Structure and activity of malate dehydrogenase from the extreme halophilic bacteria of the Dead Sea. 2. Inactivation, dissociation and unfolding at NaCl concentrations below 2 M. Salt, salt concentration and temperature dependence of enzyme stability.

The stability of halophilic malate dehydrogenase increases with increasing salt concentration and with decrease in temperature. Stabilization by various salts, at high salt concentrations, follows the Hofmeister series. The enzyme inactivation rates closely match dissociation of the dimeric enzymes into monomeric subunits and unfolding of the polypeptide chains, as followed by velocity sedimentation, light scattering and circular dichroism measurements. The alpha-helix content goes to zero upon denaturation. Unusual water and salt binding properties of the native enzyme (cf. preceding paper, in this journal) are believed to be largely lost upon enzyme dissociation and unfolding. The properties thus seem to be associated with the intact structure of the enzyme.