Abstract

The thermostability of cell-free protein extracts (from 10 microorganisms grown at 37–84°C) and of 10 fully or partially purified neutral proteases was determined in aqueous solution (Tris-HCl buffer 0.1 m. pH 7.0) using protein solubility and enzyme activity loss, respectively, as the criteria of denaturation. The stability of the protein extracts and purified proteins was also determined in an aqueous-water immiscible organic solvent two-phase system using the same criteria. There was a strong positive correlation between microorganism growth temperature and the thermostability of protein extracts in both aqueous and aqueous:organic two-phase systems. A correlation also existed between protease thermostability and the stability in the aqueous:organic two-phase system. Protein stability was higher in those aqueous:organic two-phase systems with a more hydrophobic organic phase. However, for pairs of organic solvents of similar hydrophobicity, the aqueous:organic two-phase system with the higher rated interfacial tension was more denaturing. Protein extract stability was sensitive to both aqueous:gas and aqueous:organic interface size depending on the nature of the organic phase.