Increased activity of platelet‐activating factor acetylhydrolase in low‐density lipoprotein subfractions induces enhanced lysophosphatidylcholine production during oxidation in patients with heterozygous familial hypercholesterolaemia

Patients with heterozygous familial hypercholesterolaemia (FH) have elevated plasma concentrations of low‐density lipoprotein (LDL) and develop premature atherosclerosis. There is increasing evidence that oxidative modification of LDL is important for the pathogenesis of atherosclerosis, and the LDL‐associated platelet‐activating factor acetylhydrolase (PAF‐AH) seems to play a key role in LDL oxidation by hydrolysing the oxidized phospholipids of phosphatidylcholine (PC) and producing lysophosphatidylcholine (lyso‐PC). We measured the total serum and high‐density lipoprotein (HDL) levels of PAF‐AH activity and studied the distribution of PAF‐AH activity among three LDL subfractions isolated by gradient ultracentrifugation in 15 patients with heterozygous FH and 13 normolipidaemic control subjects. We also determined the lyso‐PC production in each LDL subfraction during Cu2+‐induced oxidation in vitro. The total serum PAF‐AH activity in heterozygous FH patients was significantly higher than in control subjects, whereas the HDL‐associated PAF‐AH activity, expressed as a percentage of total serum PAF‐AH activity, was significantly lower in the FH patients than in control subjects (13.9 ± 6.6% vs. 30.6 ± 4.4%, P < 0.001). Among the LDL subfractions, the PAF‐AH activity in both normolipidaemic control subjects and FH patients, expressed as nmol mg−1 protein min−1, was significantly higher in the LDL3 subfraction (33.1 ± 4.8 and 53.4 ± 11.5 respectively) than in the LDL2 (18.6 ± 5.3 and 26.8 ± 10.4 respectively, P < 0.0001 for both comparisons) and LDL1 subfractions (5.1 ± 1.5 and 7.8 ± 2.6, respectively, P < 0.0001 for both comparisons). Additionally, the enzyme activity in each LDL subfraction of the heterozygous FH patients was significantly higher than in control subjects (P < 0.02 for LDL1, P < 0.03 for LDL2 and P < 0.0001 for LDL3). No difference was observed in the susceptibility to oxidation of each LDL subfraction among the heterozygous FH patients and the normolipidaemic control subjects. During oxidation, the PAF‐AH activity decreased, whereas the lyso‐PC levels significantly increased in all subfractions of both groups. The lyso‐PC/sphingomyelin molar ratio in each LDL subfraction of the FH patients 3 h after the onset of the oxidation was significantly higher than in control subjects [0.38 ± 0.05 and 0.27 ± 0.04, respectively, for LDL1 (P < 0.006), 0.47 ± 0.08 and 0.39 ± 0.03, respectively, for LDL2 (P < 0.04), 0.55 ± 0.11 and 0.42 ± 0.06, respectively, for LDL3 (P < 0.02)]. Our results show that heterozygous FH patients exhibit higher PAF‐AH activity than control subjects in all LDL subfractions, resulting in higher lyso‐PC production during oxidation, which confers on these subfractions higher biological potency. This phenomenon, in combination with the diminished anti‐atherogenic and antioxidant capability of HDL in these patients due to the relatively low HDL‐cholesterol levels compared with LDL‐cholesterol levels and, consequently, the relatively low HDL‐associated PAF‐AH activity, could contribute to the higher atherogenicity and incidence of coronary artery disease observed in FH patients.