Isolation and Sequence Studies of Cysteinyl Peptides from Spirulina Glutathione Reductase: Comparison of Active Site Cysteine Peptides with Those of Other Flavoprotein Disulfide Oxidoreductases

The amino acid sequences of the cysteinyl peptides of Spirulina sp. glutathione reductase were determined. Spirulina glutathione reductase was covalently bound to Thiopropyl-Sepharose 6B in the presence of 8M urea through thiol-disulfide exchange. After tryptic digestion, 4 distinct cysteinyl peptides were finally isolated from NADPH-reduced glutathi-one reductase and 2 from oxidized glutathione reductase. The amino acid sequences of the two cysteinyl peptides which could not be isolated from the oxidized glutathione reductase were very similar to those around the active site disulfide of the other flavoprotein disulfide oxidoreductases and a unique replacement of asparagine and valine by isoleucine and arginine between the two cysteine residues was found. The other two peptides isolated from both oxidized and reduced glutathione reductase also show considerable homology to the corresponding parts of human and Escherichia coli glutathione reductases. Glutathione reductase [EC 1.6.4.2] catalyzes the reduction of GSSG by NADPH and plays an essential role in the protection of cells against oxidative damage by maintaining a high GSH: GSSG ratio (1). The complete amino aci