Model studies on the mechanism of endotoxin adsorption on flat-sheet microfiltration membrane adsorbers

The adsorption of endotoxin from E. coli O111:B4 and bovine serum albumin on membrane adsorbers with the ligands polymyxin B, L-histidine, deoxycholate, poly(L-lysine), poly(ethyleneimine) and diethylaminoethyl was studied under equilibrium and dynamic conditions. Apparent association constants varied between 30 and 30000 mL/mg for endotoxin, whereas with BSA only approximately 1 mL/mg was found on average. With all ligands a reduction of the removal efficiency was experienced in the presence of BSA, which was used as model protein. Detailed analysis of the influence of BSA on the association constants as well as the equilibrium and dynamic capacities of endotoxin adsorption demonstrated that endotoxin and BSA molecules recognize distinct binding centres on the membrane adsorbers. Competition for binding sites plays a less important role than assumed so far. It is concluded that interactions of endotoxin with protein molecules in solutions are limiting endotoxin clearance from protein solutions. With net-positively charged proteins, electrostatic interactions yield protein-endotoxin complexes. Net-negatively charged proteins attract endotoxin by the mediation of calcium ions that are derived from endotoxin micelles and vesicles. In the latter case, endotoxin clearance can be significantly improved by the addition of EDTA.