Evidence that lipid hydroperoxides inhibit plasma lecithin : cholesterol acyltransferase activity

The oxidation of low density lipoproteins (LDL) has been implicated in the development of atherosclerosis, Recently, we found that polar lipids isolat ed from minimally oxidized LDL produced a dramatic inhibition of lecithin: cholesterol acyltransferase (LCAT) activity, suggesting that HDL-cholestero l transport may be impaired during early atherogenesis, In this study, we h ave identified molecular species of oxidized lipids that are potent inhibit ors of LCAT activity, Treatment of LDL with soybean lipoxygenase generated small quantities of lipid hydroperoxides (20 +/- 4 nmol/mg LDL protein, n = 3); but when lipoxygenase-treated LDL (1 mg protein/ml) was recombined wit h the d > 1.063 g/ml fraction of human plasma, LCAT activity was rapidly in hibited (25 +/- 1 and 65 +/- 16% reductions by I and 3 h, respectively). As phospholipid hydroperoxides (PL-OOH) are the principal oxidation products associated with lipoxygenase-treated LDL, we directly tested whether PL-OOH inhibited plasma LCAT activity. Detailed dose-response curves revealed tha t as little as 0.2 and 1.0 mole % enrichment of plasma with PL-OOH produced 20 and 50% reductions in LCAT activity by 2 h, respectively To gain insigh t into the mechanism of LCAT impairment, the enzyme's free cysteines (Cys31 and Cys184) and active site residues were "capped" with the reversible sul fhydryl compound, DTNB, during exposure to either minimally oxidized LDL or PL-OOH. Reversal of the DTNB "cap" after such exposures revealed that the enzyme was completely protected from both sources of peroxidized pbospholip ids. We, therefore, conclude that PL-OOH inhibited plasma LCAT activity by modifying the enzyme's free cysteine and/or catalytic residues. These studi es are the first to suggest that PL-OOH may accelerate the atherogenic proc ess by impairing LCAT activity.