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 sele ctive 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 (U F). The system protein-membrane-electrolyte was studied using electrokineti c measurements, in order to determine the charge of both free proteins and fouled membrane, depending on the physico-chemical environment (pH, ionic s trength, chemical nature of added salts). This paper shows that, in order t o achieve the extraction of a protein from a mixture, the following strateg y can be used: the target protein recovered in the permeate has to be uncha rged, whereas the retained protein has to be charged in order to exploit th e electrostatic repulsion of the membrane partly fouled by the charged prot ein. This approach was successfully used to achieve the lysozyme/lactoferri n separation with a high selectivity (lysozyme transmission/lactoferrin tra nsmission) using an anionic membrane of pore diameter close to 28 nm. The s electivity 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 c hloride, they were always greater in potassium phosphate and increased up t o 120.