Thioredoxin reductase metabolic functions

The pyridine nucleotide-disulfide oxidoreductases, lipoamide dehydrogenase (4), glutathione reductase (5), and thioredoxin reductase (6-8) share so many properties in common that they will be compared and contrasted before being considered separately. As their group name implies, they catalyze the transfer of electrons between pyridine nucleotides and disulfides. In spite of their similarities they function in widely divergent metabolic roles. 

The metabolic function of the thioredoxin reductase-thioredoxin system is to supply reducing equivalents to a wide variety of acceptors. By far the best characterized of these is the E. coli ribonucleotide reductase system (23, 261) the reductase consists of two subunits, proteins B1 and B2 (262, 263), The B1 protein contains three reactive dithiol-disulfide pairs and appears to be the immediate acceptor of reducing equivalents from thioredoxin. As isolated, the three pairs are in the reduced state and, in the presence of the B2 protein, three molecules of ribonucleotide can be reduced prior to any input of reducing equivalents from thiore-... 

As a component of glutathione peroxidase and the iodothyronine 5 -deiodinases, selenium is an essential micronutrient for humans. Its role in the deiodinase enzymes may be one reason that children require more selenium for growth than adults. Selenium is also a component of the enzyme thioredoxin reductase, which catalyses the NADPH-dependent reduction of the redox protein thioredoxin. Other selenium-containing proteins of unknown functions, including selenoprotein P found in the plasma, have also been identified. Excess selenium administered as selenite and selenate has been shown to be metabolized to methylated compounds and excreted. 

Anabaena T-1 and spinach chloroplast Tm are the most abundant thioredoxins in vivo. They exhibit approximately 50% sequence homology to thioredoxins from other nonphotosynthetic organisms (9). They can be reduced by NADPH and the flavoprotein-type reductase (10). Presumably they have intracellular functions which are common to thioredoxins in all living organisms, such as deoxynucleotide synthesis and sulfate metabolism. 

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