EFFECT OF APOLIPOPROTEIN-A-I DEFICIENCY ON LECITHIN-CHOLESTEROL ACYLTRANSFERASE ACTIVATION IN MOUSE PLASMA

Plasma cholesteryl ester (CE) synthesis by lecithin cholesterol acyltr ansferase (LCAT) is activated by apolipoprotein (apo)A-I. We studied t he effect of plasma apoA-I concen tration on LCAT activation, using no rmal, heterozygous or homozygous apoA-I-deficient mice made by gene ta rgeting. Plasma esterified cholesterol concentrations of mice fed chow diets were ordered (mean +/- SEM): 105 +/- 7 (normal)>70 +/- 5 (heter ozygotes) > 26 +/- 2 (homozygotes) mg/dl. Plasma free cholesterol conc entrations were similar among the three genotypes. Endogenous LCAT act ivity, measured as the decrease in plasma free cholesterol after a 1 h incubation at 37 degrees C, was ordered: 44 +/- 3 (normal) > 21 +/- 2 (heterozygotes) > 5 +/- 1 (homozygotes) nmol CE formed/h per ml plasm a. Using a recombinant exogenous substrate consisting of egg yolk phos pholipid, [C-14]cholesterol, and apoA-I, CE formation of normals and h eterozygotes was similar (27.4 +/- 0.6 and 28.8 +/- 1.3 nmol/h per ml plasma, respectively), but was significantly less for homozygotes (19. 2 +/- 1.7 nmol/h per ml plasma). However, using a small unilamellar ve sicle substrate particle containing phospholipid and [C-14]cholesterol , CE formation was ordered: 1.6 +/- 0.1 (normal) = 1.6 +/- 0.1 (hetero zygotes) > 0.6 +/- 0.1(homozygotes) nmol/h per ml plasma; addition of apoA-I to the plasma of homozygous animals restored CE formation to no rmal levels (1.6 +/- 0.1). CE fatty acid analysis demonstrated that pl asma from homozygous mice contained significantly more saturated and m onounsaturated and fewer polyunsaturated fatty acids compared to norma l and heterozygous mice. We conclude that mice with no detectable plas ma apoA-I have marked reductions in plasma CE: concentrations (70%) an d CE synthesis (60%) and a more saturated plasma CE fatty acid profile , which likely reflects an increased hepatic contribution to the plasm a CE pool. The decreased plasma CE synthesis in homozygous apoA-I-defi cient mice is primarily due to a deficiency in apoA-I activator protei n, but is also partly the result of a decrease in HDL substrate partic les and LCAT enzyme activity. In addition, one half of the gene dosage of apoA-I in mice is sufficient for activation of plasma LCAT.