Selective extraction of lysozyme from a mixture with lactoferrin by ultrafiltration. Role of the physico-chemical environment

Variation of the physico-chemical environment was used to optimize the selective extraction of lysozyme(14 300 g.mol(-1)) from a model protein mixture with lactoferrin (77 000 g.mol(-1), in monomer form) by ultrafiltration (UF). The system protein-membrane-electrolyte was studied using electrokinetic measurements, in order to determine the charge of both free proteins and fouled membrane, depending on the physico-chemical environment (pH, ionic strength, chemical nature of added salts). This paper shows that, in order to achieve the extraction of a protein from a mixture, the following strategy can be used: the target protein recovered in the permeate has to be uncharged, whereas the retained protein has to be charged in order to exploit the electrostatic repulsion of the membrane partly fouled by the charged protein. This approach was successfully used to achieve the lysozyme/lactoferrin separation with a high selectivity (lysozyme transmission/lactoferrin transmission) using an anionic membrane of pore diameter close to 28 nm. The selectivity of the separation was studied according to the variation of the ionic strength, in the range 1 to 150 mmol.L-1, with either sodium chloride or potassium phosphate. Whereas selectivities were close to 20 in sodium chloride, they were always greater in potassium phosphate and increased up to 120.