PAF-acetylhydrolases.

Platelet-activating factor acetylhydrolases (PAF-AHs, EC 3.1.1.47) constitute a unique subfamily of phospholipases A(2), specific for short acyl chains in the sn-2 position of the phospholipid. Their primary substrate is the platelet-activating factor, PAF, from which they cleave an acetyl moiety with concomitant release of lysoPAF. However, some acetylhydrolase will also hydrolyze other polar phospholipids with up to 6-carbons long acyl chains in the sn-2 position. PAF-acetylhydrolases are diverse enzymes, and the well-characterized isoforms are serine-dependent hydrolases, which do not require Ca(2+) for activity. Given the existence of two pools of PAF, intra- and extracellular, the acetylhydrolases can be divided into two subclasses: those found in the cytosol and enzymes secreted to blood plasma or other body fluids. Recent crystallographic studies shed new light on the complex structure-function relationships in PAF-AHs.