THERMAL-DENATURATION OF GLUTATHIONE-REDUCTASE FROM CYANOBACTERIUM SPIRULINA-MAXIMA

The thermal unfolding of glutathione reductase (NAD[P]H:GSSG oxidoredu ctase EC 1.6.4.2.) from cyanobacterium Spirulina maxima was monitored by differential scanning calorimetry and circular dichroism at neutral pH. Covalent cross-linking of enzyme at different temperatures reveal ed dimer as the species undergoing the thermal transition. A single en dotherm was observed, but its thermodynamic parameters showed dependen ce on the scan rate. In the transition zone, aggregation of the dimeri c species was observed. Analysis of the enzyme heated at 80 degrees C revealed that the resultant species retained a high content of seconda ry structure. The addition of low concentrations of guanidinium hydroc hloride resulted in a full cooperative thermal transition. A model in which the dimeric protein undergoes a partial unfolding in a kinetical ly controlled fashion is proposed, such that the experimental value of Delta H-cal results from the simultaneous occurrence of endothermic a nd exothermic events.