Study of Bacillus licheniformis α-amylase around the isoelectric point

The solubility of a modified recombinant Bacillus licheniformis alpha-amylase (mBLA) has been studied by batch crystallization. A semi-pure preparation was chosen containing five isoforms with pI values from 6 to 7.3 (weighted average of 6.6). Small amounts (<1 %) of protein impurities were also present. Solubility was studied in the pH range of 6 to 8. The lowest solubility without added salts was 60 mg.mL(-1) at pH 7. The addition of 0.1 mol.L-1 sodium salts of nitrate, sulfate, and thiocyanate had a small effect on solubility. However, solubility was lowered significantly by adding 0.5 mol.L-1 sodium sulfate at all pH values and increased with 0.5 mol.L-1 sodium thiocyanate at pH 7 and pH 8. The effect of anions on alpha-amylase solubility followed the Hofmeister series, and only weak evidence of reversal was seen below the isoelectric point. Cations had little effect on solubility. The sign and magnitude of the alpha-amylase zeta potential was determined in the presence and absence of 0.1 mol.L-1 salt. Qualitatively, zeta potential correctly predicted the different salts influence on mBLA solubility.