|
ScienceWeek
MEDICAL BIOLOGY: ON VITAMIN E
The following points are made by M.J. Friedrich (J. Am. Med. Assoc. 2004 292:671):
1) Vitamin E has become a common presence -- not surprising for a substance touted as something that can help prevent and treat maladies ranging from cardiovascular disease to cancer. A good deal of laboratory data and results from epidemiological and retrospective studies support the notion that vitamin E can help ward off myocardial infarctions and keep cancer at bay. Findings from a variety of randomized controlled trials, however, have failed to show consistent health benefits and many questions remain unanswered about this micronutrient.
2) Vitamin E is known primarily as an antioxidant that helps rid the body of damaging free radicals, which can wreak havoc on cells and DNA by increasing the oxidative stress associated with many diseases. But the vitamin also appears to perform nonantioxidant functions that may benefit health by exerting anti-inflammatory actions and anticoagulant effects and by regulating genes and immune function.
3) Perhaps one of the most perplexing questions surrounding vitamin E is its role in cardiovascular disease. Oxidative modification of low-density lipoprotein cholesterol is considered a key step in the initiation and progression of atherosclerosis. Antioxidants such as vitamin E have been studied to determine their ability to modulate such oxidative damage and thereby decrease the risk of heart disease. But the vitamin E story is a little unusual in epidemiology. While the basic science literature suggests plausible mechanisms by which vitamin E can reduce atherosclerotic events and observational studies support this association, results from clinical trials have been conflicting in that some have shown a clinical benefit (Lancet. 1996;347:781-786) while others have not (Lancet. 1999;354:447-455; N Engl J Med. 2000;342:154-160). A recent meta-analysis of 7 randomized trials of antioxidant vitamins for the prevention of cardiovascular disease concluded that the data at this time do not support the routine use of vitamin E supplements for this purpose (Lancet. 2003;361:2017-2023).
4) Most clinical trials designed to examine the ability of vitamin E to prevent disease have included the form of vitamin E called alpha-tocopherol, the most abundant form of the vitamin and the primary compound used in dietary supplements. But recent molecular and epidemiological studies have prompted researchers to look beyond alpha-tocopherol to the other forms of vitamin E such as gamma-tocopherol. Naturally occurring vitamin E is actually a complex of 8 chemical compounds: 4 tocopherols (alpha, beta, gamma, and delta) and 4 tocotrienols (alpha, beta, gamma, and delta). While gamma-tocopherol (found in corn, soybean, and sesame oils and walnuts, peanuts and pecans) is the most common form of vitamin E in the US diet, gamma-tocopherol levels are generally 10 times lower than levels of alpha-tocopherol in plasma. For this and other reasons, researchers have placed a greater focus on alpha-tocopherol.
J. Am. Med. Assoc. http://www.jama.com
--------------------------------
Related Material:
ELEVATING VITAMIN E CONTENT OF PLANTS VIA GENETIC ENGINEERING
Notes by ScienceWeek:
The *chloroplasts of higher plants produce numerous compounds important for human agriculture and nutrition. *Tocopherols, the lipid-soluble *antioxidants sometimes known collectively as vitamin E, are one such group of compounds and are synthesized only by photosynthetic organisms.
The 4 naturally occurring tocopherols, alpha-, beta-, gamma-, and delta-tocopherol, differ only in the number and position of methyl substituents on the aromatic ring. In addition to their role as antioxidants, tocopherols stabilize polyunsaturated fatty acids within *lipid bilayers by protecting them from *lipoxygenase attack.
Of tocopherol species present in foods, alpha-tocopherol is the most important to human health and has the highest vitamin E activity. Although all tocopherols are absorbed equally during digestion, only alpha-tocopherol is preferentially retained and distributed throughout the body.
Alpha-tocopherol is an essential component of mammalian diets, and intakes in excess of the US recommended daily allowances are apparently correlated with decreased incidence of a number of degenerative human diseases. Plant oils, the main dietary source of tocopherols, typically contain alpha-tocopherol as a minor component, but with high levels of biosynthetic precursor, gamma-tocopherol.
D. Shintani and D. DellaPenna (Science 1998 282:2098) report the use of genetic engineering to clone the final enzyme in alpha-tocopherol synthesis, gamma-tocopherol methyltransferase. The authors report that *overexpression of gamma-tocopherol methyltransferase in *Arabidopsis seeds shifted oil composition in favor of alpha-tocopherol. The authors suggest that similar increases in agricultural oil crops would increase vitamin E levels in the average US diet.
Science http://www.sciencemag.org
--------------------------------
Notes by ScienceWeek:
chloroplasts: Chloroplasts are cell organelles involved in photosynthesis, and are found in all photosynthetic plant cells. The typical higher plant chloroplast is lens-shaped and approximately 5 microns in diameter. The number per cell can vary from 1 to over 100, depending on the organism. There is some evidence that chloroplasts may have originated from photosynthetic bacteria that became *endosymbiotic with plant cells.
endosymbiotic: Endosymbiosis is an arrangement in which one organism lives inside another organism, but the term is usually restricted to arrangements of mutual benefit, thus not including parasite-host relationships.
Tocopherols: "Tocopherol" is a generic term for di- and trimethyltocols. Alpha-tocopherol is 5,7,8-trimethyltocol. Although the tocopherols are sometimes known collectively as "vitamin E", the usual referent for vitamin E is alpha-tocopherol.
antioxidants: In general, an antioxidant is any substance that opposes oxidation or inhibits reactions produced by dioxygen or peroxides. An antioxidant is usually effective because it can itself be more easily oxidized than the substance protected. The term is often applied to substances that can trap free radicals, thereby breaking a chain reaction that normally leads to extensive biological damage.
lipid bilayers: Lipid bilayers are spontaneously forming self-organizing bimolecular layers of certain molecules (lipids) with long nonpolar chains terminated by a polar group. Such molecules are found in cell membranes, and also in soaps. A variety of artificial lipid bilayer membrane systems can be investigated in the laboratory. The cell membrane itself is basically a lipid-bilayer structure.
lipoxygenase: In general, any member of a group of enzymes that catalyze the oxidation of polyunsaturated fatty acids to a particular corresponding hydroperoxide. Such enzymes are found widely distributed in plants and animals (including humans).
overexpression: In general, the term "expression" refers to any gene activity, but particularly to activity that results in the production of the specific protein encoded by the gene. The expression of genes is closely regulated in the cell, so that underexpression and overexpression are potential pathological abnormalities of cell function. In the context of this report, however, overexpression is genetically engineered in a plant to produce a result of potential human benefit.
Arabidopsis: (Arabidopsis thaliana) (thale cress) A weed of the mustard family with a small genome of 120 million base pairs. Arabidopsis is now an important laboratory species, and it is presently the model for physiological, biochemical, cell biological, and developmental studies of over 250,000 plant species.
ScienceWeek http://scienceweek.com
|