Platelet-activating factor acetylhydrolases: Broad substrate specificity and lipoprotein binding does not modulate the catalytic properties of the plasma enzyme

Platelet-activating factor acetylhydrolases (PAF-AHs) are a group of enzyme s that hydrolyze the sn-2 acetyl ester of PAF (phospholipase A(2) activity) but not phospholipids with two long fatty acyl groups, Our previous studie s showed that membrane-bound human plasma PAF-AH (pPAF-AH) accesses its sub strate only from the aqueous phase, which raises the possibility that this enzyme can hydrolyze a variety of lipid esters that are partially soluble i n the aqueous phase. Here we show that pPAF-AH has broad substrate specific ity in that it hydrolyzes short-chain diacylglycerols, triacylglycerols, an d acetylated alkanols, and displays phospholipase Al activity. On the basis of all of the substrate specificity results, it appears that the minimal s tructural requirement for a good pPAF-AH substrate is the portion of a glyc eride derivative that includes an sn-2 eater and a reasonably hydrophobic c hain in the position occupied by the sn-1 chain. In vivo, pPAF-AH is bound to high and low density lipoproteins, and we show that the apparent maximal velocity for this enzyme is not influenced by lipoprotein binding and that the enzyme hydrolyzes tributyroylglycerol as well as the recombinant pPAF- AH does. Broad substrate specificity is also observed for the structurally homologous PAF-AH which occurs intracellularly [PAF-AH(II)I as well as for the PAF-AH from the lower eukaryote Physarum polycephalum although pPAF-AH and PAF-AH(II) tolerate the removal of the sn-3 headgroup better than the P AF-AH from P. polycephalum does, In contrast, the intracellular PAF-AH foun d in mammalian brain [PAF-AH(ro) alpha1/alpha1 and alpha2/alpha2 homodimers ] is more selectively operative on compounds with a short acetyl chain alth ough this enzyme also displays significant phospholipase A(1) activity.