Nonuniform radiolabeling of VLDL apolipoprotein B: implications for the analysis of studies of the kinetics of the metabolism of lipoproteins containing apolipoprotein B - PubMed (original) (raw)

. 1990 Jun;31(6):1031-42.

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Nonuniform radiolabeling of VLDL apolipoprotein B: implications for the analysis of studies of the kinetics of the metabolism of lipoproteins containing apolipoprotein B

R Ramakrishnan et al. J Lipid Res. 1990 Jun.

Abstract

Radiolabeling of whole lipoproteins or individual apolipoproteins has been an essential tool for the determination of the kinetics of apolipoprotein metabolism in vivo. Mathematical analysis of specific radioactivity (SA) or total radioactivity data has demonstrated the existence of significant complexity in the plasma decay curves of several apolipoproteins. Results obtained during development of methods to study the metabolism of apolipoprotein B (apoB) in very low density lipoprotein (VLDL) subclasses isolated according to flotation (Sf) rates from whole radiolabeled (d less than 1.006 g/ml) VLDL suggested nonuniform radiolabeling of apoB in the three Sf subclasses being studied. We therefore determined apoB SA in VLDL Sf subclasses in ten hypertriglyceridemic and five normal subjects. After radioiodination of apoB in whole VLDL, different apoB SA were found in Sf 400-100, Sf 100-60, and Sf 60-20. The pattern of labeling was quite variable among subjects. On average, apoB SA in the VLDL tracer was greatest in Sf 400-100, and least in Sf 60-20. Nonuniform labeling could also be demonstrated in five studies in which samples were obtained 3 min after intravenous injection of the tracer into subjects with a wide range of plasma triglycerides. Nonuniform labeling of apoB in whole VLDL was also demonstrated in two of the subjects by isolating subclasses of their VLDL that did not bind to an anti-apolipoprotein E immunoaffinity column. These results indicate that the usual assumption of homogeneous labeling of apoB may be erroneous. We have derived a simple mathematical formula to study the consequences of this assumption in estimating kinetic parameters. It is shown that an erroneous assumption of homogeneous tracer labeling may significantly underestimate or overestimate the true production rate, even in a simple two-pool model. Identification of labeling characteristics and incorporation of this information into the mathematical analysis of the plasma radioactivity data can improve the accuracy of the analysis as well as the sensitivity of compartmental models generated by such data.

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Figures

Fig. 1

Fig. 1

Distribution of radioactivity in a 2–16% polyacrylamide gel. Aliquots of whole VLDL, Sf 400–100, Sf 100–60, and Sf 60–20 were subjected to the TMU method, the resulting apoB pellets were resolubilized, and the solubilized material was electrophoresed. The gels were cut into lanes, each lane was divided into 2- to 3-mm slices, and the radioactivity was determined in a gamma counter. There was a single peak of radioactivity for each sample in the first two slices and there were no significant differences in the remaining slices among the samples.

Fig. 2

Fig. 2

ApoB SA in VLDL subfractions for the first 60 min after a bolus injection of radiolabeled whole VLDL. Lines are drawn freehand to provide visual continuity; A, subject 12; B, subject 13.

Fig. 3

Fig. 3

A general n-pool model for the turnover of VLDL apoB. Mv is the total mass of apoB in VLDL, mi is the mass fraction in the i-th pool; ui is the initial SA in the i-th pool; R0i is the apoB flux out of the i-th pool, converted to IDL and/or LDL or removed from plasma. The dashed lines among the pools indicate possible fluxes (in any direction). The dashed box surrounds pools that are intravascular and are included in the measurement of whole VLDL SA. In this case, all VLDL pools are circulating and intravascular, by assumption.

Fig. 4

Fig. 4

An n-pool model for the turnover of VLDL apoB, similar to that in Fig. 3, but flux out of VLDL is from only one pool, the n-th pqol.

Fig. 5

Fig. 5

A two-pool cascade model for the turnover of VLDL apoB. Here, m1 is the mass fraction in the first pool; m2 (= 1 − m1) is the mass fraction in the second pool; u1 and u2 are initial SA in the two pools; R01 is the flux out of pool 1, which may be removed from plasma or converted to IDL and/or LDL; R02 is the corresponding flux out of pool 2.

Fig. 6

Fig. 6

The ratio of the calculated production rate, under the uniform labeling assumption, to the true production rate, as a function of the ratio of specific activities in the two pools of the model shown in Fig. 5; A, the case with 30% of the total mass in pool 2; B, 50%; C, 70%. In each panel, the curves numbered 1–6 correspond to six different values of R02Rv0 (the fraction of total flux that exits from pool 2 ) −0.1, 0.3, 0.5, 0.7, 0.9, and 1.0, respectively. The highlighted regions correspond to the range of specific activity ratios that we have observed.

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References

    1. Langer T, Strober W, Levy RI. The metabolism of low density lipoprotein in familial type II hyperlipoproteinemia. J Clin Invest. 1972;51:1528–1536. - PMC - PubMed
    1. Janus ED, Nicoll A, Wootton R, Turner PR, Magill PJ, Lewis B. Quantitative studies of very low density lipoprotein: conversion to low density lipoprotein in normal controls and primary hyperlipidaemic states and the role of direct secretion of low density lipoprotein in heterozygous familial hypercholesterolaemia. Eur J Clin Invest. 1980;10:149–159. - PubMed
    1. Bilheimer DW, Stone NJ, Grundy SM. Metabolism studies in familial hypercholesterolemia: evidence for a gene-dose effect in vivo. J Clin Invest. 1979;64:524–533. - PMC - PubMed
    1. Fisher WR, Zech LA, Bardalaye P, Warmke G, Berman M. The metabolism of apolipoprotein B in subjects with hypertriglyceridemia and polydisperse LDL. J Lipid Res. 1980;21:760–774. - PubMed
    1. Chait A, Albers JJ, Brunzell JD. Very low density lipoprotein overproduction in genetic forms of hypertriglyceridemia. Eur J Clin Invest. 1980;10:17–22. - PubMed

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