The synthesis of omega-3 polyunsaturated fatty acids
Within omega-3 (ω-3) polyunsaturated fatty acid family:
- alpha-linolenic acid,
- eicosapentaenoic acid (EPA),
- docosahexaenoic acid (DHA)
are important fatty acids.
Omega-3 polyunsaturated fatty acids and α-linolenic acid
Like linoleic acid (omega-6 fatty acid), alpha-linolenic acid (ALA) is a primary product of plant polyunsaturated fatty acids (PUFA) synthesis and is the precursor of all the omega-3 polyunsaturated fatty acids.
It is produced de novo from linoleic acid only by plants (by the chloroplasts of marine phytoplankton and land plants) in a reaction catalyzed by Δ15-desaturase, i.e. the enzyme that forms the omega-3 polyunsaturated fatty acid family from omega-6 one catalyzing the insertion of the double bond between carbon atoms 3 and 4, numbered from methyl end of the molecule.
Note: while many land plants lack the ability to synthesize omega-3 polyunsaturated fatty acids, aquatic ones and planktons in colder water produce abundant amounts of them.
Animals, lacking Δ15-desaturase, can’t synthesize alpha-linolenic acid, and all the omega-3 polyunsaturated fatty acid family de novo, and they are obliged to obtain it from plant foodstuff and/or from animals that eat them; for this reason omega-3 polyunsaturated fatty acids are considered essential fatty acids, so called EFA.
Omega-3 polyunsaturated fatty acids: from α-linolenic acid to EPA and DHA
Animals are able to elongate and desaturase dietary alpha-linolenic acid in a cascade of reactions to form very long polyunsaturated omega-3 fatty acids but terrestrial animals have limited ability to do it. The efficiency of synthesis decreases down the cascade: conversion of alpha-linolenic acid to EPA is limited (the activity of Δ6-desaturase is the rate limiting in humans) and to DHA is even more restricted than that of EPA. This metabolic pathway occurs mainly in the liver and cerebral microvasculature of the blood brain barrier, but also in the cerebral endothelium and astrocytes.
Fish and shellfish, unlike terrestrial animals, are able to convert efficiently alpha-linolenic acid, obtained from chloroplast of marine phytoplankton, in EPA and DHA (the last one is present in high concentration in many fish oils but pay attention: many fish oils are also rich in saturated fatty acids).
It should be noted that polyunsaturated fatty acids of the ω-3 family, and of any other n-families, can be interconverted by enzymatic processed only within the same family, not among families.
EPA and DHA are primarily found in marine algae (in genetically engineered algae DHA represents approximately 50% of the total fatty acids), fish, shellfish, and marine products (particularly oil from cold-water marine fish).
Some functions of omega-3 polyunsaturated fatty acids
- Omega-3 polyunsaturated fatty acid are capable of increasing high-density lipoprotein (HDL), “good cholesterol”, and of interleukin-2 levels. On the other hand, they decrease the levels of low-density lipoprotein (LDL), “bad cholesterol“, and very low density lipoprotein cholesterol (VLDL) and of interleukin-1 levels.
- They are essential for the normal functioning of the brain and retina, especially in premature borns.
- They are essential for growth and development throughout the life; for example if in children diet there is not enough omega-3 polyunsaturated fatty acids they may suffer dermatitis, growth retardation, neurological and visual disturbances.
- C-20 polyunsaturated fatty acids, belonging to omega-3 and also omega-6 polyunsaturated fatty acid families, are the precursors eicosanoids (prostaglandins, prostacyclin, thromboxanes, and leukotrienes), potent, short-acting, local hormones.
- While the omission in the diet of omega-6 polyunsaturated fatty acids results in a manifest systemic dysfunction, the deprivation of omega-3 polyunsaturated fatty acids causes dysfunction in a wide range of behavioral and physiological modalities.
Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008
Aron H. Uber den Nahvert (On the nutritional value). Biochem Z. 1918;92:211–233 (German)
Bender D.A. “Benders’ dictionary of nutrition and food technology”. 2006, 8th Edition. Woodhead Publishing. Oxford
Bergstroem S., Danielsson H., Klenberg D. and Samuelsson B. The enzymatic conversion of essential fatty acids into prostaglandins. J Biol Chem 1964;239:PC4006-PC4008 [Full Text]
Burr G. and Burr M. A new deficiency disease produced by the rigid exclusion of fat from the diet. J Biol Chem 1929;82:345-67 [Full Text]
Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008
Cozzani I. e Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006
Mahan L.K., Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000
Rosenthal M.D., Glew R.H. Mediacal biochemistry. Human metabolism in health and disease. John Wiley & Sons, Inc. 2009
Shils M.E., Olson J.A., Shike M., Ross A.C.: “Modern nutrition in health and disease” 9th ed. 1999
Stipanuk M.H.. Biochemical and physiological aspects of human nutrition. W.B. Saunders Company-An imprint of Elsevier Science, 2000
Van D., Beerthuis R.K., Nugteren D.H. and Vonkeman H. Enzymatic conversion of all-cis-polyunsaturated fatty acids into prostaglandins. Nature 1964;203:839-41