4.10:
Lipid-derived Compounds in the Human Body
Lipid-derived compounds, such as fat-soluble vitamins, eicosanoids, lipoproteins, and glycolipids, play an important role in human physiology.
Among fat-soluble vitamins, vitamin A or retinol is converted in the retina to retinal — part of a photoreceptor pigment essential for normal vision. In contrast, vitamin K is critical for proper blood clotting.
Membrane phospholipids are hydrolyzed by an enzyme phospholipase A2 producing arachidonic acid that acts as a precursor to prostaglandins, thromboxanes, and leukotrienes— all grouped under a class of hormones called eicosanoids.
Eicosanoids are involved in various physiological processes, including mucus production in the stomach and blood pressure regulation.
Additionally, lipoproteins are complexes made up of lipids and proteins. They aid the transport of dietary lipids to the liver through the bloodstream.
Similarly, glycolipids are complexes between carbohydrate residues and lipids attached via a glycosidic bond. These are located in the outer leaflet of the plasma membrane and play a vital role in membrane stability. They also facilitate cell-cell interaction during immune and signaling processes.
4.10:
Lipid-derived Compounds in the Human Body
Fats and lipids are crucial components in the human body. Some lipid-derived compounds, such as fat-soluble vitamins, eicosanoids, lipoproteins, and glycolipids, also play unique roles to support various biological processes .
Fat-soluble Vitamins
Fat-soluble vitamins, including vitamins A, D, E, and K, are required in minimal quantities, but their deficiencies can lead to severely abnormal physiological conditions. For example, vitamin A deficiency can cause night blindness, dry skin, delayed growth, and infertility. Similarly, vitamin K deficiency can lead to an increased risk of cardiovascular diseases, vitamin K deficiency bleeding (VKDB), osteoporosis, and poor bone development. Vitamin D deficiency can cause muscle weakness, deformities in joints, and depression; vitamin E deficiency may lead to muscle and nerve damage.
Eicosanoids
Eicosanoids are a class of cell signaling molecules derived from arachidonic acid— a polyunsaturated fatty acid with 20 carbons. Eicosanoids include three types— prostaglandins, thromboxane, and leukotrienes. Prostaglandins are pivotal in initiating the inflammatory response, including pain, redness, and swelling at the injury site. Importantly, prostaglandins also act as vasodilators promoting blood flow and facilitating the entry of white blood cells at the injury site. Conversely, thromboxanes act as vasoconstrictors and initiate platelet aggregation at the injury site for subsequent clot formation. Leukotrienes, the third type of eicosanoids, promote the production of histamines and prostaglandins and generate contractions in the smooth muscles of the bronchioles.
Lipoproteins
Lipoproteins are involved in the transport of hydrophobic molecules, such as dietary lipids and fat-soluble vitamins, through blood circulation. Lipoproteins are categorized into five major types— chylomicrons, very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Appropriate levels of lipoproteins are essential for good health. Elevated levels of LDL and HDL have been implicated in the onset of atherosclerosis, marked by plaque deposition in the arteries.