The dipalmitoylphosphatidylcholine, better known in the literature as dipalmitoyl lecithin or DPL, it is a lipidic compound belonging to the group of phospholipids, specifically the family of glycerophospholipids and the set of phosphatidylcholines.
This lipid is the main surfactant of pulmonary surfactant and in this organ it is produced essentially by alveolar macrophages from the cytidine diphosphate or CDP-choline pathway..
Lung surfactant is a complex mixture of lipids and proteins that is found more or less in a ratio of 10 to 15 milligrams per kilogram of weight in adult animals, and in a lung its concentration is equivalent to about 120 milligrams per milliliter.
Lipids, including dipalmitoylphosphatidylcholine, other phospholipids, and cholesterol, account for more than 85% of the weight of lung surfactant. This important phospholipid (DPL) is responsible for the reduction of surface tension in the alveoli during expiration..
Its biosynthesis can occur de novo through the CDP-phosphocholine pathway, or by sequential methylation of phosphatidylethanolamine (catalyzed by a phosphatidylethanolamine N-methyltransferase); or it can be synthesized by base exchange of other phospholipids such as phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine or others.
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The structure of dipalmitoylphosphatidylcholine, as its name indicates, consists of a skeleton composed of a glycerol molecule to which two palmitic acid molecules are esterified at the carbons of positions 1 and 2, and a choline portion bound to phosphate of carbon in position C3 of the same skeleton.
This structure, like that of all lipids, is characterized by its amphipathic nature, which has to do with the presence of a hydrophilic polar portion, represented by the choline attached to the phosphate group, and a hydrophobic apolar portion, represented by the two esterified aliphatic chains.
Hexadecanoic acid, palmitic acid or palmitate, is a long chain (16 carbon atoms) saturated fatty acid (only carbon-carbon single bonds), and is one of the most common fatty acids in nature (animals, microorganisms and especially in plants).
Since palmitic acid chains are saturated, dipalmitoylphosphatidylcholine or dipalmitoyl lecithin is also part of the "disaturated" lecithins that can be found in cell membranes..
Choline, an essential element in the diet of many animals, is a type of quaternary ammonium salt soluble in water and with a net positive charge; that is, it is a cationic molecule, for which phosphatidylcholines are polar lipids.
Like the rest of the phosphatidylcholines, dipalmitoylphosphatidylcholine is one of the main and most abundant components of the lipid bilayers that make up the biological membranes of all living beings.
Its conformation allows it to easily form bilayers, where the hydrophobic tails "hide" from the hydrophilic medium towards the central region and the polar heads are in direct contact with the water..
For all phosphatidylcholines, in general, it is possible to form a "lamellar" phase in aqueous dispersions. These are known as liposomes, which are concentric (spherical) lipid layers with water trapped between the bilayers..
In membranes rich in cholesterol, this lipid is associated in a ratio of seven molecules of dipalmitoyl lecithin for each molecule of cholesterol and its function is to avoid contact between two cholesterol molecules and stabilize them in the membrane structure..
The permeability of membranes rich in dipalmitoylphosphatidylcholine increases with temperature, which can mean a metabolic advantage for many cells..
As mentioned previously, dipalmitoylphosphatidylcholine is essential for the reduction of surface tension in the pulmonary alveoli during expiration..
Its hydrophilic portion (the choline) is associated with the liquid phase of the alveoli, while the hydrophobic chains of palmitic acid are in contact with the aerial phase.
This "substance" is produced and secreted by type II alveolar cells in the lungs (type II pneumocytes) and by alveolar macrophages, and its components are synthesized and assembled in the endoplasmic reticulum. They are then transferred to the Golgi complex and later form "lamellar" bodies in the cytosol..
The primary function of pulmonary surfactant, and thus of dipalmitoylphosphatidylcholine together with other associated lipids and proteins, is to counteract alveolar expansion during inspiration and support its retraction during expiration..
It also contributes to the maintenance of alveolar stability, as well as fluid balance and the regulation of capillary flow to the lungs..
At present, it is not known exactly whether the production of dipalmitoyl lecithin by alveolar macrophages is associated with the incorporation of this lipid into lung surfactant or with its phagocytic activity, although there is much research on this..
Some respiratory stress syndromes in newborn infants and adults are characterized by decreased dipalmitoylphosphatidylcholine at the air-tissue interface. For this reason, there are various research reports related to nebulization with this lipid to restore pressure-volume relationships in the lungs..
The breakdown products of dipalmitoylphosphatidylcholine are essential elements for many metabolic processes:
- The two palmitic acid chains can be used in the β-oxidation of fatty acids to obtain large amounts of energy or for the synthesis of new lipids.
- The choline residue of the polar "head" group of this phospholipid is an important precursor for the biosynthesis of other phospholipids, which are essential components for the formation of biological membranes..
- Choline is also a precursor for the neurotransmitter acetylcholine and is an important source of labile methyl groups..
- The glycerol 3-phosphate produced from the hydrolysis of the ester and phosphodiester bonds between the fatty acid chains and the choline residue, can serve as a precursor molecule for other lipids that have important functions in intracellular signaling events..
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