Effects of deletion of the carboxyl-terminal domain of ApoA-I or of its substitution with helices of ApoA-II on in vitro and in vivo lipoprotein association.

In the present study, the lipoprotein association of apoA-I, an apoA-I (DeltaAla190-Gln243) deletion mutant and an apoA-I (Asp1-Leu189)/apoA-II (Ser12-Gln77) chimera were compared. At equilibrium, 80% of the 125I-labeled apolipoproteins associated with lipoproteins in rabbit or human plasma but with very different distribution profiles. High density lipoprotein (HDL)2,3-associated fractions were 0.60 for apoA-I, 0.30 for the chimera, and 0.15 for the deletion mutant, and corresponding very high density lipoprotein-associated fractions were 0.20, 0.50, and 0.65. Clearance curves after intravenous bolus injection of 125I-labeled apolipoproteins (3 microg/kg) in normolipemic rabbits could be adequately fitted with a sum of three exponential terms, yielding overall plasma clearance rates of 0.028 +/- 0.0012 ml.min-1 for apoA-I (mean +/- S.E.; n = 6), 0.10 +/- 0.008 ml.min-1 for the chimera (p < 0.001 versus apoA-I) and 0.38 +/- 0.022 ml.min-1 for the deletion mutant (p < 0.001 versus apoA-I and versus the chimera). Fractions that were initially cleared with a t1/2 of 3 min, most probably representing free apolipoproteins, were 0.30 +/- 0.04, 0.50 +/- 0.06 (p = 0.02 versus apoA-I), and 0.64 +/- 0.07 (p = 0.002 versus apoA-I), respectively. At 20 min after the bolus, the fractions of injected material associated with HDL2,3 were 0.55 +/- 0.06, 0.25 +/- 0.03 (p = 0.001 versus apoA-I), and 0.09 +/- 0.01 (p < 0.001 versus apoA-I and versus the chimera), respectively, whereas the fractions associated with very high density lipoprotein were 0. 15 +/- 0.006, 0.25 +/- 0.03 (p = 0.008 versus apoA-I), and 0.27 +/- 0.03 (p = 0.003 versus apoA-I), respectively. The ability of the different apolipoproteins to bind to HDL3 particles and displace apoA-I in vitro were compared. The molar ratios at which 50% of 125I-labeled apoA-I was displaced from the surface of HDL3 particles were 1:1 for apoA-I, 3:1 for the chimera and 12:1 for the deletion mutant, indicating 3- and 12-fold reductions of the affinities for HDL3 of the chimera and the deletion mutant, respectively. These data suggest that the carboxyl-terminal pair of helices of apoA-I are involved in the initial rapid binding of apoA-I to the lipid surface of HDL. Although the lipid affinity of apoA-II is higher than that of apoA-I, substitution of the carboxyl-terminal helices of apoA-I with those of apoA-II only partially restores its lipoprotein association. Thus, this substitution may affect cooperative interactions with the middle amphipathic helices of apoA-I that are critical for its specific distribution over the different HDL species.