Revealing binding sites for myeloperoxidase on the surface of human low density lipoproteins

Low density lipoproteins (LDL) of human blood, once oxidized, provoke cholesterol accumulation in cells of arterial wall, which favors the development of atherosclerosis. Oxidative modification of LDL can result from their interaction with hypochlorous acid produced in the halogenation cycle of myeloperoxidase (MPO). On account that MPO is able to form complexes with LDL it seems important to learn the forces promoting such contacts and to spot the likely binding sites for the enzyme on the surface of LDL particles. In this study affinity chromatography on MPO-Sepharose showed that MPO-LDL complexes are uncoupled at ionic strength above 0.3M NaCl or when pH of solution goes below 3.6. This is an evidence of ionic interaction between MPO and LDL. We used spin probes of lipid nature embedded in phospholipid monolayer so that a variety of distances between the surface of an LDL particle and the paramagnetic center of a spin probes was provided. Since MPO interaction with labeled LDL caused no alteration of EPR spectra it was concluded that lipid components of LDL are not involved in MPO binding. Analysis of Mn(2+) distribution between LDL surface and the aqueous milieu showed that the surface negative charge of LDL is not considerably changed upon interaction with MPO. It can be suggested that interaction of LDL with MPO does not involve phospholipids that are the principal carriers of the surface charge. Among synthetic oligopeptides with amino acid sequences mimicking those of apoB-100 fragments -(1)EEEMLEN(7), (53)VELEVPQ(59) and (445)EQIQDDCTGDED(456) - only the latter could replace MPO in the complex with LDL. It is concluded that the likely site of interaction with MPO is the amino acid stretch 445-456 of apoB-100 in LDL.