Features of the acid protease partition in aqueous two-phase systems of polyethylene glycol–phosphate: Chymosin and pepsin

The partitioning of chymosin (from Aspergilus niger) and pepsin (from bovine stomach) was carried out in aqueous-two phase systems formed by polyethyleneglycol-potassium phosphate. The effects of polymer concentration, molecular mass and temperature were analysed. The partition was assayed at pH 7.0 in systems of polyethyleneglycol of molecular mass: 1450, 3350, 6000 and 8000. Both proteins showed high affinity for the polyethyleneglycol rich phase. The increase of polyethyleneglycol concentration favoured the protein transfer to the top phase, suggesting an important protein–polymer interaction. Polyethyleneglycol proved to have a stabilizing effect on the chymosin and pepsin, increasing its protein secondary structure. This finding agreed with the enhancement of the milk clotting activity by the polyethyleneglycol. The method appears to be suitable as a first step for the purification of these proteins from their natural sources.