Natural occurrence and benefits of phospholipids
Phospholipids are ubiquitous molecules that participate in countless cellular processes. The biochemical and physiological properties of phospholipids are largely related to their amphipathic property. Arranged as a lipid bilayer, phospholipids play a significant role in the structure and functionality of biological membrane. The phospholipid composition varies according to cell and organelle functions. Thus, a typical membrane of a mammalian cell consists of the following (phospho)lipids:
- 45-55% Phosphatidylcholine (PC)
- 15-25% Phosphatidylethanolamine (PE)
- 10-20% Cholesterol
- 10-15% Phosphatidylinositol (PI)
- 5-10% Phosphatidylserine (PS)
- 5-10% Sphingomyelin (SM)
- 2-5% Cardiolipin
- 1-2% Phosphatic acid (PA)
Phospholipid regulatory functions are both direct and indirect, through their metabolites. It is well established that phospholipids participate in a broad range of cellular events such as apoptosis and regulation of mitochondrial physiology, and that phospholipid-derived messenger molecules are crucial for the transduction of extracellular signals to intracellular events.1)
Phospholipids are abundant in the brain, spinal cord, and body fluids such as plasma, but are mainly found in various organelles and cell membranes, including mitochondria, endothelial cells, and aggregated and/or activated platelets. These cells are an important source of anionic phospholipids such as phosphatidylserine (PS), provide a catalytic surface for proteins, and act as modulators of coagulation pathways.2)
The distribution of phospholipids between the two halves of the cell membrane bilayer is extremely asymmetric for both phosphatidylserine (PS) and sphingomyelin (SM), the presence of the first being almost exclusively in the inner monolayer, whereas sphingomyelin is restricted to the outer monolayer. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are more evenly distributed, with PC appearing to have a preference for the outer monolayer and PE for the inner leaflet.
The result of this distribution is that the outer surface of the cells contains about 80% PC, with the remainder occupied by PE. The inner monolayer of the membrane contains virtually all of the phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidic acid (PA), and a large proportion of PE, resulting in the negatively charged phospholipids being found almost exclusively in the inner monolayer of the cell membrane.
However, under certain circumstances, such as ongoing apoptosis, cell activation, or cell injury, there may be an increased loss of phospholipid asymmetry. The phenomenon of scrambling of phospholipids has been observed in platelets activated by the combined action of the physiological activators collagen and thrombin.
Biological function of phospholipids
Phospholipids are essential for digestion and various metabolic processes in humans:
- component of bile (as monoacyl phospholipid)
- responsible for the transport of fat between the intestine and the liver (in the form of lipoproteins)
- source of acetylcholine (in the case of PC lipids)
- source of essential fatty acids, especially poly unsaturated fatty acids (PUFA)
- source of energy
In addition, certain phospholipids have other functions. For example, dipalmitoyl phosphatidylcholine (DPPC) is a component of lung surfactant, and various phosphatidylserines (PS) are involved in bone formation (lipid calcium phosphate), regulation of apoptosis, and blood coagulation.
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in: Encyclopedia of Food and Health (Eds: Caballero B, Finglas PM, Toldrá F), Academic Press, Oxford, 352-359.
39 - Immunology and anti-phospholipid antibodies and co-factors
in: Systemic Lupus Erythematosus (Fourth Edition) (Ed: Lahita GA), Academic Press, San Diego, 1081-1105.