Hypochlorite (HOCl) attacks amino acid residues in LDL making the particle atherogenic. Tryptophan is prone to free radical reactions and modification by HOCl. We hypothesized, that free tryptophan may quench the HOCl attack therefore protecting LDL. Free tryptophan inhibits LDL apoprotein modification and lipid oxidation. Tryptophan-HOCl metabolites associate with LDL reducing its oxidizability initiated by endothelial cells, Cu(2+) and peroxyl radicals. One tryptophan-HOCl metabolite was identified as 4-methyl-carbostyril which showed antioxidative activity when present during Cu(2+) mediated lipid oxidation, but did not associate with LDL. Indole-3-acetaldehyde, a decomposition product of tryptophan chloramine (the product of the tryptophan-HOCl reaction) was found to associate with LDL increasing its resistance to oxidation. Myeloperoxidase treatment of LDL in the presence of chloride, H(2)O(2) and tryptophan protected the lipoprotein from subsequent cell-mediated oxidation. We conclude that, in vivo, the activated myeloperoxidase system can generate antioxidative metabolites from tryptophan by the reaction of hypochlorite with this essential amino acid.