The primary structure of calf thymus glutaredoxin was determined by analysis of the [ ''C]carboxymethylated protein and the proteolylic fragments obtained by treatments with trypsin, chymotrypsin, CNBr and staphylococcal Glu-specific extracell ular protease.The active center has the structure Cys-Pro-Tyr-Cys, with the redox-active cysteines/half-cystines located at positions 22 and 25 in the polypeptide chain. This active center is identical in amino acid sequence and similar in position to that of Escherichia coli glutaredoxin, suggesting this structure to be typical for glutaredoxins and distinguishing them from the distantly related thioredoxins. However, the two glutaredoxins also exhibit considerable differences. Calf thymus glutaredoxin is extended at both ends and has 31 % overall residue identities with the corresponding E. cori protein. In contrast to the bacterial glutaredoxin, the calf thymus protein contains two additional half-cystineslcysteine residues at positions 74 and 78, which may be of regulatory significance.Two hydrogen donor systems are known for the NADPHdependent reduction of ribonucleotides by the enzyme ribonucleotide reductase to form the corresponding deoxyribonucleotides [l, 21. One is the thioredoxin system composed of thioredoxin, NADPH and thioredoxin reductase [I]. The other is the glutaredoxin system composed of glutaredoxin, GSH, NADPH and glutathione reductase [3]. The relative importance of the two systems for ribonucleotide reduction in cells is presently not clear [2]. Thioredoxins and glutaredoxins are both small proteins that have a redox-active disulfide/dithiol loop in the N-terminal region [4].Thioredoxin has been structurally characterized from several sources. So far, the Echerichia coli form first analyzed is similar in size and with the same active center structure as thioredoxins from bacteria, yeast, plants and mammalian cells [5 -71. However, thioredoxin froni the bacteriophage T4 differs widely from the E. coli form in primary [S] but not in tertiary [9] structure. Mammalian thioredoxin is also different, and has an additional half-cystinelcysteine pair [6].Glutaredoxin has been structurally characterized from E.

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