LDL modified by hypochlorous acid is a potent inhibitor of lecithin-cholesterol acyltransferase activity

Modification of low density lipoprotein (LDL) by myeloperoxidase-generated HOCl has been implicated in human atherosclerosis, Incubation of LDL with H OCl generates several reactive intermediates, primarily N-chloramines, whic h may react with other biomolecules. In this study, we investigated the eff ects of HOCl-modified LDL on the activity of lecithin-cholesterol acyltrans ferase (LCAT), an enzyme essential for high density lipoprotein maturation and the antiatherogenic reverse cholesterol transport pathway. We exposed h uman LDL (0.5 mg protein/mL) to physiological concentrations of HOCl (25 to 200 mu mol/L) and characterized the resulting LDL modifications to apolipo protein B and lipids; the modified LDL was subsequently incubated with apol ipoprotein B-depleted plasma (density >1.063 g/mL fraction), which contains functional LCAT. Increasing concentrations of HOCl caused various modifica tions to LDL, primarily, loss of lysine residues and increases in N-chloram ines and electrophoretic mobility, whereas lipid hydroperoxides were only m inor products. LCAT activity was extremely sensitive to HOCl-modified LDL a nd was reduced by 23% and 93% by LDL preincubated with 25 and 100 mu mol/L HOCl, respectively. Addition of 200 rho mol/L ascorbate or N-acetyl derivat ives of cysteine or methionine completely prevented LCAT inactivation by LD L preincubated with less than or equal to 200 mu mol/L HOCl. Protecting the free thiol groups of LCAT with 5,5 ' -dithio-bis-(2-nitrobenzoic acid) bef ore exposure to HOCl-modified LDL, which inhibits lipid hydroperoxide-media ted inactivation of LCAT, failed to prevent the loss of enzyme activity. Ou r data indicate that N-chloramines from HOCl-modified LDL mediate the loss of plasma LCAT activity and provide a novel mechanism by which myeloperoxid ase-generated HOCl may promote atherogenesis.