Process for purifying proteins

A method for purifying a recombinant protein of interest from gram-negative bacterial host cells, wherein said gram-negative bacterial host cells express the recombinant protein of interest and a recombinant disulphide isomerase, wherein the method comprises:

• collecting said gram-negative bacterial host cells from culture medium;
• releasing said recombinant protein of interest and said recombinant disulphide isomerase from said collected gram-negative bacterial host cells to form a gram-negative bacterial host cell extract;
• separating said gram-negative bacterial host cells from the released recombinant protein of interest in said gram-negative bacterial host cell extract to form a liquid gram-negative bacterial host cell extract, wherein the pH of the liquid gram-negative bacterial host cell extract is lower than the pI of the recombinant protein of interest;
• adjusting the pH of the liquid gram-negative bacterial host cell extract to a pH of 5 or less to precipitate the recombinant disulphide isomerase from the recombinant protein of interest, which remains in solution;
• and
• separating the precipitated recombinant disulphide isomerase from the recombinant protein of interest remaining in solution to produce a liquid recombinant protein sample containing a reduced quantity of recombinant disulphide isomerase compared to the quantity of recombinant disulphide isomerase prior to said separating step.