OXIDATION OF LOW-DENSITY-LIPOPROTEIN BY HYPOCHLORITE CAUSES AGGREGATION THAT IS MEDIATED BY MODIFICATION OF LYSINE RESIDUES RATHER THAN LIPID OXIDATION

Peroxidation of low-density lipoprotein (LDL) lipid is generally thought to represent the initial step in a series of modification reactions that ultimately transform the protein moiety of the lipoprotein into a form recognized by receptors different from those that bind native LDL. Uptake of LDL via these alternative receptors can lead to the formation of lipid-laden cells, which are typical for the early stages of atherogenesis. We have studied the oxidative modification of LDL by hypochlorite ((OCl)-O--), a powerful oxidant produced from H2O2 and chloride via the action of myeloperoxidase which is released from activated neutrophils and monocytes. Exposure of LDL to reagent or enzymically generated (OCl)-O-- at 4 or 37 degrees C resulted in immediate and preferential oxidation of amino acid residues of apolipoprotein B-100, the single protein associated with LDL. Lysine residues quantitatively represented the major target and, like tryptophan, were oxidized to approximately the same extent with reagent or enzymically generated (OCl)-O--. in contrast, LDL lipid oxidation was less favoured than protein oxidation, as judged by the amounts of lipid hydroperoxides, chlorohydrins, cholesterol or fatty acid oxidation products formed. Treatment with (OCl)-O-- caused aggregation of LDL, as shown by an increased turbidity of the oxidized LDL solution and elution from a size-exclusion h.p.l.c. column of high-molecular-mass LDL complexes. Chemical modification of lysine residues before oxidation with (OCl)-O-- prevented aggregation, while it enhanced the extent of lipid peroxidation. Treatment of LDL with (OCl)-O-- also caused the formation of carbonyl groups and release of ammonia; both these modifications were inhibited by lysine-residue modification before oxidation. These results demonstrate that aggregation reactions are dependent on initial lysine oxidation by (OCl)-O--, followed by deamination and carbonyl formation, but do not involve lipid (per)oxidation. We propose that the observed (OCl)-O---mediated aggregation of LDL is caused, at least in part, by crosslinking of apoproteins by Schiff-base formation independently of lipid peroxidation.