**3.2 Changes in lipid content of LDL particles and secondary structure of apoB-100**

The conformation of apoB-100 on the surface of LDL is more dependent on the physical and chemical state of the lipid core, which is linked to the shape and

*Apolipoproteins, Triglycerides and Cholesterol*

Treatment Program III (NCEPIII) [4].

plasma LDL in normal people is 2–4 days.

potential of self-repair [2].

B-100 (apoB-100) is composed of the amphoteric α-helical domain and β-sheet domain alternately (NH2-βα1-β1-α2-β2-α3-coOH), in which β-sheet structure accounts for 40%, which is related to the stability of LDL particles, and α-helix accounts for 25%, which is related to the amphiphilicity of LDL particles and the

LDL is the plasma metabolite of very low-density lipoprotein (VLDL). Plasma lipoprotein lipase or liver lipase catalyzes the hydrolysis of triglyceride (TG) in VLDL particles. At the same time, under the action of cholesterol ester transfer proteins (CETPs), cholesterol ester (CE) of HDL is transferred to VLDL, and phospholipids, apolipoprotein C (ApoC), and cholesterol are transferred to high-density lipoprotein (HDL) on the surface of VLDL. This process continues. In VLDL, TG decreases continuously, CE increases gradually, particles become smaller, and density increases gradually. First intermediate density lipoprotein (IDL) is formed, and then LDL is formed [3]. According to the formation process of LDL, it is easy to see that LDL is not a kind of particle but a class of particles with different sizes, densities, chemical composition, or different charges. In recent years, LDL particles have been divided into two phenotypes: type A (large and light LDL), LDL particle diameter ≥25.5 nm, and type B (small and dense LDL, sd-LDL), LDL particle diameter <25.5 nm. Compared with type A LDL, sd-LDL has a stronger ability to cause atherosclerosis and has been identified for a long time as a new risk factor of cardiovascular disease by the American Cholesterol Education Program and Adult

Natural LDL (nLDL) is in charge of the transport of endogenous cholesterol, and its metabolic process is the transport process of endogenous cholesterol. Among them, two-third were metabolized through the LDL receptor pathway [5]. LDL receptor (LDLR) is widely distributed in the whole body, especially on the cell membrane surface of the liver, adrenocortical, ovarian, testicular, and arterial wall, and specifically binds to ApoB100 on the surface of LDL. Internalization causes the membrane at the junction to sink in for endocytosis. Under the action of the proton pump (H+ -ATPase), the pH of endocytotic vesicle contents decreased, and LDL is separated from the receptor and fused with the lysosome. ApoB100 is decomposed into amino acids by a lysosomal proteolytic enzyme, and CE is hydrolyzed into free

The remaining one-third is cleared by the mononuclear macrophage pathway. As a major member of innate immunity, macrophages are endowed with an advanced arsenal of sensors, composed of various pattern-recognition receptors (PRR). It is able to identify and bind foreign substances or altered substances to inactivate and degrade them. Therefore, the monocyte–macrophage clearance pathway is mainly aimed at LDL, which has changed its structure for a variety of reasons. It can also be called modified LDL (mLDL). mLDL specifically binds to the scavenger receptors (SRs) on the surface of macrophages and is subsequently removed. The half-life of

The cells prepare their cholesterol needs via two pathways: an exogenous pathway mediated with the LDLR and an endogenous pathway activated with the substrates of mevalonate and HMG-CoA reductase [6]. When the intracellular cholesterol level is too high, sterol regulatory element-binding protein (SREBP), a nuclear transcription factor, is activated, which inhibits the expression of LDL receptor gene from the transcription level, inhibits the synthesis of the receptor protein, and reduces the further uptake of LDL by the cell [7]. It is suggested that the simple increase of the nLDL-C level cannot fully explain the occurrence of atherosclerotic disease. More significantly, contrary to LDLR, SR expression is not inhibited by elevated intracellular cholesterol levels [1]. Then, when the number of modified LDL absorbed by macrophages through SRs far exceeds their scavenging

cholesterol and fatty acids by cholesterol esterase for cell utilization.

**160**

size of the LDL particles. Related experiments have found that the conformation of ApoB-100 is associated with the coverage of the interfacial area effect existing at the LDL particle-water solution interface. The coverage of the interfacial area effect increases with the decrease of LDL particle size, which can be regarded as an adaptive conformation change. This change is to avoid or minimize the contact between hydrophobic protein residues and aqueous solutions largely. It also ensures the stable existence of cholesterol in a hydrophilic environment and leads to successfully complete the targeted transport. However, this conformation regulation also causes more β-sheet domains to be parallel to the phospholipid monolayer of LDL, making it more vulnerable to adverse factors [19]. This may be one reason why sd-LDL with higher atherosclerosis is more easily oxidized.

The effect of oxidative modification on the secondary structure of LDL is mainly characterized by the destruction of β-sheet domain in the initial stage, resulting in the generation of disordered rings and corners. This change reduces the proportion of β-sheet in ApoB-100 and destroys the stability of LDL, but the alpha-helix ratio increases, triggering the self-repairing potential of apoB-100. At this point, the physical and chemical properties of LDL particles changed very little. When the oxidant continuously penetrates into the core, the physical state and accumulation mode of the lipid inside the hydrophobic core change, leading to the loss of the secondary structure of apoB-100, and the physical and chemical properties of LDL completely change [19].

In summary, compared to nLDL, mLDL has the following characteristics:

