Differential regulation of rat liver selenoprotein mRNAs in selenium deficiency

Hormone-nuclear receptor interactions in health and disease. The iodothyronine deiodinases and 5'-deiodination

Two types of iodothyronine deiodinase (ID-I and ID-II) catalyse the 5'-deiodination of thyroxine (T4) to produce the biologically active triiodothyronine (T3). Under normal circumstances ID-I in liver and kidney provides the main source of T3 to the circulation, whilst ID-II is largely responsible for local T3 production in the CNS, brown adipose tissue and pituitary. In some circumstances ID-II in brown adipose tissue and ID-I in the thyroid may provide a significant source of plasma T3, and ID-I in the pituitary may be important for local T3 production in this gland. The IDs thus play a pivotal role in controlling the supply of T3 to the nuclear receptors. ID-I is a selenoenzyme and, although ID-II activity is reduced in selenium deficiency, this is a consequence of increased plasma T4 concentration, rather than ID-II activity being directly dependent on selenium. Changes in 5'-deiodination occur in a number of situations such as poor nutrition, illness, iodine and selenium deficiency, and drug therapy. In iodine deficiency these changes appear to have evolved to ensure that the plasma T3 level is maintained and also to provide the brain with a degree of protection from hypothyroxinaemia. Relatively little is known about the importance of selenium deficiency on thyroid function in humans but, in combination with iodine deficiency, selenium deficiency may prove to be a contributing factor in the pathogenesis of myxodematous cretinism. The changes that occur in ID-I and ID-II in illness produce abnormalities in thyroid function tests which, although of no direct clinical significance, may lead to interpretative problems.

Conserved nucleotide sequences in the open reading frame and 3' untranslated region of selenoprotein P mRNA

Rat liver selenoprotein P contains 10 selenocysteine residues in its primary structure (deduced). It is the only selenoprotein characterized to date that has more than one selenocysteine residue. Selenoprotein P cDNA has been cloned from human liver and heart cDNA libraries and sequenced. The open reading frames are identical and contain a signal peptide, indicating that the protein is secreted by both organs and is therefore not exclusively produced in the liver. Ten selenocysteine residues (deduced) are present. Comparison of the open reading frame of the human cDNA with the rat cDNA reveals a 69% identity of the nucleotide sequence and 72% identity of the deduced amino acid sequence. Two regions in the 3' untranslated portion have high conservation between human and rat. Each of these regions contains a predicted stable stem-loop structure similar to the single stem-loop structures reported in 3' untranslated regions of type I iodothyronine 5'-deiodinase and glutathione peroxidase. The stem-loop structure of type I iodothyronine 5'-deiodinase has been shown to be necessary for incorporation of the selenocysteine residue at the UGA codon. Because only two stem-loop structures are present in the 3' untranslated region of selenoprotein P mRNA, it can be concluded that a separate stem-loop structure is not required for each selenocysteine residue.