The primary structure and properties of thioltransferase (glutaredoxin) from human red blood cells

Thioltransferase (glutaredoxin) was purified from human red blood cells essentially as described previously (Mieyal JJ et al., 1991a, Biochemistry 30:6088-6097). The primary sequence of the HPLC-pure enzyme was determined by tandem mass spectrometry and found to represent a 105-amino acid protein of molecular weight 11,688 Da. The physicochemical and catalytic properties of this enzyme are common to the group of proteins called glutaredoxins among the family of thiol:disulfide oxidoreductases that also includes thioredoxin and protein disulfide isomerase. Although this human red blood cell glutaredoxin (hRBC Grx) is highly homologous to the 3 other mammalian Grx proteins whose sequences are known (calf thymus, rabbit bone marrow, and pig liver), there are a number of significant differences. Most notably an additional cysteine residue (Cys-7) occurs near the N-terminus of the human enzyme in place of a serine residue in the other proteins. In addition, residue 51 of hRBC Grx displayed a mixture of Asp and Asn. This result is consistent with isoelectric focusing analysis, which revealed 2 distinct bands for either the oxidized or reduced forms of the protein. Because the enzyme was prepared from blood combined from a number of individual donors, it is not clear whether this Asp/Asn ambiguity represents inter-individual variation, gene duplication, or a deamidation artifact of purification.