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The purpose of this study was to evaluate the mechanisms of apolipoprotein B (apoB) modification during oxidation of human low density lipoproteins (LDL) mediated either by copper or by hypochlorite (HOCl). The kinetics of protein carbonyl formation, the relationship of apoB carbonyl formation to lipid peroxidation, and the loss of apoB lysine residues were determined. During copper-mediated LDL oxidation, apoB carbonyls appeared to increase slowly, displayed saturation kinetics in response to increasing copper concentrations, and correlated with lipid peroxidation. During HOCl-mediated LDL oxidation, apoB carbonyls increased with increasing HOCl concentrations reaching plateau with time; however, lipid peroxidation was not observed. During copper-mediated but not during HOCl-mediated LDL oxidation, LDL vitamin E was depleted. ApoB carbonyls formed more efficiently during copper-mediated LDL oxidation at low (< 5 microM) copper concentrations compared with higher copper concentrations or during HOCl-mediated LDL oxidation. The differences in oxidation kinetics between copper- and HOCl-mediated LDL oxidation support the concept that the binding of copper to LDL is a site specific process, and suggest that HOCl modifies apoB amino acids randomly.