Evaluation of apoA-I kinetics in humans using simultaneous endogenous stable isotope and exogenous radiotracer methods.
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, [13C6]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 NaDodSO4-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).(ABSTRACT TRUNCATED AT 250 WORDS)
Ikewaki
K
K
Molecular Disease Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892.
Rader
D J
DJ
Schaefer
J R
JR
Fairwell
T
T
Zech
L A
LA
Brewer
H B
HB
D000328:Adult; D016632:Apolipoprotein A-I / Q000378:metabolism*; D053299:Apolipoprotein B-100; D053283:Apolipoprotein B-48; D001055:Apolipoproteins B / Q000378:metabolism; D002247:Carbon Isotopes; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007457:Iodine Radioisotopes; D007700:Kinetics; D010649:Phenylalanine; D014461:Ultracentrifugation
D016632:Apolipoprotein A-I; D053299:Apolipoprotein B-100; D053283:Apolipoprotein B-48; D001055:Apolipoproteins B; D002247:Carbon Isotopes; D007457:Iodine Radioisotopes; D010649:Phenylalanine