LDL-ASSOCIATED PHOSPHOLIPASE-A DOES NOT PROTECT LDL AGAINST LIPID-PEROXIDATION IN-VITRO

The irreversible proteinase inhibitor Pefabloc (4-[2-aminoethyl] benze nesulfonyl fluoride) inactivates LDL-catalyzed hydrolysis of the short -chain fluorescent phospholipid C-6-NBD-PC -2-(N-4-nitrobenzo-2-oxa-1, 3-diazole)-aminocaproyl phosphatidylcholine). The dose-dependence of t his inactivation is similar to that obtained previously for the inhibi tory effect of Pefabloc on the hydrolysis of platelet activating facto r (PAF) by the LDL-associated PAF acetylhydrolase (PAF-AH), in agreeme nt with the notion that the hydrolysis of C-6-NBD-PC and PAF is cataly zed by the same enzyme (LDL-associated phospholipase A; LDL-PLA). This conclusion is also supported by the finding that hydrolysis of C-6-NB D-PC by LDL becomes inactivated by LDL oxidation only at late stages o f the oxidation, similar to the effect of oxidation on the hydrolysis of PAF by the LDL-associated PAF-AH. Under conditions of complete inac tivation of this enzyme towards C-6-NBD-PC, the kinetics of lipid pero xidation, induced either by copper ions or by the free radical generat or AAPH at varying doses of the prooxidant, was similar to that observ ed when the PLA was active (i.e., in the absence of Pefabloc). Hence, LDL-associated PLA (PAF-AH) does not protect LDL lipids from peroxidat ion. Similar results were obtained with fractionated LDL in albumin-co ntaining buffer and for non-fractionated serum, in which copper-induce d peroxidation was also not influenced by inactivation of the enzyme r esponsible for hydrolysis of C-6-NBD-PC. Phospholipolysis of short cha in phospholipids by LDL-PLA may still play a protective role against t he toxic effects of oxidized phospholipids by reducing their internali zation into cells ( Schmitt et al. 1995). (C) 1998 Elsevier Science In c.