Platelet-activating Factor Acetylhydrolase, and Not Paraoxonase-1, Is the Oxidized Phospholipid Hydrolase of High Density Lipoprotein Particles

Abstract

Paraoxonase-1 (PON1), an high density lipoprotein (HDL)-associated organophosphate triesterase, suppresses atherosclerosis in an unknown way. Purified PON1 protects lipoprotein particles from oxidative modification and hydrolyzes pro-atherogenic oxidized phospholipids and the inflammatory mediator plateletactivating factor (PAF). We find human PON1 acted as a phospholipase A 2 but not as a phospholipase C or D through cleavage of phosphodiester bonds as expected. PON1 requires divalent cations, but EDTA did not block the phospholipase A 2 activity of PON1. In contrast, a serine esterase inhibitor abolished phospholipase activity even though PON1 has no active-site serine residues. PAF acetylhydrolase, an oxidized phospholipid phospholipase A 2 , is a serine esterase associated with specific HDL particles. Western blotting did not reveal detectable amounts of PAF acetylhydrolase in PON1 preparations, although very low amounts of PAF acetylhydrolase might still account for PON1 phospholipase A 2 activity. We revised the standard PON1 purification by first depleting HDL of PAF acetylhydrolase to find PON1 purified in this way no longer hydrolyzed oxidized phospholipids or PAF. Serum from a donor with an inactivating mutation in the PAF acetylhydrolase gene did not hydrolyze oxidized phospholipids or PAF, yet displayed full paraoxonase activity. We conclude that PAF acetylhydrolase is the sole phospholipase A 2 of HDL and that PON1 has no phospholipase activity toward PAF or pro-atherogenic oxidized phospholipids. Paraoxonase-1 (PON1)1 catalyzes the hydrolysis of organophosphorous triesters and aryl carboxyl esters; it is of interest because of its hydrolysis of paraoxon (a metabolite of the insecticide parathion) and neurotoxins such as sarin and diisopro-

High density lipoproteins and endothelial functions: mechanistic insights and alterations in cardiovascular disease

“…Several experimental studies have consistently demonstrated the capacity of HDL to modulate eNOS expression and to stimulate endothelial NO production in vitro and in vivo ( 10,13,15,(97)(98)(99). Moreover, in human studies, administration of reconstituted HDL has been shown to improve endothelial function in subjects with can hydrolyze oxidized phospholipids ( 77,78 ). In arteries of non hyperlipidemic rabbits, local expression of PAF-AH reduced the accumulation of oxidatively modifi ed LDL without changing plasma levels of PAF-AH and reduced the expression of endothelial cell adhesion molecules ( 79 ).…”

Dysfunctional HDL: From Structure-Function-Relationships to Biomarkers

“…In vivo study in mice deficient for LDL receptor and leptin revealed that LCAT overexpression decreased autoantibodies to oxLDL (Mertens et al 2003). Besides LCAT and PON1, PAF-AH, another HDL-associated enzyme, nowadays also known as lipoproteinassociated phospholipase A2 (LpPLA2), is able to hydrolyze oxidized phospholipids (Marathe et al 2003;Noto et al 2003). The local expression of PAF-AH in arteries of non-hyperlipidemic rabbits reduced the accumulation of oxidatively modified LDL without changing plasma levels of PAF-AH and reduced the expression of endothelial cell adhesion molecules (Arakawa et al 2005).…”

Genotype‐Phenotype Correlations

“…14 PON has also been identified as a homocysteine thiolactonase 15 and possesses PAF-AH-like activity. 16 Although a recent study has shown that PON1 had no phospholipase A 2 activity, 17 conflicting results have been reported. 18 Therefore, the protecting role of PON is the subject of considerable debate.…”