Selenoprotein P (SELENOP) and selenoprotein P autoantibodies

Selenoprotein P (SELENOP) is a protein made mainly in the liver that carries several selenium atoms in the form of the amino acid selenocysteine. It is the main transport form of the trace element selenium in the blood. Selenoprotein P is measured in serum. The test catalogue also offers the determination “Selenoprotein P AAK” – AAK stands for autoantibodies (Autoantikörper), i.e. antibodies of the body's own immune system against selenoprotein P. Both values are looked at in chronic fatigue syndrome and long/post-COVID. Selenoprotein P is considered a meaningful measure of how well the tissues are actually supplied with selenium.

What selenoprotein P does in the body

The liver incorporates absorbed selenium into selenoprotein P and releases it into the blood. The brain, testes, kidneys and other tissues take it up via their own receptors and use the selenium for their selenoenzymes. These include the glutathione peroxidases, which render hydrogen peroxide and fat peroxides harmless, the thioredoxin reductases for the balance of oxidation and reduction in the cell, and the deiodinases, which convert the thyroid hormone T4 into active T3. When selenium is scarce, the brain and hormone glands are supplied preferentially. Selenoprotein P itself also has a weak antioxidant effect.

Low selenoprotein P – what happens in the body?

A low value means that less selenium is distributed to the tissues. The selenoenzymes then work to a limited extent: defence against oxidative stress weakens, and the conversion of T4 into T3 can decline. The most common reason is a low selenium intake – the soils in large parts of Europe are low in selenium, and foods grown there contain correspondingly little selenium. Selenoprotein P also falls during inflammation, however, because the liver then changes its protein production, as well as with liver disease and absorption disorders in the gut. Studies have observed low selenoprotein P levels in severe COVID-19 courses. Possible consequences of a persistent deficiency are tiredness, brittle nails, hair loss and weakened defence against infection.

High selenoprotein P – what happens in the body?

As selenium intake rises, selenoprotein P increases until the liver cannot raise its production any further – the curve then flattens. A high value therefore usually shows that the tissues are adequately supplied with selenium. If a great deal of selenium is added beyond that, usually via high-dose preparations, it is incorporated non-specifically into other proteins; with a persistent excess, garlic-like breath, brittle nails, hair loss and skin changes can occur. Independently of intake, studies have observed raised selenoprotein P levels in insulin resistance and type 2 diabetes: as a messenger from the liver it can weaken the effect of insulin in muscle cells.

Autoantibodies against selenoprotein P – what do they do?

Autoantibodies against selenoprotein P were described only a few years ago. They can bind to the transport protein and thereby impair the uptake of selenium into the tissues. Studies have found them more often in people with autoimmune thyroid inflammation, and they were associated with lower glutathione peroxidase activity in the blood. This may explain why the selenium supply to the tissues is limited in some people even though enough selenium is taken in. How common these autoantibodies are and what significance they have in states of exhaustion and after COVID-19 is still being researched. A negative result is the normal case.

The interpretation is given in your report.