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
P. Holvoet et al., 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, The Journal of biological chemistry, 271(32), 1996, pp. 19395-19401
In the present study, the lipoprotein association of apoA-I, an apoA-I
(Delta Ala(190)-Gln(243)) deletion mutant and an apoA-I (Asp(1)-Leu(1
89))/apoA-II (Ser(12)-Gln(77)) chimera were compared. At equilibrium,
80% of the I-125-labeled apolipoproteins associated with lipoproteins
in rabbit or human plasma but with very different distribution profile
s. 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, a
nd corresponding very high density lipoprotein-associated fractions we
re 0.20, 0.50, and 0.65. Clearance curves after intravenous bolus inje
ction of I-125-labeled apolipoproteins (3 mu g/kg) in normolipemic rab
bits 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 chime
ra). 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 inj
ected material associated with HDL(2,3) were 0.55 +/- 0.06, 0.25 +/- 0
.03 (p = 0.001 versus apoA-I), and 0.09 +/- 0.01 (p < 0.001 versus apo
A-I and versus the chimera), respectively, whereas the fractions assoc
iated 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 a
poA-I), respectively. The ability of the different apolipoproteins to
bind to HDL, particles and displace apoA-I in vitro were compared, The
molar ratios at which 50% of I-125-labeled apoA-I was displaced from
the surface of HDL, particles were 1:1 for apoA-I, 3:1 for the chimera
and 12:1 for the deletion mutant, indicating 3- and 12-fold reduction
s of the affinities for HDL, of the chimera and the deletion mutant, r
espectively. These data suggest that the carboxyl-terminal pair of hel
ices 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 hi
gher than that of apoA-I, substitution of the carboxyl-terminal helice
s of apoA-I with those of apoA-II only partially restores its lipoprot
ein association. Thus, this substitution may affect cooperative intera
ctions with the middle amphipathic helices of apoA-I that are critical
for its specific distribution over the different HDL species.