Modification of protein moiety of human low density lipoprotein by hypochlorite generates strong platelet agonist

Conflicting reports exist about the effects of mildly or extensively oxidiz ed low density lipoproteins (LDLs) on the reactivity of human platelets. Th is platelet response is mainly caused by modification of the protein and li pid moiety, giving rise to very differently modified species with hardly pr edictable properties. The aim of this study was to prepare oxidized LDL wit h modifications essentially restricted to the protein moiety and to determi ne the eventual platelet responses. We treated LDL at 0 degrees C for 10 mi nutes with a 60- to 1000-fold molar excess of sodium hypochlorite in berate buffer in the presence of the radical scavenger butylated hydroxytoluene. Under these conditions, neither fragmentation of apolipoprotein B-100 nor f ormation of LDL aggregates was observed, and lipid oxidation products did n ot exceed the amount present in untreated LDLs. The degree of modification and the respective effects on platelet function were highly reproducible. H ypochlorite-modified LDLs act as strong platelet agonists, inducing morphol ogical changes, dense granule release, and irreversible platelet aggregatio n. The evoked platelet effects are completely suppressed by inhibitors of t he phosphoinositide cycle but not by EDTA or acetylsalicylic acid. Most lik ely, these effects are transmitted via high-affinity binding to a single cl ass of sites, which does not recognize native or acetylated LDL. Obviously, modified lysines, and the secondary lipid modifications derived from them, are not essential for this interaction. We conclude that bioactive oxidize d lipids are not directly involved in the stimulation of platelets by hypoc hlorite-modified LDLs.