SELENOCYSTATHIONINE FROM THE PARADISE NUT
Selenium is an
element which is detrimental to most plants if it occurs in the soil.
Plants absorb selenium as selenate or selenite and convert
it in analogy to the common biochemical pathway of sulfate. In
fact, most plants cannot differentiate between selenium and sulphur
and therefore incorporate selenium instead of sulphur into amino
acids: They synthezise thus selenocysteine along with cysteine and
selenomethionine along with methionine. Most plants do also not
differentiate between sulphur and selenium containing amino acids and
incorporate both into their proteins. There are slight differences
between sulphur amino acids and selenium amino acids: The selenium is
larger and behaves
chemically somewhat differently. The Se-H bond in selenocysteine is
more acidic (that is, it ionizes at
lower pH) than
the S-H bond in cysteine and also it is more prone to oxidation. Thus,
in proteins Se-H groups form readily -Se-Se- crosslinking bridges.
This, together with the higher degree of ionization contorts proteins
and makes them - from a certain Se concentration on - dysfunctional:
The organism – be it plant or animal - dies if it has absorbed too
much selenium.
Normal plants as
for instance wheat absorb selenium passively; in contrast there are
plants which accumulate the element actively. These so-called
accumulator plants have found a way not only to survive on highly
seleniferous soils, they even make good use of the selenium. In order
to prevent the protein distortion by selenocystein and -methionin
they interrupt the seleno amino acid biosynthetic pathway at a
precursor. The precursor is selenocystathionine:
HOOC-CH(NH2)-CH2-CH2-Se-CH2-CH(NH2)-COOH
The advantage of
this compound is that it
cannot be incorporated into proteins. Thus proteins cannot be damaged.
Rather, the uncommon amino acid is
“compartmentalized" in the free form in certain parts
of the plant rendering these parts less attractive to browsing
animals. Selenium is thus used as a protectant.
Thare are
numerous selenium accumulator species in the mid Western United
States which are of botanical inerest but otherwise not commonly
known.
In South America
all selenium accumulators belong to the family of the Lecythidaceae.
Among them is Bertholettia excelsa, the Brazil nut tree.
Brazil nuts do occasionally have a
high selenium concentration but most of them are free of
selenium. This is due to the fact that the distribution range of the
Brazil nut tree is more or less the entire Amazone basin where
seleniferous soils are not common. Most Brazil nuts remain therefore
free of selenium.
There are,
however, species related to the Brazil nut, namely the genus Lecythis
which are more reliable selenium accumulators (mostly on account of a
more narrow range of distribution). Seeds of these species, if
harvested in certain areas, have been found to have an amazingly high
selenium content. The species are similar to each other so that the
local names do often not differentiate between them. Common English
names are Sapucaia and Paradise nut.
The oil from
Paradise nuts can be removed by pressing and ethanol extraction to
obtain a light colored, dry flour with a variable but often high
selenium concentration.
This nut flour
is a completely natural selenium source.
Selenocystathionine cannot be incorporated into proteins (in contrast to selenomethionine), but being a natural precursor it is easily converted into selenocysteine, the only selenium form used by the body. Paradise nut flour is thus not only the most natural but also the physiological most favourable form of selenium.