*3.1.1 Apolipoprotein A-I*

*Apolipoproteins, Triglycerides and Cholesterol*

lipoprotein particle [7].

**3.1 Apolipoproteins**

apoproteins (Apo).

• Hilomicrones

Pre-beta-lipoproteins

Beta-lipoproteins

Alpha-lipoproteins

*Basic blood lipoprotein particles.*

and their further catabolism [2, 15, 16].

• Very low-density lipoproteins (VLDL)

• Low-density lipoproteins (LDL)

• High-density lipoproteins (HDL)

The lipid part of the lipoprotein particles consists of cholesterol, cholesterol esters, triglycerides, and phospholipids, and their protein parts are very different in structure and are called apolipoproteins. Depending on the type and amount of lipids, on the one hand, and the amount and type of protein part, or apolipoprotein,

The four basic types of lipoprotein particles present in the blood of all persons are chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins

Today, the labeling of lipoprotein particles with abbreviations deriving from English names, i.e., as VLDL, LDL, and HDL particles, is accepted worldwide. They should also be supplemented with chylomicrons, which have the largest circulating

Certain lipids (cholesterol, triglycerides, phospholipids) are found in the blood in the composition of all lipoprotein particles, in different amounts, and in combination with other constituents of lipoprotein particles. Chylomicrons and VLDLs contain predominantly triglycerides, LDL mainly cholesterol, while HDL particles are most abundant in the protein moiety and lipids in phospholipids. One person's blood cholesterol represents the sum of cholesterol present in all lipoprotein particles, in chylomicrons, VLDL, LDL, and HDL, although its largest amount (about 70%) is found in LDL particles. Depending on the physiological role of individual lipoprotein particles, the increase in cholesterol in one of them will have a different significance for the organism. Cholesterol contained in the LDL particle leads to atherosclerosis, while the increase in cholesterol in HDL particles has a protective effect against atherosclerosis [6]. In addition to chylomicrons, VLDL, LDL, and HDL particles, some other types of lipoproteins are normally present in the blood. Among these lipoprotein species is the so-called lipoprotein Lp(a). It is a special type of lipoprotein, which has been shown to be present in the blood of every person at quite minimal concentrations (up to 0.25 g/l). Lipoprotein Lp(a) together with the LDL particle represents the most serious lipid risk factor for atherosclerosis [7].

Lipids are mostly insoluble in water. In plasma, they can be in the form of stable complexes if they are attached to specific protein moieties called apolipoproteins or

The protein part of the lipoprotein particle was previously thought to be a lipid transport agent only. However, they have been found to have other, very significant roles in the body. Apoproteins provide stability of plasma lipid transport. They are cofactors of individual enzymes that participate in the metabolism of lipoprotein particles. Their most important function is to bind to specific receptors on cell membranes, thereby ensuring the entry of plasma lipoprotein particles into cells

on the other, different types of lipoproteins are present in the blood [4].

(LDL), and high-density lipoproteins (HDL) (**Table 1**) [7].

**6**

**Table 1.**

Apolipoprotein A-I (Apo A-I) is the major apolipoprotein of HDL particles. About 90% of Apo A-I is in HDL fractions. In the HDL particle alone, it accounts for 65% of the total protein portion. It is present in small amounts in the chylomicrons and the VLDL fraction. It is synthesized in the liver and small intestine wall. Its most important role is the activator of the enzyme LCAT, which allows esterification of free cholesterol on the surface of HDL particles. In this way, small discoidal HDL3 particles accept excess cholesterol from the cells and accumulate it in the form of esterified cholesterol, thereby transforming it into larger, more soluble, lower-density HDL2 particles. Since it allows the uptake of free cholesterol and subsequently its esterification, it is logical that it has a protective effect on the process of atherosclerosis [18].
