthesized in the intestine in response to dietary fat, apoB-48 is produced as a result of a premature stop codon at the apoB-100 codon 2153 by tissue-specific mRNA processing and secreted within chylomicrons.4 Both chylomicrons and VLDL are the major triglyceride carriers in plasma, and the triglycerides therein are hydrolyzed by lipoprotein lipase to form chylomicron remnants and VLDL remnants, respectively. ApoB-48 does not contain an LDL receptor–binding domain; therefore, the chylomicron remnants are most likely taken up by the liver by receptors that recognize apo E.5 6
Increasing evidence suggests that chylomicron and VLDL remnants are atherogenic; however, chylomicron remnant clearance has not been examined extensively in humans because of the difficulty in studying apoB-48 kinetics. Furthermore, the interrelationships between intestinal apoB-48 production and apoB-100 secretion from the liver have not been studied in humans. Using different approaches, both Nestel7 and Grundy and Mok8 showed that the triglyceride component of chylomicrons has a half-life of 5 to 8 minutes in healthy subjects and a longer half-life of 26 minutes in hypertriglyceridemic subjects with coronary heart disease. Although a substantial mass of the triglyceride is rapidly hydrolyzed by lipoprotein lipase in chylomicrons and VLDL, the nontriglyceride core components (cholesteryl ester) and apoB-48 remain with the lipoprotein particle until receptor-mediated uptake occurs. Previous kinetic studies of triglyceride-rich lipoproteins (TRLs) using radiolabeled particles have suggested that catabolism of both apoB-48 and apoB-100 within TRL is very rapid in normal humans, ie, 15 to 30 minutes.9 10
Kinetic studies using stable isotopes of amino acids allow for
the direct evaluation of endogenously synthesized protein
and therefore have an advantage over radioactive studies in which a
protein is isolated, labeled, and reinjected. Using stable isotopes and
a monoexponential analysis, Lichtenstein et
al11 studied 8 subjects to determine whether an intestinal
apolipoprotein such as apoB-48 would have a similar level of enrichment
at plateau compared with a hepatically derived protein such as VLDL
apoB-100. They did not study IDL and LDL kinetics. They observed that
the tracer-tracee ratio at plateau for TRL apoB-48 was 
50%
of that for VLDL apoB-100 and concluded that maximal enrichment at
plateau differed between the liver and intestine. Using a different
methodology to prepare apoB-48 and apoB-100 samples for mass
spectrometry,12 we observed that the plateau for TRL
apoB-48 approached that of VLDL apoB-100 in the present study. We
then investigated the kinetics of TRL apoB-48 and apoB-100 within
VLDL-, IDL-, and LDL-containing lipoproteins in 19 humans by using a
primed-constant infusion of deuterated leucine with the subject in the
constantly fed state and performingmulticompartmen-tal modeling
to determine apoB kinetic parameters and correlations
between apoB-48 and apoB-100 kinetics.
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Methods |
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Subjects
Nineteen subjects, 8 women and 11 men, underwent a medical history and physical examination. They had no evidence of any chronic illness, including endocrine, hepatic, renal, thyroid, or cardiac dysfunction. They did not smoke and were not taking any medications known to affect lipid levels. All female subjects were postmenopausal. The experimental protocol was approved by the Human Investigation Review Committee of the New England Medical Center and Tufts University.
Experimental Protocol for In Vivo Stable Isotope Kinetics
To determine the kinetics of TRL apoB-48 and VLDL, IDL, and LDL
apoB-100, the subjects underwent a primed-constant infusion of
deuterated leucine while they were in the fed state as previously
described.11 12 13 14 15 Starting at 6 AM, the
subjects received 20 identical small hourly meals, each equivalent to
1/20th of their daily food intake, with 15% of calories as protein,
49% carbohydrate, 36% fat (15% saturated, 15%
monounsaturated, 6% polyunsaturated), and 180 mg
cholesterol/1000 kcal. At 11 AM, with 2
intravenous lines in place, 1 for the infusate and 1 for
blood sampling,
(5,5,5-2H3)-L-leucine
(10 µmol/kg body wt) was injected as a bolus IV over 1 minute
and then by continuous infusion (10 µmol · kg body
wt-1 · h-1) over a
15-hour period. Blood samples (20 mL) were collected at hours 0, 1, 2,
3, 4, 6, 8, 10, 12, and 15.
Plasma Lipid and Lipoprotein Characterization
Blood was collected in sterile tubes containing EDTA (0.1%
final concentration). Plasma was separated from red cells in a
refrigerated centrifuge at 3000 rpm for 30 minutes at 4°C.
Plasma and lipoprotein fractions were assayed for total
cholesterol and triglyceride with an Abbott
Spectrum analyzer with enzymatic reagents.16 17
HDL cholesterol was measured as previously
described.18 Lipid assays were standardized through the
Centers for Disease Control Lipid Standardization Program.
The VLDL (d<1.006 g/mL), IDL (d=1.006 to 1.019 g/mL), and LDL (d=1.019 to 1.063 g/mL) fractions were isolated from fresh plasma by ultracentrifugation.19 ApoB was assayed in plasma and lipoprotein fractions with a noncompetitive ELISA using immunopurified polyclonal antibodies.20 The coefficient of variation for the apoB assay was <5% within runs and <10% between runs.20
Quantification and Isolation of the Apolipoproteins
ApoB-48 and apoB-100 were isolated from lipoproteins by
preparative SDS-PAGE using a Tris-glycine buffer system as previously
described.21 22 Based on the assumption that both apoB-100
and apoB-48 have the same chromogenicity, apoB
concentrations within individual apoB species were assessed by scanning
each gel with laser densitometry as previously
described.11 12 13 14 15 23 We scanned VLDL fractions from each
time point and averaged all 10 to calculate ratios and to estimate
concentrations of apoB-48 and apoB-100 using the total apoB
concentration as determined by ELISA.
Isotopic Enrichment Determinations
ApoB-48 and apoB-100 bands were excised from the
polyacrylamide gels. Plasma (0.3 mL) and the excised apoB-48
and apoB-100 bands were hydrolyzed in 12N HCl at 100°C for 24
hours.11 12 13 14 15 The free amino acids were isolated from
plasma by Dowex AG-50W-X8 100- to 200-mesh cation exchange
chromatography as previously
described.11 12 13 14 15 The free amino acids for apoB-48 and
apoB-100 were initially isolated with the Dowex columns and
subsequently with centrifugation at 2000g
for 5 minutes as previously described for apoB-67.12
The amino acids were converted to the n-propyl ester
N-heptafluorobutyramide derivatives before analysis
on a Hewlett-Packard 5890/5988A gas chromatograph/mass
spectrometer.
Isotope enrichment (%) and tracer/tracee ratio (%) were calculated from the observed ion current ratios by the method of Cobelli et al.24 Data in this format are analogous to specific radioactivity in radiotracer experiments. The isotopic enrichment of leucine in the apolipoproteins was expressed as tracer/tracee ratio (%).24
Kinetic Analysis
The kinetics of apoB-100 in the VLDL, IDL, and LDL fractions
were described by a multicompartmental model (Figure 1
) as previously
described.15 The SAAM II program was used to fit the model
to the observed tracer data by a weighted-least-squares approach to
find the best fit as previously described.15 25
Compartment 1 is the plasma amino acid forcing function. Compartment 2
is an intracellular delay compartment representing the
synthesis of apoB in the liver. Compartments 3, 4, 5, and 6
represent plasma VLDL; compartment 7, IDL; and compartment 8,
LDL. The details of the model have been described
previously.15
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The kinetics of apoB-48 in the VLDL fraction were described by the
multicompartmental model shown in Figure 2. The model consists of a precursor
compartment (compartment 1), which is the plasma leucine pool.
Compartment 2 is an intracellular delay compartment accounting for the
synthesis of apoB-48 and the assembly of lipoproteins. Compartment 3
accounts for the kinetics of the TRL containing apoB-48. The fractional
catabolic rate (FCR) of TRL apoB-48 corresponds to the rate of
irreversible loss from compartment 3, k (0,3). The SAAM II
program was used to fit the model to the observed tracer data by use of
a weighted-least-squares approach to find the best
fit.25
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It is assumed that plasma leucine (compartment 1) is the
source of the leucine that is incorporated into apoB and that all apoB
enters the plasma via compartment 3. Therefore, transport rates into
compartment 3 correspond to total apoB-100 and apoB-48
production in both models. Figure 3 shows the mean tracer-tracee ratio for
plasma, VLDL apoB-100, TRL apoB-48, IDL apoB-100, and LDL apoB-100 for
the 19 subjects at each time point. The mean tracer-tracee ratio for
plasma leucine at plateau is 30% higher than that for VLDL apoB-100
at its plateau. Fisher et al26 showed that the labeling of
intracellular and extracellular leucine is not always similar;
therefore, plasma leucine may not accurately reflect the precursor pool
and may not be a suitable forcing function, especially for
slowly-turning-over lipoprotein particles. We assumed a constant
enrichment of the precursor pool and used the VLDL apoB-100 plateau for
liver-derived VLDL apoB-100 as previously described27 and
the TRL apoB-48 plateau for intestinally derived apoB-48. Calculation
of the plateau values was based on the solution of an exponential model
assuming a single compartment. Thus, the VLDL apoB-100 plateau and TRL
apoB-48 plateau tracer-tracee ratio data were used as the forcing
functions to drive the appearance of tracer into apoB-100 and apoB-48,
respectively.
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), TRL apoB-48 processed with the new
methodology (
), IDL apoB-100 (
), TRL apoB-48 processed with the
old methodology (
), and LDL apoB-100 (
). Results are shown as
mean±SEM for all 19 subjects.It was assumed that each subject remains in steady state with respect to apoB-48 and apoB-100 metabolism during the study as previously shown.11 12 13 14 15 Under this condition, the FCR is equivalent to the fractional synthetic rate. ApoB production rates (PRs) were determined by the formula PR (mg · kg-1 · d-1)=[FCR (pools/d)x apoB concentration (mg/dL)xplasma volume (L)]/body wt (kg).13 14 Plasma volume was estimated as 4.5% of body weight.
Statistical Analysis
Data were analyzed with the SysStat program and
presented as mean±SD. Unpaired t tests were
performed. Spearman correlation coefficients were determined.
Probability values 
0.05 were considered to be significant.
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Results |
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Characteristics of the Subjects
Table 1
shows the
characteristics of the subjects. The men were significantly younger and
had significantly lower levels of HDL cholesterol than the
women. The nonfasting plasma apoB-100 concentrations in the VLDL, IDL,
and LDL lipoprotein fractions and apoB-48 in chylomicrons (Table 2) represent means of measures at
all 10 time points during the study period.
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In Vivo Kinetics of ApoB
During the kinetic studies, plasma apoB and lipid concentrations
did not change significantly throughout the infusion period, indicating
steady-state conditions as previously shown.11 12 13 14 15
When cation-exchange columns were used to separate SDS gel fragments
from amino acids in apoB-48 samples, the plateau for TRL apoB-48
was 50% lower than the plateau for VLDL apoB-100 (Figure 3
).
Lichtenstein et al11 reported similar results in 8
subjects. When centrifugation was used to separate SDS
gel fragments from amino acids rather than cation exchange columns, the
plateau for TRL apoB-48 (Figure 3) approached the plateau for
VLDL apoB-100. We previously reported similar findings for apoB-67, a
protein also present in low concentrations, and showed that the
differences in tracer-tracee ratio significantly affected the FCRs for
apoB-67.12 Therefore, centrifugation was
performed to remove SDS gel fragments from all samples obtained from
SDS gels rather than cation exchange chromatography as
previously described for apoB-67.12
Representative VLDL, IDL, and LDL apoB-100 leucine
tracer/tracee ratios and model-predicted values are shown in Figure 4. Representative apoB-48
leucine tracer-tracee ratios and model-predicted values are shown in
Figure 5.
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)
versus time after a primed-constant infusion of
[5,5,5-2H3]leucine over 15 hours in 3 study
subjects: A, subject 1; B, subject 2; and C, subject 3. Observed values
are given as symbols and model-predicted values as lines. No data
points were excluded from the fitting process.
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Table 3 shows the pool sizes (PS),
FCRs, and PRs for apoB-48 and apoB-100 grouped by sex. There were no
sex differences. The remainder of the results are given for the entire
group. The PS of TRL apoB-48, VLDL apoB-100, and LDL apoB-100 were
17±10, 273±167, and 3325±1146 mg, respectively; therefore, the mean
TRL apoB-48 PS was only 6.2% of that of VLDL apoB-100. There was a
trend toward a faster FCR for VLDL apoB-100 than for TRL apoB-48
(6.73±3.48 versus 5.02±2.07 pools/d, respectively,
P=0.06). The mean FCRs for IDL and LDL apoB-100 were
10.07±7.28 and 0.27±0.08 pools/d, respectively. The mean PR of TRL
apoB-48 was 6.5% of VLDL apoB-100 (1.3±0.90 versus 20.06±6.53
mg · kg-1 ·
d-1, P<0.0001). The mean PRs of IDL
and LDL apoB-100 were 7.6±3.9 and 11.3±2.3 mg ·
kg-1 · d-1,
respectively.
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TRL apoB-48 PS was correlated with apoB-48 PR (r=0.780, P<0.0001) but not FCR (r=-0.1810, P=0.458). VLDL apoB-100 PS was correlated with both PR (r=0.713, P=0.0006) and FCR (r=-0.692, P=0.001) of VLDL apoB-100 and with apoB-48 PR (r=0.728, P=0.0004). LDL apoB-100 PS was correlated with FCR (r=-0.549, P=0.015). TRL apoB-48 PR and PS were correlated inversely with VLDL apoB-100 FCR (r=-0.597, P=0.007 and r=-0.721, P=0.0005, respectively) and directly with VLDL apoB-100 PR (r=0.507, P=0.027 and r=0.534, P=0.018). ApoB-48 PS was directly correlated with VLDL apoB-100 PS (r=0.837, P<0.0001).
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Discussion |
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The present study provides information on the interrelationships between TRL apoB-48 and VLDL, IDL, and LDL apoB-100 kinetics in 19 human subjects on an average American diet by multicompartmental modeling. Cell culture studies have shown that the regulation of apoB-containing lipoprotein secretion occurs mainly posttranslationally, because de novo synthesized apoB molecules are targeted for either secretion or intracellular degradation.28 29 30 The availability of lipid appears to play a major role in determining the amount of apoB that is secreted from the liver. Oleic acid significantly stimulates apoB secretion from cultured HepG2 cells29 by facilitating the translocation of newly synthesized apoB away from proteases31 that appear to be associated with the endoplasmic reticulum29 and thereby protecting newly synthesized apoB from intracellular degradation.
On the basis of these in vitro results, we hypothesized that humans who have higher rates of apoB-48 production would have more fat delivered to their liver in the form of chylomicron remnants and therefore have higher rates of production of VLDL apoB-100 from the liver. The positive correlation between apoB-48 PS and VLDL apoB-100 PR observed in the present study suggests that when more lipid and/or cholesteryl ester is delivered to the liver in the form of chylomicron remnants, the secretion of VLDL apoB-100 from the liver is increased. Thus, our results provide support for our hypothesis.
Both chylomicrons and VLDL increase after a fat-rich meal. Cohn et al13 22 showed that the apoB-48 PS is 10-fold higher and VLDL apoB-100 PS is 22% higher in the fed state than in the fasted state. The data in the present study indicating that TRL apoB-48 PS is correlated with PR and not FCR suggest that the increase in chylomicrons is due to an increase in PR. In contrast, VLDL apoB-100 PS is correlated with both production and catabolism of VLDL apoB-100. In addition, the inverse correlation between TRL apoB-48 PS and PR and VLDL apoB-100 FCR suggests that an increase in the apoB-48 PS slows down the catabolism of VLDL apoB-100, a finding suggesting that TRL apoB-48 and VLDL apoB-100 may be competing for the same catabolic pathway. Thus, the increase in VLDL apoB-100 PS after a fat-rich meal is due to increased production and decreased clearance of VLDL apoB-100. These results are supported by previous observations with fat feeding in humans. Bjorkegren et al32 infused a chylomicron-like triglyceride emulsion intravenously for 60 minutes to healthy young men. Using simultaneous stable isotope studies, they observed that concomitant with a 3-fold increase in plasma triglycerides, large VLDL increased and small VLDL decreased. They concluded that the formation of chylomicrons and their remnants during feeding inhibited lipolysis of large VLDL to small VLDL and accounted for the increase in VLDL after meals. In another study, after an oral fat load, Karpe et al33 observed an increase in both small and large chylomicron remnants in control subjects and normotriglyceridemic patients and an increase in only large chylomicron remnants in hypertriglyceridemic patients. They also concluded that chylomicrons compete with VLDL for removal of triglycerides by lipoprotein lipase.
In the present study, we observed that LDL apoB-100 PS was correlated with FCR but not PR. This finding indicates an important role for the LDL receptor in regulating LDL levels in humans. It is important to note that the correlations we observed between apoB-48 and apoB-100 kinetics provide insight into possible mechanisms that can now be examined by studying subjects under various metabolic and dietary conditions.
Our second hypothesis was that the FCRs of apoB-48 and apoB-100 would be similar, a finding Lichtenstein et al11 observed in a monoexponential analysis. However, we observed a trend toward faster catabolism for VLDL apoB-100 than for TRL apoB-48 (P=0.06). ApoB-48 kinetics have been difficult to perform because of the low protein concentration of apoB-48, especially in the fasting state, and difficulty in measuring the concentration of apoB-48.22 34 Some investigators have used retinyl esters as a marker of chylomicron remnant clearance. However, it has been noted that retinyl ester can exchange between lipoproteins and therefore may not be an ideal marker of chylomicron remnants.35 36 ApoB-48 appears to be a better marker for chylomicrons and their remnants in plasma than retinyl palmitate because there is no exchange of apoB-48 between lipoproteins, and the liver secretes only apoB-100 in humans.2
Several studies have used radioactivity to study the kinetics of
apoB-100 and apoB-48 within TRL simultaneously. These
studies are summarized in Table 4. Using
labeled pleural chylomicrons, Schaefer et al37 showed that
<1% of apoB labeled within TRL remained at 6 hours in the fasting
state with virtually no transfer to LDL. Nestel et al9
labeled TRL from hypertriglyceridemic
subjects with 125I and reinjected it into 4
hypertriglyceridemic subjects. ApoB-100 was
cleared more rapidly than apoB-48, whereas the rates were similar when
reinjected into 2 hypertriglyceridemic and
2 normal subjects.9 When
125I-labeled TRL from a
hypertriglyceridemic subject was reinjected
into 2 normal subjects, Stalenhoef et al10 observed rapid
clearance of most of the apoB-48 in 15 minutes and most of the apoB-100
within 30 minutes, whereas in their
hypertriglyceridemic subject, clearance of
both proteins was markedly delayed. In a second study, Stalenhoef et
al38 used 125I-TRL from a
dysbetalipoproteinemic subject; clearance of both TRL apoB-100 and
apoB-48 was impaired. When Schaefer et al39 reinjected
125I-TRL from an apoE-deficient subject, delayed
clearance of both the apoB-48 and apoB-100 occurred in the
apoE-deficient subject compared with rapid clearance in the normal
subjects.
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In these early studies,9 10 38 39 the half-life of VLDL apoB-100 and apoB-48 was much shorter than the longer half-lives observed in the present study and many other recent studies using either radioactivity or endogenous labeling techniques.11 12 13 14 27 40 41 Differences in half-lives between the present study and the previous studies may be due to differences in the design of the studies. In the previous studies, either TRL isolated from hypertriglyceridemic subjects or pleural chylomicrons were used; thus, the particles were abnormal. The TRL was then radioactively labeled and reinjected into both abnormal and normal subjects who were fasting. The isolation, labeling, and concentration of the lipoproteins for radioactive studies may alter the protein and thus affect its FCR. In addition, the populations of particles studied with exogenous labeling may differ from those studied with endogenous labeling. In exogenous labeling studies, lipoproteins are removed at 1 point in time and concentrated; therefore, the population of particles that are labeled are representative of the distribution at that time. As a result, a large proportion of these lipoproteins may consist of remnant-like particles rather than nascent particles, and thus, the kinetics of the nascent particles may not be apparent. In contrast, endogenous labeling ensures that all particles are labeled proportionally, and thus, the kinetics of both nascent and remnant particles should be apparent.
In summary, we have shown that apoB-48 PS is only 6.2% of that of VLDL apoB-100 and is correlated with PR, not FCR; VLDL apoB-100 PS is correlated with both PR and FCR of VLDL apoB-100 and with PR of apoB-48; and LDL PS is correlated with FCR. In addition, there is a trend toward faster catabolism for VLDL apoB-100 than for TRL apoB-48. The analytic and modeling approach in the present study and the improvement in the methodology for preparation of apoB-48 samples for mass spectrometry should permit the use of stable isotopes to elucidate key features of both apoB-48 and apoB-100 metabolism in normal and pathological states and to examine factors influencing dietary responsiveness. Determination of these factors will have important implications for cardiac risk reduction in the primary and secondary prevention of coronary heart disease.
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Acknowledgments |
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This research was supported by grants HL-39326 (E.J.S.), HL-56895 (F.K.W.), HL-49110 (P.H.R.B.), and RR-12609 (P.H.R.B.) from the National Institutes of Health and contract 53-3K06-5-10 from the US Department of Agriculture Research Service (E.J.S.). This work was done during the tenure of a Grant-in-Aid from the American Heart Association and Pfizer, Inc (F.K.W.). Dr Welty is the Irving and Charlotte Rabb Harvard Scholar in Medicine in memory of Dr Grete Bibring and in honor of the 50th anniversary of admission of women to Harvard Medical School. We are extremely grateful to the study subjects for participating in the kinetic studies.
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Appendix 1 |
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TopAbstractIntroductionMethodsResultsDiscussionAppendix 1References |
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For Appendix, please see Tables 5
, 6, and 7. Received February 5, 1999; accepted July 8, 1999.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionAppendix 1References |
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