Immobilised metal affinity chromatography of β-galactosidase from unclarified Escherichia coli homogenates using expanded bed adsorption

The development of an expanded bed process for the direct extraction and partial purification of β-galactosidase from unclarified Escherichia coli homogenates using its natural affinity for metal loaded STREAMLINE Chelating is described. Small packed beds were used to determine the effect of chelated metal ion (Cu 2+, Ni 2+, Co 2+ or Zn 2+), loading pH and ionic strength on the selective binding capacity, and recovery of β-galactosidase from clarified homogenates. An elution protocol was developed using the competitive displacer, imidazole, to recover β-galactosidase in 87% yield and 3.4-fold purification. These results were then used to develop a separation for the recovery of β-galactosidase from unclarified homogenates in a 2.5-cm diameter expanded bed. Although Ni 2+ loaded STREAMLINE Chelating had a 5% dynamic capacity for β-galactosidase of just 118 U ml −1 (0.39 mg ml −1), the low capacity was thought to be due to the large size of the target (464 000) relative to the exclusion limit of the macroporous adsorbent. Despite this low capacity, Ni 2 STREAMLINE Chelating was used successfully to recover β-galactosidase from an unclarified homogenate in 86.4% yield and at 5.95-fold purification. The degree of purification relative to a commercial standard, as assessed using the purification factor and sodium dodecyl sulphate–polyacrylamide gel electrophoresis was high suggesting that this pseudo-affinity procedure compared favourably with alternative methods.