Release of free F₂-isoprostanes from esterified phospholipids is catalyzed by intracellular and plasma platelet-activating factor acetylhydrolases

F₂-isoprostanes are produced in vivo by nonenzymatic peroxidation of arachidonic acid esterified in phospholipids. Increased urinary and plasma F₂-isoprostane levels are associated with a number of human diseases. These metabolites are regarded as excellent markers of oxidant stress in vivo. Isoprostanes are initially generated in situ, i.e. when the arachidonate precursor is esterified in phospholipids, and they are subsequently released in free form. Although the mechanism(s) responsible for the release of free isoprostanes after in situ generation in membrane phospholipids is, for the most part, unknown, this process is likely mediated by phospholipase A ₂ activity(ies). Here we reported that human plasma contains an enzymatic activity that catalyzes this reaction. The activity associates with high density and low density lipoprotein and comigrates with platelet-activating factor (PAF) acetylhydrolase on KBr density gradients. Plasma samples from subjects deficient in PAF acetylhydrolase do not release F₂- isoprostanes from esterified precursors. The intracellular PAF acetylhydrolase II, which shares homology to the plasma enzyme, also catalyzes this reaction. We found that both the intracellular and plasma PAF acetylhydrolases have high affinity for esterified F₂-isoprostanes. However, the rate of esterified F₂-isoprostane hydrolysis is much slower compared with the rate of hydrolysis of other substrates utilized by these enzymes. Studies using PAF acetylhydrolase transgenic mice indicated that these animals have a higher capacity to release F₂-isoprostanes compared with nontransgenic littermates. Our results suggested that PAF acetylhydrolases play key roles in the hydrolysis of F₂-isoprostanes esterified on phospholipids in vivo.