HDL and LDL/VLDL Cholesterol Assay Kit

Cholesterol is an essential lipid component of mammalian cell membranes, maintaining membrane integrity and fluidity and forming membrane microstructures. The metabolic homeostasis of intracellular cholesterol constitutes a complex network that regulates cholesterol biosynthesis, uptake, efflux, conversion, esterification and cholesterol transport. The sources of cholesterol consist of endogenous synthesis and exogenous uptake. Endogenous synthesis begins with acetyl-CoA, which is ultimately converted to cholesterol through nearly 30 enzymatic reactions, including the mevalonate (MVA) pathway, squalene biosynthesis, and subsequent conversion. Among these enzymatic reactions, 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (HMGCR) and squalene epoxidase (SQLE) are the two key rate-limiting enzymes that reduce HMG-CoA to MVA and catalyze the oxidation of squalene to 2,3-epoxy squalene, respectively. Exogenous uptake, on the other hand, involves the uptake of low-density lipoproteins (LDL) from the circulation via the low-density lipoprotein receptor (LDLR). The LDL binds to the LDLR and then the complex is internalized into the endosomes of the cell by endocytosis. LDL in endosomes is further transferred to lysosomes where it is hydrolyzed to release free cholesterol. With the help of sterol transfer proteins, cholesterol is eventually transferred to the cell membrane or endoplasmic reticulum (ER) membrane. The liver is the major site of cholesterol biosynthesis, and it delivers endogenously synthesized and exogenously obtained cholesterol to the bloodstream in the form of very low-density lipoproteins (VLDLs), where VLDLs are generated into circulating LDLs, which are able to be taken up by peripheral cells through receptor-mediated endocytosis.

Figure 1. The homeostasis of intracellular cholesterol metabolism
(Source: Xu H, et al. 2020)

The structure of cholesterol can be divided into a proximal hydrophilic end, a distal hydrophobic end, and a central four-ring structure that gives the molecule a very high degree of rigidity, and based on this structural feature it is involved in a number of important processes that affect cellular function. Embedded in the cell membrane, cholesterol provides rigidity to the cell, eliminating the need for a cell wall in some plants and bacteria. The more cholesterol in the cell membrane, the lower its permeability, so cholesterol can determine the permeability of cells to ions and gases, and its concentration varies with the function of the membrane, for example, the mitochondrial membrane has low cholesterol concentration and high membrane permeability, while the external membrane has high cholesterol and low permeability. Cholesterol also organizes signaling molecules, membrane protein transport, and neurotransmitter regulation by forming lipid rafts on cell membranes that segregate cellular processes. Cholesterol homeostasis is important for normal physiological processes, and its expression level is closely linked not only to the risk of atherosclerosis and cardiovascular disease, but also underlies the pathogenesis of a growing number of other diseases, such as neurodegenerative diseases and cancer.

Figure 2. The structure of the cholesterol molecule demonstrating the three important domains of the molecule
(Source: Schade DS, et al. 2020)