Analysis of the complement C3 fragments associated with hemodialysis membranes

During hemodialysis with cuprophan membranes, bioactive peptides are generated because the alternative pathway of complement is activated. When cellulose acetate membranes are employed, complement activation is attenuated. The molecular basis for this improved biocompatibility is unknown. It has been postulated, however, that the complement activating potential of dialysis membranes is influenced by the availability of free hydroxyl groups which would provide an acceptor site for activated C3. To investigate this hypothesis, the forms of C3 associated with cellulose acetate and cuprophan membranes have been analyzed. By Western blot, the predominant form of C3 present on both types of membranes is C3c, a degradation product of C3 that lacks the thiolester necessary for covalent bonding. Minimal amounts of C3d (the region of C3 which contains the thiolester) were observed on both membranes; however, by ELISA, there was no difference in the amount bound to cellulose acetate compared to cuprophan. Further, membrane-associated C3d could be removed by urea, suggesting that it was not bound covalently. These studies indicate that the complement activating potential of dialysis membranes is not determined primarily by the availability of potential covalent binding sites for activated C3b.