Glutamate dehydrogenases from thermophiles and hyperthermophiles have been reported to be expressed as inactive forms in a mesophile Escherichia coli and differ in their form from the native enzymes. We have recently found that the recombinant Pyrobaculum islandicum glutamate dehydrogenase (Pis-GDH) expressed in E. coli hardly exhibits the activity, and that the heat treatment of the inactive enzyme at 90℃ or the addition of 5M urea to the inactive enzyme solution dramatically activated to the activity level comparable to that of the native enzyme. Our small-angle X-ray scattering measurements showed that the size of inactive molecule markedly decreases and the arrangement of quaternary structure changes accompanying the activation without change in the molecular weight of the enzyme. This finding indicates that the subunit rearrangement - i.e., a change in the quaternary structure of the recombinant hexameric Pis-GDH - is essential for creation of the active enzyme.