Plasma metabolism of apolipoprotein A-IV in humans

As assessed by molecular sieve chromatography and quantitation by a specific radioimmunoassay, apoA-IV is associated in plasma with the triglyceride-rich lipoproteins, to a high density lipoprotein (HDL) subfraction of smaller size than HDL₃, and to the plasma lipoprotein-free fraction (LFF). In this study, the turnover of apoA-IV associated to the triglyceride-rich lipoproteins, HDL and LFF was investigated in vivo in normal volunteers. Human apoA-IV isolated from the thoracic duct lymph chylomicrons was radioiodinated and incubated with plasma withdrawn from normal volunteers after a fatty meal. Radioiodinated apoA-IV-labeled triglyceride-rich lipoproteins, HDL, and LFF were then isolated by chromatography on an AcA 34 column. Shortly after the injection of the radioiodinated apoA-IV-labeled triglyceride-rich lipoproteins, most of the radioactivity could be recovered in the HDL and LFF column fractions. On the other hand, when radioiodinated apoA-IV-labeled HDL or LFF were injected, the radioactivity remained with the originally injected fractions at all times. The residence time in plasma of ¹²⁵I-labeled apoA-IV, when injected in association with HDL or LFF, was 1.61 and 0.55 days, respectively. When ¹²⁵I-labeled apoA-IV was injected as a free protein, the radioactivity distributed rapidly among the three plasma pools in proportion to their mass. The overall fractional catabolic rate of apoA-IV in plasma was measured in the three normal subjects and averaged 1.56 pools per day. The mean degradation rate of apoA-IV was 8.69 mg/kg·day. The results are consistent with the conclusions that: 1) apoA-IV is present in human plasma in three distinct metabolic pools; 2) apoA-IV associated with the triglyceride-rich lipoproteins is a precursor to the apoA-IV HDL and LFF pools; 3) apoA-IV in LFF is not a free protein and its turnover rate is faster than that of apoA-IV in HDL; 4) since no transfer of apoA-IV from the HDL or the LFF occurs, these pools may represent a terminal pathway for the catabolism of apoA-IV; and 5) the catabolism of apoA-IV in HDL is dissociated from that of apoA-I although both apoproteins may reside on the same lipoprotein particles.