51] Apolipoprotein B protein oxidation in low-density lipoproteins

Abstract

Publisher Summary The oxidative modification of low-density lipoprotein (LDL) plays an important role in pathogenesis of atherosclerosis in humans. It is well known that oxidatively modified LDL is recognized by a scavenger receptor of monocyte-derived macrophages rather than the LDL receptor, which results in its unregulated uptake and eventual formation of cholesteryl ester-loaded cells. The oxidation of lipids in LDL proceeds by a free radical-mediated mechanism, and it can be inhibited by antioxidants. During the oxidation of LDL, several oxidative modifications of apolipoprotein B100 (apoB) are observed: (1) increase in fluorescence, (2) decrease in amino groups, (3) increase in negative charge, (4) aggregation, and (5) fragmentation. Recognition of oxidized LDL by a scavenger receptor may result from derivatization of apoB lysine residues by peroxidation products or breakdown products, leading to a net increase of negative charge. Alternatively, another possibility that reactive oxygen species generated during oxidation of LDL may attack positively charged residues directly and convert these residues to negatively charged residues and also cause fragmentation of apoB. The increase in negative charge and fragmentation of the protein may result in conformational changes in apoB, which can lead to an increase in recognition by a scavenger receptor and loss of the epitope required for the LDL receptor. The quantitative evaluation of these alterations of apoB during oxidation of LDL is crucial to prove the role of oxidized LDL in the mechanism of atherosclerosis. This chapter presents methods for measuring the negative surface charge and fragmentation of the apoB protein.