Structure of lipoproteins and the role of apolipoproteins
Highly lipophilic compounds—cholesterol (CH), triglycerides (TG), and phospholipids (PL)—cannot dissolve in blood plasma or move freely through the bloodstream. Lipoproteins are formed to transport them.
All classes of lipoprotein particles share a fundamentally similar structure but differ in physicochemical properties. The differences between them are determined by surface proteins—apolipoproteins (Apo).
Functions of apolipoproteins:
- Serve as recognition elements (markers) for target cells;
- Ensure the selective interaction of particles with receptors and enzymes.
Exogenous pathway of dietary fat transport
The exogenous pathway is responsible for delivering dietary lipids from the small intestine to body tissues:
- Absorption in enterocytes: The entry of cholesterol into intestinal cells occurs with the participation of a special transport system—the NPC1L1 (Niemann-Pick C1-Like 1) protein.
- Formation and secretion: Chylomicrons are formed in the intestine and enter the blood plasma.
- Vascular phase: The enzyme lipoprotein lipase is located on the endothelial cells of capillaries (particularly in adipose tissue). It breaks down the triglycerides of chylomicrons, releasing fatty acids and glycerol for tissue needs.
- Utilization: The remaining particles—chylomicron remnants—are captured by the liver and broken down in the lysosomes of hepatocytes.
Endogenous pathway: synthesis and metabolism of VLDL and LDL
The endogenous pathway ensures the distribution of lipids produced within the body:
- Cholesterol synthesis: Occurs in the liver from acetyl-CoA. The main regulating enzyme is HMG-CoA reductase (3-hydroxy-3-methylglutaryl-coenzyme A reductase).
- VLDL secretion: The liver releases very-low-density lipoproteins (VLDL) into the bloodstream.
- Cascade of transformations: Under the action of endothelial lipoprotein lipase, VLDL loses fatty acids and glycerol, turning into remnants—intermediate-density lipoproteins (IDL), and then into low-density lipoproteins (LDL).
- Tissue uptake: LDL particles bind to receptors in peripheral tissues (e.g., adipocytes) or return to the liver via receptor-mediated endocytosis.
Receptor regulation in the liver and cholesterol excretion
The clearance of blood plasma from lipoproteins is controlled by hepatocytes via feedback:
- Receptor dynamics: Lipoprotein receptors are located on the membranes of liver cells. When intracellular cholesterol is deficient, the density of these receptors increases, which activates the extraction of cholesterol-containing lipoproteins from the blood.
- Elimination from the body: Cholesterol entering the liver is transformed into bile acids. Then, cholesterol and bile acids are secreted through the bile duct into the intestine, where they partially participate in enterohepatic circulation.
Briefly: answers to topical questions
Which specific classes of apolipoproteins serve as markers for chylomicrons?
The marker protein for chylomicrons is apolipoprotein B-48 (apoB-48). Immature (nascent) chylomicrons synthesized in the mucosa of the small intestine contain only this class of apolipoproteins. In the bloodstream, apoC-II and apoE are additionally transferred to them from high-density lipoproteins, but it is apoB-48 that acts as a specific marker for these particles.
Which group of pharmacological drugs inhibits the enzyme HMG-CoA reductase?
The enzyme HMG-CoA reductase is inhibited by a group of drugs called statins (HMG-CoA reductase inhibitors). They block the key enzyme of cholesterol synthesis in the liver.
The main representatives of this pharmacological group include:
- Simvastatin
- Atorvastatin
- Rosuvastatin
Inhibition of cholesterol synthesis occurs inside hepatocytes.
Which drug selectively blocks the NPC1L1 transport system in the intestine?
The NPC1L1 transport system in the intestine is selectively blocked by the drug ezetimibe. It selectively inhibits cholesterol absorption without affecting the absorption of other steroids.
The mechanism of action at the molecular level includes:
- Inhibition of entry — blocking the specific NPC1L1 transporter (Niemann-Pick C1-Like 1), which is responsible for transferring cholesterol from the intestinal lumen into the enterocyte.
- Activation of efflux — stimulation of ATP-binding cassette transporters that pump sterols back into the intestinal lumen.
Which lipoproteins carry out reverse cholesterol transport from peripheral tissues to the liver?
Reverse cholesterol transport from peripheral tissues to the liver is carried out by high-density lipoproteins (HDL). They remove excess free cholesterol from the membranes of peripheral tissue cells and deliver it to hepatocytes for utilization.
This process is the main antiatherogenic factor of vascular protection. HDL do not contain apolipoprotein B-100, and therefore are not taken up by peripheral tissues, but only remove excess lipids, preventing the development of atherosclerosis.
Sequence of the endogenous particle size reduction cascade: Liver -> VLDL -> IDL -> LDL -> Receptor uptake.
Class and exam questions
What is the function of the NPC1L1 protein in lipid metabolism?
The NPC1L1 protein (Niemann-Pick C1-Like 1) is located in the enterocytes of the small intestine and represents a transport system that ensures the absorption of dietary cholesterol.
How does the liver regulate cholesterol uptake from blood plasma?
By the feedback principle: upon depletion of internal cholesterol stores, hepatocytes increase the density of lipoprotein receptors on the membrane, activating the endocytosis of LDL and other particles from the blood.
What happens to chylomicron and VLDL triglycerides in capillaries?
The enzyme lipoprotein lipase on the capillary endothelium hydrolyzes triglycerides into fatty acids and glycerol, which then transfer into tissues.
What else to study on this topic
- Pharmacological inhibition of HMG-CoA reductase and the NPC1L1 protein
- Molecular mechanism of receptor-mediated LDL endocytosis
- The role of lipoprotein lipase in adipocyte metabolism
- Enterohepatic circulation of bile acids
- Comparative characteristics of apolipoprotein markers
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