The redistribution and metabolism of iodinated apolipoprotein A-IV in rats

The metabolic fate of 125I-labeled apolipoprotein A-IV has been studied in rats. This was achieved by incorporating labeled apolipoprotein A-IV into lymph chylomicrons and plasma high density lipoprotein by in vitro exchange or by association, in vivo of apolipoprotein A-IV with plasma chylomicrons and high density lipoprotein following injection of 125I-labelled apolipoprotein A-IV into fat-fed rats. After separating lipoprotein-bound from unbound apolipoprotein A-IV, the interrelationship of apolipoprotein A-IV between lipoproteins was studied by administering 125I-labeled apolipoprotein A-IV/lipoproteins into rats and bleeding at various times thereafter. Apolipoprotein A-IV was rapidly removed from chylomicrons and transferred into high density lipoprotein and lipoprotein-free fractions. The time course of transfer, measured both as absolute radioactivity and as specific radioactivity was consistent with a precursor-product relationship between apolipoprotein A-IV in the lipoprotein-free fraction and high density lipoprotein, suggesting that the rapid transfer of apolipoprotein A-IV from chylomicrons into the lipoprotein-free form precedes transfer into the high density lipoprotein fraction. Since it could be demonstrated by electroimmunoassay that approximately half the total apolipoprotein A-IV in rat plasma was not associated with lipoproteins, it is concluded that a pool of lipoprotein-free apolipoprotein A-IV exists which may have an important physiological role in the lipoprotein transport system.