involvement of mitogen-activated protein kinase and Fas pathways.
Br
J Nutr. 2004;91(1):11-28. (PubMed)
phil.whanger@orst.edu
Selenomethionine (Semet) is the major seleno-compound in cereal grains
and enriched yeast
whereas Se-methylselenocysteine (SeMCYS) is the major seleno-compound
in Se-accumulator plants
and some plants of economic importance such as garlic and broccoli
exposed to excess Se.
Animals can metabolize both Semet and SeMCYS.
Epidemiological studies have indicated an inverse relationship
between Se intake and the incidence of certain cancers.
Blood or plasma levels of Se are usually lower in patients with cancer
than those without this disorder,
but inconsistent results have been found with toenail-Se values
and the incidence of cancer.
There have been eight trials with human subjects conducted
on the influence of Se on cancer incidence or biomarkers,
and except for one,
all have shown a positive benefit of Se on cancer reduction
or biomarkers of this disorder.
This is consistent with about 100 small-animal studies
where Se has been shown to reduce the incidence of tumours in most of
these trials.
Se-enriched yeast is the major form of Se used in trials with human
subjects.
In the mammary-tumour model,
SeMCYS has been shown to be the most effective seleno-compound
identified so far in reduction of tumours.
Several mechanisms have been proposed on the mechanism whereby Se
reduces tumours.
Even though SeMCYS was shown to be the most effective seleno-compound
in the reduction of mammary tumours,
it may not be the most effective seleno-compound for reduction of colon
tumours.
Proc Nutr Soc.
2005 Nov;64(4):527-42.
Selenium in cancer prevention: a review of the evidence and mechanism
of action.
Se is an unusual trace element in having its own codon in mRNA
that specifies its insertion into selenoproteins as selenocysteine
(SeCys),
by means of a mechanism requiring a large SeCys-insertion complex.
This exacting insertion machinery for selenoprotein production
has implications for the Se requirements for cancer prevention.
If Se may protect against cancer, an adequate intake of Se is desirable.
However, the level of intake in Europe and some parts of the world
is not adequate for full expression of protective selenoproteins.
The evidence for Se as a cancer preventive agent
includes that from geographic, animal, prospective and intervention
studies.
Newly-published prospective studies on oesophageal, gastric-cardia and
lung cancer
have reinforced previous evidence,
which is particularly strong for prostate cancer.
Interventions with Se have shown benefit in reducing the risk of
cancer incidence and mortality in all cancers combined,
and specifically in liver, prostate, colo-rectal and lung cancers.
The effect seems to be strongest in those individuals with the lowest
Se status.
As the level of Se that appears to be required for optimal effect
is higher than that previously understood to be required to maximise
the activity of selenoenzymes,
the question has been raised as to whether
selenoproteins are involved in the anti-cancer process.
However, recent evidence showing an association between Se,
reduction of DNA damage and oxidative stress
together with data showing an effect of selenoprotein genotype on
cancer risk
implies that selenoproteins are indeed implicated.
The likelihood of simultaneous and consecutive effects at different
cancer stages
still allows an important role for
anti-cancer Se metabolites such as methyl selenol
formed from gamma-glutamyl-selenomethyl-SeCys and selenomethyl-SeCys,
components identified in certain plants and Se-enriched yeast
that have anti-cancer effects.
There is some evidence that Se may affect
not only cancer risk but also progression and metastasis.
Se-broccoli similar to se-garlic
J Med Food. 2003
Spring;6(1):19-26.
Reduction of cancer risk by consumption of selenium-enriched plants:
enrichment of broccoli with selenium increases the anticarcinogenic
properties of broccoli.
jfinley@gfhnrc.ars.usda.gov
Plant-based diets and phytochemicals present in plants
are associated with decreased risk of cancer.
Brassica species, and broccoli in particular,
are associated with reduced risk of several important cancers.
Broccoli accumulates Se many-fold beyond the concentration of Se in the
soil,
and the chemical form of Se in broccoli
is similar to the chemical form in high-Se garlic,
a food with unique chemoprotective properties.
Se from broccoli grown to accumulate more than 500 micro g Se/g
did not accumulate in rat tissues
or increase glutathione peroxidase enzyme activity
to the same extent as Se salts or seleno-amino acids.
Se from high-Se broccoli decreased the incidence of aberrant crypts in
rats
with chemically induced colon cancer by more than 50%, compared with
controls.
Se from high-Se broccoli also decreased the incidence of mammary tumors
in rats
treated with 7,12-dimethylbenz(a)anthracene (DMBA)
and tumor number and volume in APC(min) mice.
These results suggest that development of methods to increase
the natural accumulation of Se in broccoli
may greatly enhance its health-promoting properties.
Med Hypotheses.
2006;67(2):318-22. Epub 2006 Mar 30.
Cancer chemoprevention: selenium as a prooxidant, not an antioxidant.
Rocky Mountain Selenium, Inc.,
endrake@airbits.com
Although the average daily dietary selenium (Se) intake in the United
States
is consistently above the adult RDA of 55 microg Se/day,
supranutritional supplements of 200 microg Se/day have been shown to
provide
chemopreventive benefits against several cancers, particularly prostate
cancer.
The hypothesis herein contends
that selenium compounds with the greatest anticarcinogenic potency
are likely to be sodium selenite with Se in the +4 oxidation state
and methylseleninic acid.
These compounds exert their cancer chemopreventive effects
by directly oxidizing critical thiol-containing cellular substrates,
and are more effective
than the more frequently preferred (used) supplements of
selenomethionine and
Se-methylselenocysteine
that lack oxidation capability.
Selenate (+6 Se) the immediate precursor of selenite (+4 Se)
can be metabolically reduced,
and although less potent than the +4 Se compounds cited above,
appears to be a more effective anticarcinogen than organic forms of
dietary selenium.
Apoptosis, an important, Se-induced anticarcinogenic mechanism,
is accomplished by the direct oxidation of
vicinal sulfhydryl groups in cysteine clusters within the catalytic
domains of
cellular enzymes (e.g., protein kinase C),
and by the production of CH3Se-,
which reacts with O2 to generate superoxide
and other reactive oxygen species (ROS).
Activated oncogenes "prime" cells for Se-induced prooxidative apoptosis
thereby providing the needed margin for "killing" cancer cells
while leaving normal, healthy cells unharmed.
Selenoethers, such as selenomethionine and Se-methylselenocysteine
are not oxidizing agents,
and first, must be converted to methylselenol (CH3Se-)
that can be directly oxidized to methylseleninic acid.
The addition of methioninase, to selenomethionine,
or beta-lyase to Se-methylselenocysteine,
rapidly produces significant amounts of methylselenol,
which may be oxidized to methylseleninic acid
or may react with O2 to produce superoxide and ROS,
resulting in anticarcinogenic activities comparable to selenite
or methylseleninic acid.
The relatively large amounts of selenomethionine or
Se-methylselenocysteine needed
to produce apoptosis in cancer cells
compared with selenite or methylseleninic acid
are a probable consequence of low tissue levels of the required enzymes.
Even though many studies have consistently shown that
selenomethionine is an ineffective anticarcinogen
at doses corresponding to those currently allowed by the FDA,
it has been chosen as the Se intervention agent in the
32,500-man (phase III), NCI-funded SELECT trial,
which tests the effectiveness of dietary supplements of dietary
supplements of Se and tocopherol,
individually or in combination, in the prevention of prostate cancer.
In 2013, when the data are in,
the value of using Se supplements for cancer chemoprevention
is likely to be underestimated.
11.4:
Lessons from Basic Research in Selenium and Cancer Prevention
Journal of Nutrition Vol. 128 No. 11 November 1998, pp. 1845-1854
Selenium, in the form of selenite or selenomethionine,
functions as an essential micronutrient at levels of
[approx] 0.1 ppm (mg/kg) in the animal diet,
but it becomes a toxin at levels of 8-10 ppm (Jacobs and Frost 1981).
At the other extreme,
selenium deficiency is customarily induced in laboratory animals by
[a dose of less than ] 0.01 ppm Se.
the effect of selenium deficiency on carcinogenesis.
The information in this particular topic is not only sketchy
but also inconsistent.
[anti-cancer effects unrelated to resolving deficiency issues
involve dosing at levels ] above dietary requirement,
usually in the range of 1-5 ppm Se.
More than 90% of the selenium cancer chemoprevention experiments
have used either sodium selenite or selenomethionine
as the test reagent because they are commercially available.
Both of these compounds are known to suppress carcinogenesis in many
animal models
(Combs 1997, El-Bayoumy 1991, Ip 1986, Medina and Morrison 1988).
The effect is not organ specific,
because tumor inhibition has been reported in
mammary gland, liver, skin, pancreas, esophagus, colon and a few other
sites.
[. similar effects have been shown in humans,
with significant reductions in occurrences of the major cancers in the
U.S. population:
(
lung: 54 % (P = 0.04),
colon: 37 % (P = 0.002)
prostate: 42 % (P = 0.03)
)
. the study was documented by Clark et al. (1996):
Effects of selenium supplementation for cancer prevention
in patients with carcinoma of the skin.
J.
Am. Med. Assoc. 1996; 276:1957-1985
. the Clark et al. (1996) trial used a [a non-methionine form]
(
Selenomax¨ from Nutrition 21 La Jolla, Calif.)
--
. according to
Life
Extension: A Practical Scientific Approach (p#671)
Selenomax¨ is not your typical selenized yeast:
. most selenized yeasts are primarily selenomethionine
which is less effective than non-organics (eg selenite)
at elevating deficient levels of glutathione peroxidase;
an exception is selenized yeast from Nutrition 21
which exhibits good results in this test .
. while being organic, Selenomax¨ is most likely a seleno-trisulfide
and not primarily selenomethionine .
[or a
di-methyl-di-selenide
. according to
Dr.Whanger's
ref's
yeast's se forms may vary like so:
0...4 % selenate
0...25 % selenite
6...20 % Se-methylselenocysteine
23...63 % (Nutrition 21: 16 %) se-methionine
13...21 % (Nutrition 21: 11 %) se-cysteine
13...51 % (Nutrition 21: 81 %) other:
seleno-lanthionine,
seleno-cystathionine,
{gamma}-glutamyl-seleno-ethionine,
Se-adenosyl seleno-homocysteine,
Se-ally selenocysteine,
Se-propenyl selenocysteine,
and unknown.
#46 Speciation of selenoamino acids and organoselenium compounds in
selenium-enriched yeast
using high-performance liquid chromatography-inductively coupled plasma
mass spectrometry.
J Anal Atomic Spect12 :785 Ð788,1997 .
#47 Analytical selenoamino acid studies
by chromatography with interfaced atomic mass spectrometry and atomic
emission spectral detection.
Fresenius J Anal Chem362 :447 Ð456,1998 .
#50 Identification of the principal selenium compounds in
selenium-enriched natural sample extracts
by ion-pair liquid chromatography with inductively coupled plasma- and
electrospray ionization-mass spectrometric detection.
Anal Commun36 :249 Ð252,1999 .
#54 Characterization of selenium species in biological extracts
by enhanced ion-pair liquid chromatography with inductively coupled
plasma-mass spectrometry and by reference electrospray ionization-mass
spectrometry.
Spectrochimica Acta54 :1573 Ð1591,1999 .
what are selenotrisulfides?
Selenocompounds in Plants and Animals and their Biological
Significance
Journal of the
American College of Nutrition, Vol. 21, No. 3, 223-232 (2002)
. when selenite was given to animals,
it represented only 14% of the overall quantity of selenium [found in
tissues]
and the seleno-trisulfide metabolites,
as a percentage of total selenium absorbed, were:
( seleno-di-cysteine: 10 %;
, mixed seleno-tri-sulfide of cysteine and reduced glutathione: 14 %;
, seleno-di-glutathione: 8 %
). [seleno-tri-sulfides are apparently 1 part se, and 2 parts
sulfer-based organic ]
. selenized yeast often contains mostly Selenomethionine
but the amount can vary markedly depending upon the growth conditions.
[ eg, incorporation of selenite with cysteine, cystine and glutathione
into Seleno-di-cysteine and seleno-di-glutathione
is greatly affected by pH. {
Czauderna,
Samochocka}
] [
and may vary
on storage conditions:
All the yeast-based supplements and one yeast-free supplement
formed S-(methylseleno)cysteine on heating
(from a reaction between di-methyl-di-selenide and cysteine or cystine)
-- there were similar speciation patterns observed between
yeast-based supplements subject to extended storage
and those heated briefly at elevated temperatures . ]
. plants and animals may contain some inorganic se;
when inorganics are found, they are usually selenate .
. most plants contain se-methionine,
which animals can uses as either se or methionine
[hence the reason animals short on protein (esp'ly methionine)
will still be deficient in se after getting se-methionine]
. animals contain mostly variants of se-cysteine;
any se-methionine used as se rather than protein
will soon be reduced and reconstructed as some se-cysteine .
. the few plants that contain mostly se-cysteine (esp'ly
Se-methylselenocysteine)
-- when they contain any se at all --
include garlic, onions, wild leeks, and broccoli florets or sprouts .
]
. the Nutrition 21 Selenomax¨ trial was
double-blind, randomized, placebo-controlled, and involving
1312 basal cell or squamous cell carcinoma of the skin patients (mostly
men)
from a selenium-deficient area (eastern coastal plains of USA).
. the blinded phase of the trial was stopped early
primarily because of the apparent reductions in total cancer mortality
and total cancer incidence in the selenium group .
. it was originally set up to determine whether
selenium would decrease the incidence of skin cancer;
and while it failed to show any effect there,
it did show that giving se to se-deficient people (200 µg of selenium
per day)
was capable of significant harm reduction:
reductions in total cancer incidence:
(77 cancers in the selenium group
vs 119 in controls [RR, 0.63; 95% CI, 0.47-0.85])
reductions in total cancer deaths:
(29 deaths in the selenium treatment group
vs 57 deaths in controls [RR, 0.50; 95% CI, 0.31-0.80]) .
the scientific conclusion:
[11.4:
. what was proven here was not that increasing se decreases cancer risk,
but that being se deficient does increase cancer risk .
. it's an essential mineral, and plants don't need it,
so the only sure source is seafood and or supplements .
]
. one reason for the variation of success could be
the degree to which apoptosis is involved or prevented;
eg, calcium channel blockers can inhibit apoptosis
and increasing doses of them are known to
increase the risk of cancer in the elderly . [48]
Ganther H. E., Lawrence J. R.
Chemical transformations of selenium in living organisms.
Improved forms of selenium for cancer prevention.
