SELENOPROTEINS

Glutathione (GSH)

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Glutathione peroxidase

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• GSHPx-1: cytosolic or classical; found in all cells
• GSHPx-2 (GI): mainly in cells of gut tissues
• GSHPx-3: extracellular; present in plasma and milk
• GSHPx-4 (phospholipid hydroperoxide glutathione peroxidase): catalyses reduction of fatty acid hydroperoxides

Peroxides are produced in the body as a result of oxidative stress in the organism. They can be detected in plasma. GSHPx removes peroxides and so protects. But reactive oxygen species (ROS) also play a role in cell signalling. ROS may mediate proinflammatory signals (stress or cytokines) and growth stimulation (by growth factors). GSHPx may be important in regulating the balance of oxidants involved in signalling.

Selenoprotein P

Extracellular glycoprotein found in plasma (where it carries much of the plasma Se) and also associated with endothelial cells (heparin-binding domains allow it to coat cell membranes).

Its coding sequence contains several UGAs indicating selenocysteine incorporation.

The concentration of plasma selenoprotein P declines in Se deficiency and so it can be used as a biomarker of Se status.

Its role may be to protect endothelial cells from oxidant molecules generated during the inflammatory response.

Iodothyronine deiodinase

Converts thyroxine (T4) to triiodothyronine (T3), and further removal of I from T3. Thus involved in regulation of the level of the active hormone T3 in cells.

Selenium and iodine are linked biochemically, since both are involved in thyroid hormone production.

Other selenoproteins

• Selenophosphate synthetase: produces selenophosphate, which is the Se donor in the reaction that produces selenocysteine.

• Selenoprotein W: biochemical function unknown.