In peripheral blood, native low-density lipoprotein (LDL) is a major carrier of acetylhydrolase, the enzyme that hydrolyzes the sn-2 acetate of PAF-acether, converting it to lyse PAF-acether. By controlling the level of PAF-acether, the acetylhydrolase may regulate the biologic effects of this potent inflammatory and thrombotic mediator. The biologic oxidation of LDL appears to underlie its atherogenicity. We report here that oxidative modification of LDL led to progressive loss of associated acetylhydrolase activity. Reductions of approximately 90% and 40% of acetylhydrolase activity occurred respectively in LDL oxidized for 24 hours by copper ions (2.5 mu mol/L) in phosphate-buffered saline and in LDL incubated with human monocyte-like THP1 cells in Ham's F-10 medium. Acetylhydrolase activity decreased as a function of the degree of LDL oxidation and was correlated with an increase in net negative charge and in the content of thiobarbituric acid-reactive substances (r=-.94 and r=-.88, respectively; P less than or equal to.001). The acetylhydrolase of mildly oxidized LDL displayed a similar K-m for PAF-acether compared with native LDL, whereas its V-max was lower. Thus, acetylhydrolase conserved its affinity for PAF-acether, whereas a nondefined and noncompetitive inhibitor, apparently produced during oxidation, might account for the observed loss in enzymatic activity. Acetylhydrolase activity was totally recovered in LDL modified by both acetylation and malondialdehyde. In contrast, a reduction of 75% or more in acetylhydrolase activity was observed in LDL modified by 4-hydroxynonenal; this effect may be due to either a direct modification of amino acid side chains of acetylhydrolase by 4-hydroxynonenal or a modification of the phospholipid environment of the enzyme at the surface of the LDL particle. Because PAF-acether is probably synthesized and released by cells present in the atheromatous plaque, it may play a potentially proatherogenic role. Thus, the loss of PAF-acether-degrading acetylhydrolase activity in oxidized LDL confers this lipoprotein with a new atherogenic role.
