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brassica vegetables
Brassica Vegetables
Brassicaceae as paradigm
Plants belonging to the mustard family or Brassicaceae are widely
known as cruciferous vegetables. They contain a very powerful group of
phytochemicals called glucosinolates. The glucosinolates are largely
responsible for the taste and smell of cruciferous vegetables and are
important anti-cancer agents. The active compounds are not the
glucosinolates themselves but their hydrolysis breakdown products,
isothiocyanates. It has been shown that an extract of cruciferous
vegetables added to human breast cancer cells strongly inhibits growth
of the cells, and the degree of inhibition is related to the amount of
extract in the growth medium. Epidemiological studies confirm that
consumption of cruciferous vegetables provides protection against colon
cancer. A diet containing 10 grams of crucifers per day reduces the risk
of cancer by 8%.
Brassica vegetables contain glucobrassin, an
indolylmethylglucosinolate. Levels of this thioglucoside are reported to
be as high as 1100 mg/g in some cultivars. There are three hydrolysis
products with indole-3-carbinol being the most effective as an
anti-carcinogen. It suppresses the occurrence of stomach, lung, breast
and liver tumors in rats. Indole-3-carbinol is capable of acting as a
scavenger of free radicals in an in vitro system. It also induces
enzymatic systems which facilitate the metabolism and elimination of
chemical carcinogens.
Brassica as a selenium source
Most recently, scientists have discovered a way to accumulate highly
bioavailable forms of selenium in specially cultivated plants of the
Crucifer family. Plants in the crucifer family such as broccoli contain
particular antioxidants including sulforaphanes, isothiocyanates and
indole carbinols that have been linked to improved immune functions and
are widely sold as nutritional supplements. Some natural supplement
advertisers claim that products containing cruciferous plants or plant
extracts can help prevent breast and prostate cancer.
Conventional technology for production of trace mineral supplements
involves the use of select algae or synthetic methods of concentrated
mineral production. New developments in hyperaccumulation technology
allows the production of botanical, organic sources of nutritional
supplements in plants belonging to the crucifer family. The trace
mineral supplements produced by plants may have significant inherent
advantages such as bioavailability and higher concentrations of natural
minerals.
Plants are able to transform inorganic selenium in soil to organic
selenium compounds following the sulfur assimilatory scheme. Dietary
selenium from plant sources is probably largely in the form of
selenomethionine, whereas selenium derived from animal sources comprises
selenocysteine, selenomethionine and other forms of selenium. Higher
tissue concentrations in animals fed selenomethionine rather than
selenite is not surprising since selenomethionine is an amino acid that
is not distinguished from methionine in the body and so enters the
methionine pool. However, incorporation of selenomethionine into
proteins does not contribute any benefits of true functional
selenoproteins because the selenium is not in the form of selenocysteine.
Selenomethionine can actually have a negative impact during periods of
excessive protein catabolism (dieting or starvation) when the body can
be flooded with harmful Se intermediates formed from a glut in SeMet
being released from protein breakdown (Ip
and Lisk 1994b).
Selenomethionine is converted into selenocysteine via the
transsulfuration pathway. Selenocysteine B-lyase degrades selenocysteine
so efficiently that there is probably no free selenocysteine in the
cell. Selenocysteine and selenite both enter the central selenium pool
and are metabolized into compounds that can be incorporated into either
tRNAs or intermediate compounds, with ultimate incorporation into
proteins. Selenocysteine is mostly found in the major antioxidant enzyme
glutathione peroxidase (GSHPx), first described in 1973 by Rotruck et
al. (1973).
GSHPx protects red blood cells, cell membranes and cell components from
reacting with soluble peroxides (Zachara
et al. 1997). Se-methylselenocysteine [CH3SeCH2CH - (NH2)COOH] is
considered superior to either selenite or selenomethionine in cancer
protection. This protection may be related to the rapid conversion of
Se-methylselenocysteine to methylselenol, which can act as the signal in
events associated with the suppression of neoplastic development.
Chemical form of plant selenium
In the late eighteen hundreds biologists first noted the similarity
between sulfur and selenium. For example in 1880 C. A. Cameron wrote,
"the analogy between sulphur and selenium suggests that selenium may
wholly or partly replace sulphur as a constituent of vegetables" (Cameron
1880). In the following years Cameron's statement has been confirmed
and it is now widely accepted that Se is able to substitute for sulfur
in both cysteine and methionine forming selenocysteine and
selenomethionine (for review see
Brown and Shrift, 1982). Incorporation of these selenoamino acids
into proteins is the major cause of selenium toxicity in plants.
Interestingly, certain specialized Se accumulating plants appear to
avoid these toxic effects by funneling Se into the non protein amino
acids Se-methylselenocysteine and selenocystathionine (Brown
and Shrift, 1982).
Delivery of methylselenocysteine
Research in selenium enrichment of foodstuffs was pioneered by
Clement Ip, Donald Lisk, H.J. Thompson and coworkers with garlic (Ip
and Lisk 1993,
1994a,
1994b,
1996;
Ip et al. 1992,1994,
1996;
Stoewsand et al. 1989). The normal concentration of selenium in
fruits and vegetables and grain are in the 0.01 to 0.07 ppm range. These
researchers were routinely able to grow garlic using selenate and
selenite as fertilizers with yielding garlic 150 up to 1355 PPM selenium
(Ip
et al. 1992;
Ip and Lisk 1995).
The major source of selenium present in the selenium-enriched garlic
as well as broccoli was determined to be MSC (Block
1996;
Ge et al. 1996) and that it functions in the same manner as
chemically synthesized MSC. Selenium supplied as selenium-enriched
garlic at 2 ppm in the diet of rats was able to restore maximal
activities of the two key selenoenzymes, glutathione peroxidase and type
I 5'-deiodinase more effectively than selenite as the selenium source (Ip
and Lisk 1993).
