Large-scale purification of Leuconostoc mesenteriodes NRRL B512F dextransucrase for use in the biosynthesis of dextran by batch and continuous chromatography

The extracellular enzyme dextransucrase was produced from Leuconostoc mesenteriodes NRRL B512F and purified by ultracentrifugation and cross-flow ultrafiltration for use in the biosynthesis of the macromolecule dextran by ion exchange chromatographic reaction—separation techniques. The two-stage purification process yielded over 90% pure dextransucrase with overall enzyme recovery of over 60%. A second stage of centrifugation was required to achieve complete cell removal. The purified enzyme contained 1–2 g l⁻¹ of solute ions, which affected the operation of the chromatographic system. Gel filtration removed over 93% of the remaining ions but resulted in high activity losses. Two-phase separation with polyethylene glycol (PEG) and purification by ion exchange chromatography were less successful in desalting the enzyme. PEG precipitation was successful in concentrating the enzyme, but the ions remained predominantly with the enzyme portion of the two phases. The purified enzyme was found to be unstable during storage.

Use of the enzyme in chromatographic reactor—separators for the production of dextran resulted in over 33% more high molecular weight dextran (the desired product) and a useful pure fructose byproduct being obtained than for a conventional reactor. Sodium and potassium ions in the enzyme hampered continuous operation by displacing calcium ions from the resin and thus reducing the separation efficiency of the system. Partial regeneration of the resin with calcium nitrate rather than complete enzyme desalting, which was very expensive and resulted in high activity losses, helped overcome this effect.