Enzyme thioredoxin reductase

Deoxynucleotides for DNA synthesis are made at the nucleoside diphosphate level and then have to be phosphorylated up to the triphosphate using a kinase and ATP. The reducing equivalents for the reaction come from a small protein, thioredoxin, that contains an active site with two cysteine residues. Upon reduction of the ribose to the 2 -deoxyri-bose, the thioredoxin is oxidized to the disulfide. The thioredoxin(SS) made during the reaction is recycled by reduction with NADPH by the enzyme thioredoxin reductase. 

Arsenic trioxide is a chemotheraputic agent used to treat leukemia that is unresponsive to first line agents. It is suspected that arsenic trioxide induces cancer cells to undergo apoptosis. The enzyme thioredoxin reductase has recently been identified as a target for arsenic trioxide. Due to the toxic nature of arsenic, this drug carries signihcant risks. 

The pKa assignments, which have been controversial, are discussed in Chapter 7. This disulfide loop is reduced by NADPH through the action of the flavoprotein enzyme thioredoxin reductase. 

One possibihty for minimizing oxidized protein damage is the thiol repair (Fig. 3). This repair system requires either glutathione or the thioredoxin system. The thioredoxin/thioredoxin reductase repair system [10] is able to reduce disulfide bonds. It can dethiolate protein disulfides and thus is an extremely important regulator for redox homeostasis in the cells. Thioredoxin is a smaU ubiquitous protein that contains a pair of cysteines that undergo reversible oxidation and are re-reduced by the enzyme thioredoxine reductase. The thioredoxin reductase transfers electrons from NADPH to thioredoxin via a flavin. 

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. 

Thioredoxin reductases have also been purified from yeast (43, 134) and from rat fiver (135). Thioredoxin reductase from yeast is a flavo-protein with a molecular weight of approximately 75,000 and consists of two subunits, each containing one molecule of FAD. Although the amino acid composition of this thioredoxin reductase is quite different from that of E. coli, both enzymes contain 5 half-cystine residues and have almost identical absorption spectra. Like the E. coli enzyme, thioredoxin reductase from yeast is completely inhibited by p-chloro-mercuriphenylsulfonate (PCMS) only in the presence of NADPH suggesting that the yeast enzyme also contains a disulfide bridge at the catalytic site. 

Mammalian thioredoxin reductases are a family of selenium-containing pyridine nucleotide-disulfide oxidoreductases. These enzymes catalyze NADPH-dependent reduction of the redox protein thioredoxin (Trx), which contains a redox-active disulfide and dithiol group and by itself may function as an efficient cytosolic antioxidant [77]. One of the functions of Trx/ thioredoxin reductase system is the NADPH-catalyzed reduction of protein disulfide [78] ... 

Mammalian thioredoxin reductase is able to reduce many substances in addition to thioredoxin such as insulin, vitamin K, alloxan, and others, while Escherichia coli enzyme is a... 

Glutathione peroxide was the first selenium-containing mammalian enzyme identified over 35 years ago," the same year that the selenoprotein A was identified in C. sticklandtL With the advent of the human genome sequence we now know that there are 25 selenoproteins encoded in humans, " with similar numbers encoded by other mammals. Some of the best-studied selenoproteins include five isoenzymes of thioredoxin reductase, three... 

The deoxyribonucleotides, except for deoxythymidine nucleotide, are formed from the ribonucleotides by the action of an enzyme complex, which comprises two enzymes, ribonucleoside diphosphate reductase and thioredoxin reductase (Figure 20.11). The removal of a hydroxyl group in the ribose part of the molecule is a reduction reaction, which requires NADPH. This is generated in the pentose phosphate pathway. (Note, this pathway is important in proliferating cells not only for generation... 

In the first step, thioredoxin reductase reduces a small redox protein, thioredoxin, via enzyme-bound FAD. This involves cleavage of a disulfide bond in thioredoxin. The resulting SH groups in turn reduce a catalytically active disulfide bond in nucleoside diphosphate reductase ( ribonucleotide reductase ). The free SH groups formed in this way are the actual electron donors for the reduction of ribonucleotide diphosphates. 

All deoxyribonucleotides (used to synthesize DNA) are synthesized from ribonucleotides by the enzyme ribonucleotide reductase, which requires thioredoxin as a cofactor. This enzyme is highly regulated, for example, it is strongly inhibited by dATP—a compound that is overproduced in bone marrow cells in individuals with adenosine deaminase deficiency (see below). 

In the search for possible targets the inhibition activity of a range of RAPTA compounds was studied for two specific enzymes - namely thioredoxin reductases (TrxR) and cathepsin B (cat B), important targets in cancer chemotherapy [28]. TrxR was chosen as it is highly relevant with respect to gold-based drugs (see below) and cat B is implicated in various stages of metastasis which correlate to the observed in vitro effects of RAPTA-T [29], It turned out that while the selected compounds (Fig. 3) are not inhibitors of TrxR with the exception of carboRAPTA-C, they are active inhibitors of cat B [28],... 

---