Are all LDL molecules a health risk? Atherogenic LDL particles and how to study them
Lipids and proteins non-covalently associate to form lipoproteins, which function in blood plasma as transport vehicles for cholesterol and triacylglycerols. Lipids, such as phospholipids, triacylglycerols, and cholesterol, are poorly soluble in aqueous solution. Therefore, they are transported through the circulation as components of lipoproteins (Voet, 2004).
Various families of lipoproteins have been described, each of which plays specific roles in lipid transport. These families are classified according to their density, each class contains characteristic apoproteins and have a distinctive lipid composition. Since lipids have a much lower density than proteins, the lipid content of a class of lipoprotein is inversely related to its density. Thus, the higher the lipid abundance, the lower the density (Mathews, 2002).
The standard classification of lipoproteins includes, in increasing order of density: chylomicrons, very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Despite their differences in lipid and protein composition, all lipoproteins share common structural characteristics, especially a spherical shape. As shown in Figure 1, the hydrophobic parts, both lipids and apolar amino acids, form an inner nucleus, and the hydrophilic protein structures and polar head groups of phospholipids are found outside (Mathews, 2002).
Figure 1. General structure of a plasma lipoprotein. The spherical particle, part of which is shown in the figure, contains neutral lipids on the inside and phospholipids, cholesterol and proteins on the surface (taken from Mathews, 2002).
Cardiovascular risk factors have been shown to be closely associated with excess cholesterol and triglycerides.
Heterogeneity in particle size and density of LDL is an evaluator of cardiovascular risk. This heterogeneity has led to the definition of two phenotypes: phenotype A, with a predominance of large and light LDL particles, and phenotype B, with a predominance of small and dense LDL particles, in addition to an intermediate phenotype AB.
LDL are hydrolyzed on the arterial surface, to smaller and denser particles, which are more atherogenic, because they penetrate the intima more easily, where the chemical modification of these molecules occurs, which are more susceptible to oxidation by free radicals, by have a low amount of antioxidant and a high content of polyunsaturated fatty acids. The oxidative modification of LDL leads to an increase in the uptake of these modified particles by macrophages and to the formation of foamy cells (Witztum and Steinberg, 1991; Esterbauer et al., 1992).
It is accepted that high plasma LDL values are strongly associated with the formation of atherosclerotic lesions, the same is true with low HDL levels, also with values greater than 5 of the total cholesterol / HDL index, whereas when these values are Lower than 5 are associated with a low incidence of coronary heart disease (CHD). The imbalance between LDL and HDL in plasma has been shown to prevail in those sites in the intima of the coronary arteries where cholesterol accumulates (Smith, 1990).
Research on the size of LDL has been carried out for years, there is no standardized method for determining it. Most of the methods used for their study are based on separation by electrophoresis using polyacrylamide gels with fixed and gradient concentrations. There are also more methods, some classic, such as density gradient ultra centrifugation (UC), and others, such as magnetic resonance imaging or one based on simple lipoprotein precipitation. On the other hand, it has been suggested that calculations such as the LDL / Apo B ratio, HDL / triglycerides, total cholesterol / triglycerides or others may be simple estimators of the existence of small and dense LDL particles (Jorba and Ordóñez, 2006).
Electrophoresis is a technique that is based on the separation of charged particles in an electric field with respect to their charge. Gel electrophoresis is probably the most widely used and important method in molecular biology.
Polyacrylamide gel electrophoresis allows different LDL subfractions to be separated directly, with better resolution than the other techniques described. The polyacrylamide gel can be of a fixed or gradient concentration. The advantages of separating LDL subfractions using electrophoretic methods are clear: the equipment is inexpensive, the separations are relatively fast, the lipoproteins do not degrade during electrophoresis and the technique is relatively simple, while requiring a small volume. sample (Jorba and Ordóñez, 2006).