Lecture 4 - Selenium Flashcards
selenium is a what type of element and what does this mean
trace element = found in very small amounts in the body
what is the geographic intake of selenium intake like
large variations from deficiency to toxicity
what is the RDI of selenium for men and women
women is 60ug/day
men is 70ug/day
what is the New Zealand upper limit for selenium
400ug/day
there are different species of selenium, what are the ones that we need to remember
the organic ones
what are the organic species of selenium and their sources
- selenocysteine (SeCys) = animal products
- selenomethionine (SeMet) = plant products
in where is the selenium incorporated in a selenium protein
selenium is incorporated in the amino acid in place of sulfur
where does selenium enter the food chain and how does it get to us
enters the food chain through plants
- animals eat plants
- we eat animals and plants
Se content of plants is affected by what
availability of Se in soil
what is the availability of Se in soil limited by
total soil Se content
bioavailability of Se to the plant
what makes Se less bioavailable to a plant
organic matter in soil absorbs Se ( makes it not available for the plant to take up )
Fe hydroxides, clay minerals bind Se (makes them unavailable)
how does pH of soil and selenium species affect the bioavailability
- alkaline soils favour selenate > more bioavailable
- acidic neutral soils favour selenide = less bioavailable
what are hyperaccumulators of selenium
accumulate > 1000mg Se/Kg DW in Se rich soils
example of hyperaccumulators of selenium
brazil nuts = very efficient way to increase intake of selenium
what are secondary accumulators of selenium
accumulate up to 100-1000 mg Se/Kg DW
examples secondary accumulators of selenium
brassica (broccoli, cabbage)
allium (garlic, onions, leeks)
what are non accumulators of selenium
accumulate <100 mg Se/Kg DW
will only draw up small amount of selenium = how ever we eat a lot of them
examples non accumulators of selenium
wheat crops (wheat, oats, barley, rye)
rice
examples of Se low regions
NZ, Korea, Finland
examples of Se rich regions
USA, Canada, Japan, China
examples of Se deficient regions
certain regions of China, Nepal, UK, France, India
common food sources of selenium
brazil nuts
mussels
lamb kidney
blue cod
mushroom
what is the intake of Se like in New Zealand
low soil in NZ = low intakes however this changes throughout the country
but the intake in 2008 nutrition survey was pretty close to the RDI
in the 2008 nutrition survey what was the main source of selenium in NZ
bread = not super high in selenium but we consume a lot of it
why does selenium intake heavily change throughout the country
south island has lower levels of Se
why is the Se content of animal products more reliable than plants
animals have an absolute requirement for Se, so the content will reflect what you eat but plants depend on bioavailability
what is the absorption of selenium like (%)
absorption of selenium is high (80% from food)
where is the main place selenium is absorbed
in the duodenum
what are the inorganic forms of selenium
selenate (SeO2 - 4)
selenide (SeO2 - 3)
inorganic and organic selenium is converted to what in the enterocyte
converted to hydrogen selenite
hydrogen selenite is packaged into what and transported where from the enterocyte
packaged into LDL and VLDL (lipoproteins) that transport selenium to the liver
what is hydrogen selenite converted to in the liver
converted to functioning selenoprotein
what is the form of selenium that does the biological functions of selenium
selenoprotein
what does a selenoprotein do
transport protein = takes the selenium from the liver to the tissues in the body that need it
how many known classes are there of selenoproteins and what do they mediate
mediate Se biological functions
- 25 known classes
biological processes that selenoproteins are involved in
- antioxidant defences
- inflammation
- muscle development and function
- reproduction
- thyroid hormone metabolism
- immune function
two important selenoprotein classes
glutathione peroxidases
thioredoxin reductases
selenoprotein synthesis is what
high regulated
up to a certain level of selenium intake your what will increase until you get to a certain level of intake
your selenoprotein synthesis will increase until you get to a certain intake where it will not increase anymore (it is maximised)
what is the active part of selenoprotein
selenocysteine (inorganic selenium species)
what can impact the functions of the selenoprotein
within the 3 UTR part of the mRNA, you can sometimes get differences in single nucleotide polymorphisms where you have different types of amino acids placed in different areas
what are the isoforms of GPx (glutathione peroxidases) and their locations
GPx1 - cytosol
GPx3 - extracellular (plasma, extracellular fluid)
GPx4 - phospholipid
what are the antioxidant defences of glutathione peroxidases
- reduce hydrogen peroxide and other organic hydroperoxides
- protect tissues from inflammatory and oxidative damage
in the glutathione peroxidase cycle H2O2 oxidises the selenol of SeCys to form what
selenic acid (SeOH)
in the glutathione peroxidase cycle what reacts with selenic acid and what does this form
glutathione molecule reacts with selenic acid to form Se-SR intermediate and water
in the glutathione peroxidase cycle what reduces the Se-SR intermediate back to selenol and what is released
another glutethione molecule reduces the Se-SR intermediate back to selenol releasing oxidised gluthethione
in the glutathione peroxidase cycle glutathione is restored by what
NADPH dependent glutathione reductase
thioredoxin reductases are involved in what
vital cell processes
what influence does liver disease have on selenoprotein synthesis
reduces SelP
what influence does kidney disease have on selenoprotein synthesis
reduces GPx3
what influence does inflammation have on selenoprotein synthesis
plasma Se reduced during the acute phase
what up-regulates selenoprotein synthesis
oxidative stress
selenium containing compounds differs from what
differs from selenoproteins
what are selenium containing compounds
SeMet is non-specifically incorporated into body proteins
when Se availability is limited what is prioritised
incorporation of Se into individual selenoproteins is prioritised
when Se availability is limited what selenoprotein is prioritised and what isnt (what happens to activities of them)
GPx4 near top of hierarchy (activities maintained)
GPx1 near bottom of hierarchy (activities drop rapidly)
what body tissues are prioritised when Se availability is limited
brain, endocrinological, reproductive tissues top of hierarchy (Se levels preserved)
heart, liver, kidney, lung tissues bottom of hierarchy (Se levels decrease)
once selenium is absorbed by the body where is it taken
to the liver
in the liver selenium is converted to what or excreted when ……. or ….
converted to selenoproteins (the essential active enzymes)
or excreted if too much
or incorporated into other body proteins (no biological importance of these)
you can have high levels of body tissue selenium methionine but that doesn’t mean ….
that doesn’t mean you have high functioning selenium status
long term Se concentration can be measured in
urine, toenails and hair
what is medium / short term selenium intake usually measured in
plasma / serum concentration
what are functional biomarkers of selenium
plasma selnoproteins (SelP - reflects liver and GPx3 - reflects kidneys)
what can use to estimate selenium intake from plasma selenium concentrations
formulas
study on men with low baseline selenium status found what reduction in prostate cancer due to supplementation
50% reduction in prostate cancer
what is the shape of the relationship between Se and disease risk and why
U-shaped relationship
when you are deficient or have excessive levels you are most at risk
