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Inorganic sulfate salts are used to form aqueous two-phase systems with polyethylene glycol (PEG) for enzyme purification. Two enzymes, L-aspartase and fumarase produced by Escherichia coli are efficiently separated into different phases in spite of the high degree of similarity in molecular weight and amino acid sequence between them. The ratio of Laspartase to fumarase in the PEG-rich phase is more than sixty (60) times the ratio before extraction. A high degree of purification in a single extraction step can be achieved by careful selections of PEG molecular weight, pH, cation of the salts, and sodium chloride levels. Cations of sulfate-containing salts in the following order: NH + >Na + >Mg 2+ tend to increase the partition of L-aspartase in the PEG-rich phase. The maximal degree of enzyme purification is obtained by using PEG 4000 and ammonium sulfate as a phase system at a stable pH for both enzymes.

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