*3.1.4 Apolipoprotein C*

The apolipoprotein C (Apo C) group is represented by three different proteins that are synthesized in the liver [20]. There are chylomicrons, VLDLs, and newly formed HDL particles bound in the circulation. During intravascular metabolism of lipoprotein particles, there is an intense exchange of Apo C. From HDL particles, these apolipoproteins transfer to lipoprotein particles rich in triglycerides, namely, chylomicrons and VLDL. Apparently, Apo C plays a significant role in the catabolism of triglyceride-rich lipoproteins [24].

### *3.1.5 Apolipoprotein E*

Apolipoprotein E (Apo E) is a protein molecule composed of 229 amino acids, which contains 10% of the amino acid arginine, and is therefore called the "arginine-rich protein" [16]. The main site of its synthesis is the liver, but it is also created in other organs: brain, spleen, lungs, adrenal gland, kidneys, ovaries, muscles. It plays a significant role in the metabolism of various lipoprotein particles: chylomicrons, VLDL, IDL, and HDL containing Apo E.

Apolipoprotein E participates in the reversible transport of excess cholesterol from peripheral tissues into the liver via HDL particles, which when incorporated into cholesterol incorporate Apo E [25].

Apo E also appears to be involved in reparative responses to tissue damage. An increase in its concentration is found in sites of peripheral nerve damage and regeneration [25]. It has significance in nerve regeneration, growth, and/or differentiation of neurons [26].

### *3.1.6 Apolipoprotein (a)*

Apolipoprotein (Apo A) is a relatively large molecular weight protein that is incorporated into the lipoprotein composition of Lp(a).

If Apo A is isolated, the rest of Lp(a) in its composition is almost identical to the LDL particle because it has a similar lipid composition and contains a single molecule of Apo B-100. The gene that regulates the synthesis of this apolipoprotein is located on the sixth chromosome near the plasminogen gene. Apo A has a great structural similarity and shows immune cross-reactivity with plasminogen, which could be a link between atherosclerosis and fibrinolysis [27].
