Biochemical and Molecular Characterisation of Wheat Chloroplastic Glutathione Reductase

Wheat leaves contain two charge/mass-separable isoforms of glutathione reductase (GR, EC 1.6.4.2), one chloroplastic and the other probably cytosolic. The chloroplastic GR was purified to homogeneity, and its biochemical and molecular characterisation showed features very similar to the other plant GRs. In its native conformation the enzyme is composed by two subunits of 56 kDa and an associated polypeptide of 32 kDa, with an overall molecular mass of approximately 150 kDa. Optimum activity was observed at pH 8.00 and with an ionic strength between 60 to 100 mM. GR activity is highly sensitive to temperature changes, exhibiting an exponential increase up to 45 °C. It showed a high affinity for oxidised glutathione and an intermediate affinity for NADPH at pH 8.0. Inhibition tests with thiol and histidine modifiers demonstrated that -SH groups and histidine residues are essential for the catalytic properties of the enzyme. T study the origin of GR isoforms, the number of GR gene copies and the number and size of GR transcripts were determined. Southern analyses showed that wheat GR isoforms are encoded by multiple gene copies. However, a single size transcript of approximately 1.4 kb was observed, suggesting that different GR isoforms could be generated by post-transcriptional and/or post-translational modifications.