ABSTRACT
Thioredoxin (Trx), NADPH and thioredoxin reductase (TrxR), collec tively called the thioredoxin system, is a hydrogen donor for ribonucle otide reductase (RNR), the essential enzyme providing the deoxyribonucleotides for DNA synthesis (1-3). The allosterically regulated ribonucleotide reductase operates by a radical mechanism (4), and for each ribonucleotide reduced, a disulfide in the enzyme is generated, which has to be reduced by thioredoxin at the expense of NADPH (13) (Fig. 1). Thioredoxin, which exists in all living cells, is a general pro tein disulfide oxidoreductase operating by a dithiol mechanism. The relatively strong reductive capacity of thioredoxin makes it a cellular equivalent of dithiolthreitol (DTT). However, thioredoxin is a sophis ticated catalyst; it is specific and has a large number of functions in different biological systems through binding interactions with other proteins. A general role in redox regulation by thiol redox control and cellular signaling has received a great deal of attention.
