A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins and toxins are lipoproteins. Examples include the high density (HDL) and low density (LDL) lipoproteins which enable fats to be carried in the blood stream, the transmembrane proteins of the mitochondrion and the chloroplast, and bacterial lipoproteins.
The function of lipoprotein particles is to transport lipids (fats) (such as cholesterol) around the body in the blood.
All cells use and rely on fats and cholesterol as building blocks to create the multiple membranes which cells use to both control internal water content, internal water soluble elements and to organize their internal structure and protein enzymatic systems.
The lipoprotein particles have hydrophilic groups of phospholipids, cholesterol and apoproteins directed outward. Such characteristics makes them soluble in the salt water-based blood pool. Triglyceride-fats and cholesterol esters are carried internally, shielded from the water by the phospholipid monolayer and the apoproteins.
The interaction of the proteins forming the surface of the particles with (a) enzymes in the blood, (b) with each other, and (c) with specific proteins on the surfaces of cells determine whether triglycerides and cholesterol will be added to or removed from the lipoprotein transport particles.
Regarding atheroma development and progression as opposed to regression, the key issue has always been cholesterol transport patterns, not cholesterol concentration itself.
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