Dynamic properties of human high density lipoprotein apoproteins

This study was designed to identify a method for the measurement of human high density lipoprotein subfraction (HDL ₂ and HDL ₃) metabolism. Apolipoproteins A-I, A-II, and C, the major HDL apoproteins, were radioiodinated and incorporated individually into HDL ₂ and HDL ₃ in vitro. Using a double label technique, the turnover of apoA-I in HDL ₂ and HDL ₃ was measured simultaneously in a normal male. The apoprotein exchanged rapidly between the two subfractions evidenced by equilibration of their apoA-I specific activity. Radiolabeled apoA-II, incorporated into the subfractions, showed a similar exchange in vitro. Incubation of ¹³¹I-labeled very low density lipoproteins (VLDL) with HDL or its subfractions resulted in transfer of C proteins from VLDL to the HDL moiety. The extent of transfer was dependent on the HDL subfraction present; 50% of the VLDL apoC was transferred to HDL ₃, while the transfer to total HDL and HDL ₂ was 69% and 78%, respectively. ApoC also exchanged between HDL ₂ and HDL ₃, again showing a preference for the former and suggesting a primary metabolic relationship between VLDL and HDL ₂. Overall, the study indicates that apoA-I, apoA-II, and the C proteins exist in equilibrium between HDL ₂ and HDL ₃. This phenomenon precludes their use as probes for HDL subfraction metabolism in humans.