Human Apolipoprotein (Apo) B-48 and ApoB-100 Kinetics With Stable Isotopes

Abstract

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 (5,5,5-2 H 3 ) 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 17Ϯ10, 273Ϯ167, and 3325Ϯ1146 mg, respectively. There was a trend toward a faster fractional catabolic rate (FCR) for VLDL apoB-100 than for TRL apoB-48 (6.73Ϯ3.48 versus 5.02Ϯ2.07 pools/d, respectively, Pϭ0.06). The mean FCRs for IDL and LDL apoB-100 were 10.07Ϯ7.28 and 0.27Ϯ0.08 pools/d, respectively. The mean production rate (PR) of TRL apoB-48 was 6.5% of VLDL apoB-100 (1.3Ϯ0.90 versus 20.06Ϯ6.53 mg ⅐ kg. TRL apoB-48 PS was correlated with apoB-48 PR (rϭ0.780, PϽ0.0001) but not FCR (rϭϪ0.1810, Pϭ0.458). VLDL apoB-100 PS was correlated with both PR (rϭ0.713, Pϭ0.0006) and FCR (rϭϪ0.692, Pϭ0.001) of VLDL apoB-100 and by apoB-48 PR (rϭ0.728, Pϭ0.0004). LDL apoB-100 PS was correlated with FCR (rϭϪ0.549, Pϭ0.015). 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 Ⅲ metabolism Ⅲ stable isotopes Ⅲ LDL cholesterol Ⅲ lipoproteins T he 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 chylomi...

Methodology for studying postprandial lipid metabolism

“…From blood, plasma or lipoprotein samples, fatty acids (Bickerton et al, 2007), cholesterol (Beaumier-Gallon et al, 2001) or fat-soluble vitamins (Sauvant et al, 2003) or alternatively apoB-100 or apoB-48 (Welty et al, 1999) can be extracted and specific analyses made using various methods involving mass spectroscopy (GC-MS, GC-C-MS, LC-MS/MS-MS, etc). Determination of isotopic ratio in expired air (breath test) can also be done.…”

Effects of ApoE Genotype on ApoB-48 and ApoB-100 Kinetics With Stable Isotopes in Humans

“…The baseline kinetics for these subjects were reported previously. 18 The experimental protocol for in vivo stable-isotope kinetics, plasma lipid and lipoprotein characterization, quantification and isolation of the apolipoproteins, isotopic enrichment determinations, kinetic analysis, and statistical analysis were performed as previously described. 18 ApoE genotypes were determined by polymerase chain reaction amplification as previously described.…”

Stable isotopes in obesity research

“…The steady state enrichment of the plasma amino acid during the infusion may also be used; however, since its enrichment is usually greater than intracellular enrichment, a correction factor based on the apoB-100 plateau must be used. Welty et al (1999) have shown that the enrichment of VLDL apoB-100 at plateau is approximately 75% of that of plasma. Of note, VLDL apoB-100 does not reach a plateau in subjects with high VLDL (triglyceride) levels; therefore, one may assume that the relationship between the VLDL apoB-100 plateau and plasma amino acid enrichment is the same as in normal subjects assuming no abnormalities of amino acid metabolism.…”

“…For further discussion of apoB-100 models, see the articles by Foster et al (1993) and Cobelli et al (1987). Welty et al (1999) present models for apoB-100 and apoB-48. apoA-I, A-II, and apoE models are currently under development by Welty and will be similar to the apoB-48 model (Welty et al, 1999).…”

“…Welty et al (1999) present models for apoB-100 and apoB-48. apoA-I, A-II, and apoE models are currently under development by Welty and will be similar to the apoB-48 model (Welty et al, 1999).…”