The accumulation of cyclitols in plants is a widespread response that provides protection against various environmental stresses. The capacity of myo-Inositol, pinitol, quercitol, and other compatible solutes (i.e., sorbitol, proline, and glycinebetaine) to protect proteins against thermally induced denaturation and deactivation was examined. Enzymatic activity measurements of L-glutamine synthetase from Escherichia coli and Hordeum vulgare showed that the presence of cyclitols during heat treatment resulted in a significantly higher percentage of residual activity. CD spectroscopy experiments were used to study thermal stabilities of protein secondary structures upon the addition of myo-Inositol, pinitol, and glucose. 0.4 M myo-Inositol was observed to raise the melting temperature (T-m) of GS from E. coli by 3.9 degrees C and MDH from pig heart by 3.4 degrees C, respectively. Pinitol showed an increase in T-m of MDH by 3.8 degrees C, whereas glucose was not effective. Our results show a great potential of stabilizing proteins by the addition of cyclitols. (c) 2006 Elsevier Inc. All rights reserved.