E. Anne Edwards

The ascorbate-glutathione cycle involves successive oxidations and reductions of ascorbate, glutathione, and NADPH by the enzymes ascorbate peroxidase, glutathione reductase, and dehydroascorbate reductase. Ascorbate peroxidase activity has been identified in a number of different tissues, and the enzyme has been purified from tea, spinach, and pea. The activities of some of the ascorbate-glutathione cycle enzymes have been reported to be influenced by various environmental factors known to increase oxyradical formation. Amino-terminus peptide sequence data have been determined for putative chloroplastidic ascorbate peroxidases from tea and spinach, and for the presumed cytosolic ascorbate peroxidase from pea. A large body of evidence has been accumulated over the last decade or so which demonstrates a clear role for glutathione reductase and ascorbate peroxidase in protecting plants against damage associated with oxyradical production under “normal” conditions. In contrast to the data for tea ascorbate peroxidase, the isozymes from spinach leaves were found to be identical in size.

A cDNA for pea glutathione reductase has been cloned and sequenced. The derived amino acid sequence of 562 residues shows a high degree of homology to the previously published GR sequences from human erythrocytes and from two prokaryotes: Escherichia coli and Pseudomonas aeruginosa. The pea enzyme differs from other GRs in having an N-terminal leader sequence of about 60-70 residues which may be a chloroplast transit peptide and a 20 amino acid C-terminal extension of unknown function.