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

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, IC50 0.9 mM) and with the fluorescent and active site-directed probe 4-hexyl-(6′-O-butyl-(4′-pyrenyl))-benzoic estersulfonyl fluoride (2) (KI(inact) = 525 μM), 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 Cu2+ (20 μM, 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.