Enhanced Association of Platelet-activating Factor Acetylhydrolase with Lipoprotein (a) in Comparison with Low Density Lipoprotein

Abstract

Paired samples of human Lipoprotein (a) (Lp(a)) and low density lipoprotein (LDL) were assayed for their platelet-activating factor (PAF) acetylhydrolase activity. Lp(a) displayed markedly enhanced PAF acetylhydrolase activity (approximately 7-fold based on equal particle concentrations) in comparison to LDL isolated from the same individual. Lp(a)-associated acetylhydrolase exhibited properties observed for LDL-associated acetylhydrolase as well as for the purified enzyme; significant inhibition was obtained by treatment with diisopropylfluorophosphate (1 mM, 90%) and phenylmethanesulfonyl fluoride (5 mM, 50%). Furthermore, the hydrolytic activity of both lipoproteins was abolished with paraoxon (6 mM, IC 50 0.9 mM) and with the fluorescent and active site-directed probe 4-hexyl-(6-O-butyl-(4-pyrenyl))-benzoic estersulfonyl fluoride (2) (K I(inact) ‫؍‬ 525 M), a novel irreversible inhibitor of PAF acetylhydrolase. Treatment with 2 and subsequent quantitation of protein-bound fluorescence suggests an increased concentration of enzyme associated to Lp(a) rather than alterations of kinetic constants due to the additional apolipoprotein apolipoprotein (a). Exposure of Lp(a) to Cu 2؉ (20 M, 37°C) was followed by a concomitant decrease of hydrolytic activity. A reduction of the basal activity by 91% was found after 15 h. Whereas immunoprecipitation with anti-apoB antiserum could remove enzymatic activity of Lp(a) regardless of a reductive treatment with dithiothreitol, precipitation with anti-apolipoprotein (a)-antibodies was accompanied by a minor reduction (approximately 30%) of the PAF-hydrolyzing ability. These results suggest that PAF acetylhydrolase exhibits an enhanced association with Lp(a) due to an increased affinity to Lp(a) apolipoprotein B. Lp(a)1 is a lipoprotein from human plasma differing from low density lipoprotein by apo(a) an additional unique glycoprotein (Ref. 1; for review see Ref.2). Apo(a) is linked to apoB, the main protein component of LDL, by a single disulfide bond (3, 4) and exhibits a homology to plasminogen (5). In clinical studies, elevated Lp(a) plasma concentrations have been shown to correlate with the incidence of stroke and coronary heart disease (6 -8), and an accumulation in atherosclerotic lesions has been demonstrated (9), features that provide evidence for elevated Lp(a) levels as a substantial risk factor. Moreover, an overexpression of apo(a) promotes the development of lesions in transgenic animals (10).

Platelet-Activating Factor Acetylhydrolase Activity Indicates Angiographic Coronary Artery Disease Independently of Systemic Inflammation and Other Risk Factors

“…22 Furthermore, the LDL radius in the present study was estimated from the density range 1.006 to 1.063 kg/L, thus comprising IDL as well. Although PAF-AH was shown to reside on Lp(a), 30 it remains controversial how strongly PAF-AH activity is associated with Lp(a); recently, it was shown that the fluvastatin-induced decrease of PAF-AH activity in patients with type 2 diabetes was independent of Lp(a). 22 One might speculate how PAF-AH mediates increased CAD risk independently of other risk factors.…”

Platelet-Activating Factor Acetylhydrolase Is Mainly Associated With Electronegative Low-Density Lipoprotein Subfraction

“…7,8 Finally, the intensity of this band by protein staining ( Figure 3A) was much higher than when revealed by Western blot ( Figure 3B); this observation is inconsistent with the finding that Lp(a) contains a 7-fold higher amount of PAF-AH than LDL. 11 Hence, we concluded that this band of greater than normal-sized LDL was due to small aggregates.…”

“…7 This density range was chosen to avoid contamination with lipoprotein(a) [Lp(a)], which is known to have a high amount of PAF-AH. 11 Composition of LDL fractions, including total and free cholesterol, triglyceride, phospholipid, apolipoprotein (apo) B, and NEFA, was determined as described previously. 7 Because ultracentrifugation was reported to promote the displacement of PAF-AH from LDL to HDL or lipoprotein-deficient serum, 12 an alternative method to ultracentrifugation was used.…”

Lysine-Phosphatidylcholine Adducts in Kringle V Impart Unique Immunological and Potential Pro-inflammatory Properties to Human Apolipoprotein(a)

“…Second, based on the observations made in three laboratories, apo(a) partially prevents LDL from undergoing oxidation (4,25,26). Third, Lp(a) has a higher concentration of platelet-activating factor acetylhydrolase than authentic LDL (27), a situation that would promote a decrease rather than accumulation of oxPtdPC. According to this concept, the transport of oxPtdPCs in apo(a) would be dependent on their ability to link to the reactive lysines in KV.…”