They are lipoprotein particles that consist of triglycerides (85-92%), phospholipids (6-12%), cholesterol (1-3%), and proteins (1-2%).
The term “chylomicrons” comes from the Greek χυλός, chylous, which means juice (of plants or animals), and micron, which means small particle.
Chylomicrons are one of the five main groups of lipoproteins:
- Chylomicrons (ultra-low density lipoproteins [ULDL]).
- Very low-density lipoprotein.
- Intermediate density lipoprotein.
- Low-density lipoprotein.
- High-density lipoprotein allows fats and cholesterol to move within the liquid solution of the bloodstream.
Chylomicrons are produced only in intestinal cells, while very-low-density lipoproteins are also synthesized in the liver.
To form a chylomicron, triglycerides, fat-soluble vitamins, and cholesterol are coated with a layer of apolipoprotein (apo types A and B), cholesterol esters, and phospholipids.
Chylomicrons are produced in the endoplasmic reticulum and then processed in the Golgi complex, where glycosylation of apoprotein takes place.
Apo B has been suggested to be involved in the movement of chylomicrons from the endoplasmic reticulum to the Golgi apparatus since lipids accumulate in the former in patients with abetalipoproteinemia.
Very low-density lipoproteins are smaller than chylomicrons. They are synthesized through a different pathway and appear to be predominant in fasting states. Chylomicrons exit the enterocyte by exocytosis.
Although they are too large to pass through capillary pores, chylomicrons and very-low-density lipoproteins easily cross in the dairy endothelial gaps present in the postprandial phase.
Medium-chain triglycerides move directly into the portal circulation.
Chylomicrons transport absorbed lipids from the intestine to adipose, cardiac, and skeletal muscle tissue, where their triglyceride components are hydrolyzed by lipoprotein lipase activity, allowing the released free fatty acids to be absorbed by the tissues.
When a large part of the triacylglycerol nucleus has been hydrolyzed, chylomicron remnants are formed and are absorbed by the liver, thus transferring dietary fat to the liver.
Omega-3 fatty acids
Chylomicrons are the most significant and highest triglyceride-rich lipoproteins in the blood. Chylomicrons are generally present only in the postprandial state because they carry dietary fat from the intestines to other tissues.
Because omega-3 fatty acids are known to significantly lower fasting triglyceride concentrations, their effects on postprandial triglyceride metabolism have been examined.
As expected, treatment with omega-3 fatty acids markedly reduced the rise in serum triglycerides after a high-fat meal.
The effect was not the result of malabsorption of omega-3 fatty acids because the fish oil-rich test meals produced postprandial triglyceride curves similar to the control fats.
Its effect on a postprandial excursion of serum triglycerides only appeared weeks after omega-3 prehydration. Fatty acids and blunting of postprandial triglyceridemia occurred regardless of the type of fat in the test meal.
The dose of omega-3 fatty acids required to reduce postprandial lipemia is as low as 1 g / day.
It is unknown to what extent the reduction in postprandial lipemia contributes to the decrease in cardiovascular risk, but this may be a mechanism by which these fatty acids mitigate the development of coronary heart disease (CHD, for its acronym in English).
Chylomicrons are formed in the endoplasmic reticulum in the small intestine’s absorbent cells (enterocytes). The villi, lined with the brush border microvilli, provide a large surface area for absorption.
The newly formed chylomicrons are secreted through the basolateral membrane into blood vessels, where they join with lymph to become chyle. The lymphatic vessels carry the chyle to the venous return of the systemic circulation.
The chylomicrons supply the tissue with the fat absorbed from the diet.
Therefore, unlike the saccharides and amino acids that digestion releases from carbohydrates and proteins in the diet (respectively), lipids bypass the hepatic portal system, which means that the liver does not receive the « first crack ‘on them.
There are three stages in the “life cycle” of the chylomicron:
- Nascent chylomicron.
- Mature chylomicron.
- Chylomicron remnants.
Triglycerides are emulsified by bile and hydrolyzed by the enzyme lipase, resulting in a mixture of fatty acids and monoglycerides. These then pass from the intestinal lumen to the enterocyte, where they are re-esterified to form triglycerides.
The triglycerides combine with phospholipids, cholesteryl esters, and apolipoprotein B-48 to form a nascent chylomicron.
These are then released by exocytosis from the enterocytes to the lateral blood vessels and lymphatic vessels that originate in the small intestine’s villi and are then secreted into the bloodstream at the connection of the thoracic duct with the left subclavian vein.
The main component of apolipoprotein is apolipoprotein B-48 (apo B-48).
Once the triglyceride stores are distributed, the chylomicron returns the apolipoprotein C-II to the high-density lipoproteins (but keeps the apolipoprotein E) and thus becomes a chylomicron residue, now only 30-50 nm.
Chylomicrons are too large to cross the endothelial barrier; therefore, their lipolysis before the remains fulfills a double function: transport energy to the tissues and decrease in size to facilitate terminal catabolism.
Experiments in hepatocytes perfused rat livers and, more recently, transgenic and knockout mouse studies have shown that the remnant transport of chylomicrons in the liver is mediated by cell surface receptors.
Although there is a consensus that apo E, recognized by the low-density lipoprotein receptor, is the component on the surface that directs chylomicron residues to their uptake site.
It was predicted from studies in low-density lipoprotein receptor-deficient model systems. This elimination of the remaining chylomicron would be independent of the low-density lipoprotein receptor.
Individuals with homozygous familial hypercholesterolemia, lacking functional low-density lipoprotein receptors, show no evidence of delayed clearance of chylomicron residues.
The same is true in an animal model for human familial hypercholesterolemia, the Watanabe hereditary hyperlipidemic rabbit.
In addition, intravenous infusion of apo E lowers plasma cholesterol levels in these animals, supporting the notion that apo E mediates lipoprotein uptake via low-density lipoprotein receptor-independent pathways.
Finally, evidence for a separate mechanism for hepatic chylomicron clearance comes from studies showing that dietary, pharmacological, and hormonal factors regulate the number of hepatic low-density lipoprotein receptors without significantly affecting the clearance rate of chylomicron remnants.
Since the low-density lipoprotein receptor and the proposed chylomicron remnant receptor must share at least one property, namely the binding of apo E, attempts to isolate this receptor were based on the putative homology of their ligand-binding region of the receptor—low-density lipoprotein.
Indeed, homology cloning resulted in the characterization of a substantial membrane protein composed exclusively of structural elements found in the low-density lipoprotein receptor molecule.
Therefore, it has been named low-density lipoprotein receptor-related protein.
It contains (among other structural elements found in the low-density lipoprotein receptor) 31 repeats of the type that form the ligand-binding domain in the low-density lipoprotein receptor and 22 repetitions of the type growth factor (A, B, and C ).
The substantial membrane protein binds to lipoproteins in an apo A-dependent manner and is sensitive to the balance between apo Cs and apo E.
Shortly after cloning, the low-density lipoprotein receptor-related protein was identical to the receptor for α2-macroglobulin, a major plasma protein that functions in the ‘capture’ and thus inactivation of cells. Cellular proteases that have entered the plasma compartment.
Since then, many more plasma complex proteins and protein complexes have been identified that, at least in vitro, bind to proteins related to lipoprotein receptors.