Activated neutrophils oxidize target molecules by mechanisms that include degranulation with release of MPO, which oxidizes CT to HOCI with H2O2. We reacted human low density lipoproteins (LDL) with HOCI, treated the native and oxidized LDL with 2,4-dinitropheynylhydrazine (DNPH), delipidated and trypsinized the protein and analyzed the products by HPLC. The chromatograms of tryptic peptides from oxidized LDL showed several discrete peaks at 365 nm, characteristic of DNPH-derived hydrazones from protein carbonyls; peptides from native LDL showed no peaks at 365 nm. Of the 12 peptides from oxidized LDL showing absorbance at 365 nm isolated and identified, 7 contained Cys residues in the native sequences. The data indicate oxidative modifications at or near Cys, but the product structures remain to be elucidated. Other DNPH-positive peptides suggested oxidation of Lys, Trp, and Met. All of the peptides identified thus far are trypsin-releasable from LDL, which suggests that they are located on the particle surface. In contrast with the 12 peptides isolated and characterized from oxidation with reagent HOCI, oxidation with MPO in vitro produced a single major tryptic peptide absorbing at 365 nm, which was isolated and characterized as VELEVPQL(C*)SFILK (residues 53-66 on apoB-100). These results indicate that oxidation of LDL by MPO in vitro is not mediated by free solution HOCI, but a more specific interaction is implicated.
