Lecithin

Lecithin is a phospholipid which consists of glycerol , two fatty acids , a phosphate group and choline.

Lecithin was first found in eggs in 1846, so its name was coined from the Greek word for egg yolk, lekithos. Though lecithin is its common name, chemists refer to it as phosphatidylcholine. It is a yellow-brown fatty substance. In contrast to fats, which function as fuel molecules, lecithin serves a structural role in cell membranes. It is found in all cells. Without lecithin and other membrane phospholipids, cells would be unable to maintain their structure and probably would dissolve back into their surroundings. Lecithin is apparently vital for life in mammals , because no hereditary diseases in its biosynthesis are known. (A genetic defect involving a vital substance is lethal to the organism and therefore cannot be passed on.) The lecithin we purchase in a store is actually a mixture of lecithin and other phospholipids as well as fatty (soy bean) oil. The fatty acid components in lecithin can vary, depending on the number of carbon atoms they contain and whether they are saturated or unsaturated. The nature of the fatty acid components in a lecithin molecule greatly influence its role. For example, a lecithin molecule in which both fatty acids are saturated aids oxygen uptake in the lungs. Another "species" of lecithin, which contains two unsaturated fatty acids is involved in the transport of cholesterol in the blood .

Structure and properties

The structure of lecithin is illustrated here. Glycerol, which contains three carbon atoms, serves as the backbone of the lecithin molecule. The two fatty acids are linked to glycerol at carbon atoms 1 and 2 and the phosphate group is linked to carbon atom 3. Choline is, in turn, linked to the phosphate group. Typically, though not always, the fatty acid attached to carbon 1 of glycerol is saturated, while that attached to carbon 2 is unsaturated.

To get a crude idea of the structure of the lecithin molecule, imagine a balloon with two long paper streamers attached to it. The balloon or "head" region corresponds to the polar portion of the molecule, the negatively-charged phosphate group, and the positivelycharged choline, which readily dissolve in water . The streamers or "tails" represent the nonpolar part, the long chain of 12 to 18 carbon atoms in the two fatty acids, which are insoluble in water. As a result of the nature of its head and tail groups, lecithin molecules tend to disperse themselves in water with their nonpolar tails back-to-back to form bilayers, or double layers, in which the polar heads project outward into the water. This arrangement sequesters, or conceals, the nonpolar tails away from the water, forming a structural barrier to the passage of polar and ionic molecules as shown.

Dietary and commercial sources

Because lecithin is found in all living organisms, it is readily available in foods. Egg yolks, liver, peanuts, corn, spinach , and whole grains are good dietary sources. Soy beans are by far the most important commercial source of lecithin, because they are an excellent source, and because such huge amounts are produced. Commercial lecithin is widely used to process food products, including baking mixes, candy, chewing gum, chocolate, ice cream, macaroni and noodles, margarine, whipped toppings, and so on. It is generally used as an emulsifier, a substance that can bring water and oil together. However, the amounts used as a food additive in such products are not enough to be a good dietary source of lecithin.

Role in health and disease

Lecithin is the most abundant membrane phospho-lipid in our cells. A study involving cells with a temperature-sensitive genetic defect in lecithin biosynthesis illustrates how essential it is for cell survival. When grown above a certain temperature , these cells were unable to make lecithin. Under these conditions the cells began to burst open and eventually died.

Several studies suggest that lecithin is involved in cell signaling, the process by which one cell initiates changes in another. For example, a hormone, neuro-transmitter , or growth factor secreted by one cell communicates with another by altering its cell membrane in some way, usually by activating an enzyme which breaks down phospholipid in the membrane. The breakdown products interact with an enzyme which sets into motion a domino effect of changes in cell growth, metabolism , function, and so on. Disruptions in this process may give rise to certain diseases. Some recent evidence suggests that lecithin deficiency may interfere with cell signaling and so may be a factor in the development of liver and colon cancer .

Lecithin plays an important role in the transport of fats and cholesterol from the liver to sites where they can be either used or stored. Since fats do not dissolve in water solutions like blood plasma , they are transported in spherical particles called lipoproteins. These particles can mix with water solutions because the water-friendly proteins , cholesterol and phospholipids are on the outside surface. The nonpolar fats associated with them make up the core, which is unexposed to water. Because lecithin is required for lipoprotein synthesis, a lecithin deficiency results in fats accumulating in the liver and leads to liver damage. Lecithin deficiency also leads to increased amounts of cholesterol in the blood and atherosclerosis, a disease in which narrowing of the arteries is caused primarily by the deposit of fats from the bloodstream.

Lecithin is the primary source of choline, the precursor of the neurotransmitter acetylcholine . Recent findings suggest a relationship between the lack of availability of lecithin to nerve cells which produce acetylcholine, and the progress of Alzheimer disease .

Commercial importance

Commercial lecithin is actually a mixture of lecithin, other related phospholipids and oil. Due to its ability to break up fat and oil globules and its antioxidant properties, lecithin extracted from soy beans is important commercially for processing food and other products. In food it aids in the mixing of vegetable oils, butters, and other fatty ingredients so that they are uniformly distributed throughout the product. Without lecithin these ingredients tend to separate out. A familiar example is the separation of fat out of chocolate, leaving a light oily film on the surface. Lecithin stabilizes oils against oxidation during processing, resulting in better flavor and longer shelf life. Lecithin is also used in the manufacture of paints, dyes, inks, leather goods, plastics , cosmetics, textiles , and pharmaceutical products, among others.

Resources

periodicals

Bretscher, Mark S. "The Molecules of the Cell Membrane." Scientific American (October 1985): 100-08.

Canty, David J., and Steven H. Zeisel. "Lecithin and Choline in Human Health and Disease." Nutrition Reviews, 52, (1994): 327.

Delaney, Lisa, and Cemela London. "Dictionary of Healing Techniques and Remedies-part 35." Prevention, 44, (February 1992): 135.

Patricia V. Racenis

KEY TERMS

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Lipid

—Any fatty substance which tends not to dissolve in water but instead dissolves in relatively nonpolar organic solvents.

Phospholipid

—A lipid which contains a phosphate group.

Saturated

—Containing a single bond between the carbon atoms in a chain.

Unsaturated

—Containing multiple bonds between the carbon atoms in a chain. Unsaturated fatty acids contain at least one double bond between two carbon atoms.