Selenium
- Chemistry similar to sulphur
- Can be substituted for S in the amino acids methionine & cysteine
- Exerts biological effects through
‘selenoproteins
Organic forms
Selenomethionine
Selenocysteine
Inorganic forms
Selenide
Selenite
Selenate
Selenium – Absorption & transport
- Efficiently absorbed in the _____ ______:
* Selenomethionine shares the same transport mechanism as methionine
* Selenocysteine not well understood
* Inorganic forms absorbed passively (selenite) or actively (selenate)
- Absorption rate in humans ~__%
- Se appears to be bound to _____ & other ______ for transport through the blood to
liver and other organs - Se amino acids can travel _____
small intestine
80
albumin
proteins
freely
Selenium- Metabolism &
Excretion
Selenium can be:
* Stored as Selenomethionine in
an amino acid pool (see protein
notes from week 4)
* Used for protein synthesis just
like methionine
* Catabolised to Selenocysteine
then degraded to give free Se
which can be incorporated into
enzymes for functional uses
- found in multiple tissues including
thyroid, kidney, liver, heart, pancreas
and muscles - Excess Se is excreted via urine &
faece
Well characterised Summary of Selenium Functions
- Antioxidant defence
- Cellular redox state maintenance
- Thyroid hormone synthesis and metabolism
Less well understood Summary of Selenium Functions
- Muscle growth
- Calcium regulation
- Protein folding
Selenium- functions:
- Essential cofactor for ______ peroxidase
* GPX 1-2 found in different body ______, GPX 3 in _____, GPX 4 in _____
* GP functions in vivo to remove _____ ____
* GP prevents ________ and ______- (works in tandem with vitamin _)
* Therefore an important ________
glutathione
tissues, plasma, membranes
hydrogen peroxide
peroxidation and oxidati
on
E
antioxidant
Selenium- functions:
- Selenium component of the enzyme 5’-Iodothyronine deiodinase transforms T4 to T3
* selenium deficiency may exacerbate _____ deficiency
iodine
Selenium- functions:
Selenoprotein P (a ____)
* Antioxidant
* Removes peroxynitrite radical which can cause ____ damage and _____ peroxidation
glycoprotein
DNA
lipid
Selenium-deficiency
- Very _____
- Often occurs in tandem with _______ exposure and/or
_____ stress (e.g. low vit E)
- Seen in the past in people receiving ________ nutrition
(intravenous nutrition) which didn’t have Se added
- _________ disease
* Cardiomyopathy
* Seen in parts of China, central Africa with Se deficient soil
* Se deficiency not the only cause but predisposes –
coxsackie virus is a co-factor
* Improves with selenium supplementation
rare
chemical
oxidant
parenteral
Keshan
Selenium and Cancer
- Mechanisms of possible prevention
- Se deficiency → lack of selenoprotein expression
- A number of selenoproteins have important anti-inflammatory and
anti-oxidant roles - These include protection against damage to lipids, lipoproteins and
DNA - Evidence
- Probably a U-shaped association with disease risk (cancer, heart disease):
low levels (<120ug/L) increase risk and high levels (>160 ug/L) increase risk - Increased selenium intake advised for those with poor selenium status, but no
evidence for benefit of high levels (achieved through supplementation)
Selenium- dietary sources
- Brazil nuts, fish, organ meats richest sources
- Muscle meats, eggs, cereals and grains good sources
- Fruits and vegetables mostly poor sources
- Content in plant foods varies depending on where they
are grown
Selenium- toxicity (______):
- Can occur in:
* Users of ________
* _____ workers
* In _______ areas when selenium-rich foods consumed
- Symptoms- nausea, vomiting, fatigue, loss of hair and nails,
muscle cramps, skin and teeth may also be affected
selenosis
supplements
Mine
selenoferous
Assessment of Se Status
Biochemistry
* Se plasma [ ] reflects dietary intake
* > 0.8 μmol/L/ 70ng/mL
* Selenoprotein P and GPX activities and [ ] in tissue and plasma
* Urinary selenium excretion
* Toe nail or hair analysis (commonly used in health studies) indicative of long-
term status
