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Evaluation of apoA-I kinetics in humans using simultaneous endogenous stable isotope and exogenous radiotracer methods



Evaluation of apoA-I kinetics in humans using simultaneous endogenous stable isotope and exogenous radiotracer methods



Journal of Lipid Research 34(12): 2207-2215



Apolipoprotein A-I is the major apolipoprotein constituent of high density lipoproteins (HDL). Methods used to investigate in vivo kinetics of apoA-I include exogenous labeling with radioiodine and endogenous labeling with stable isotopically labeled amino acids. We report here a direct comparison of these methods to determine the in vivo kinetics of apoA-I in four normal subjects. Purified apoA-I was labeled with 125I, reassociated with autologous plasma, and injected into study subjects. At the same time, (13C-6)phenylalanine was administered as a primed constant infusion for up to 14 hours. The kinetic parameters of apoA-I were determined from the 125I-labeled apoA-I plasma curves. For the analysis of data from stable isotope studies, very low density lipoprotein (VLDL) apoB-100, VLDL apoB-48, and total apoA-I were isolated by ultracentrifugation and subsequent preparative NaDodSO-4-PAGE, hydrolyzed, and derivatized. The tracer/tracee ratio was determined by gas chromatography-mass spectrometry. Monoexponential function analysis was used to determine the tracer/tracee curves of VLDL apoB-100 and VLDL apoB-,48, and total apoA-I. The mean plateau tracer/tracee ratio of VLDL apoB-100 (primarily liver-derived) was 5.19%, whereas that of VLDL apoB-48 (intestinally derived) was only 3.74%. Using the VLDL apoB-100 plateau tracer/tracee ratio as the estimate of the precursor pool enrichment for apoA-I, the mean apoA-I residence time (RT) was 5.14 +- 0.41 days, compared with 4.80 +- 0.30 days for the exogenous labeling method. The apoA-I RTs using these two methods were highly correlated (r = 0.874). We also used several different assumptions about the relative contribution of the liver and the intestine to the total plasma apoA-I pool and compared the kinetic parameters obtained with each of these assumptions to those obtained with the exogenous radiotracer method. The assumption that the liver contributed 90% of the total apoA-I pool resulted in the closest agreement between methods (RT 4.85 +- 0.35 days by stable isotope). In addition, the mean RT of VLDL apoB-48 was 3.9 hours, significantly longer than that of VLDL apoB-100 of 1.9 hours. These data indicate that endogenous labeling of apoA-I by primed constant infusion using the VLDL apoB-100 plateau tracer/tracee ratio as the estimate of the precursor pool tracer/tracee ratio for apoA-I provides kinetic parameters that are highly comparable with those obtained by the exogenous labeling method.

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Accession: 002374658

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PMID: 8301239


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