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

Abstract

Variation of the physico-chemical environment was used to optimize the selective extrac- tion 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 mem- brane, 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 fol- lowing 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 mem- brane partly fouled by the charged protein. This approach was successfully used to achieve the lysozyme/lactoferrin separation with a high selectivity (lysozyme transmission/lactoferrin trans- mission) using an anionic membrane of pore diameter close to 28 nm. The selectivity of the separa- tion 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. ultrafiltration / protein / selectivity / electrophoretic mobility / specific adsorption