Influence of the charge of low molecular weight proteins on their efficacy of filtration and/or adsorption on dialysis membranes with different intrinsic properties

Hemodialysis membranes eliminate by filtration low-molecular-weight toxic metabolites (urea and creatinine) with minimum interactions between blood components and the membrane itself. However, the ability of a membrane to adsorb specific proteins could be beneficial if the accumulation of these same proteins is implicated in the genesis of a pathological condition. β-amyloidosis which accompanies the elevation of β2-microglobulin (11.8 kDa) in the plasma of dialysed patients is one such condition (Biochem. Biophys. Res. Commun. 129 (3) (1985) 701–706; Lancet 1 (1986) 1240–1311). To determine whether increases in plasma β2-microglobulin levels were due to differences in filtration efficacy of the membrane used and/or to certain characteristics of this protein, e.g. its charge (pI 5.7) the adsorption and filtration of [3H] β2-microglobulin and [3H] lysozyme of similar MW 14.5 kDa, but pI: 10.8 were compared on different membranes. It was found that, neither [3H] β2-microglobulin nor [3H] lysozyme are removed by cuprophan, whereas over 75% of β2-microglobulin is removed by filtration on polyacrylonitrile, polyacrylonitrile–polyethyleneimine, polysulfone and >95% by adsorption to polymethylmethacrylate–BK. For lysozyme, removal by adsorption is >95% on polyacrylonitrile and polyacrylonitrile–polyethyleneimine, 72% on polymethylmethacrylate–BK and by filtration is 95% on polysulfone. Hemodialysis membranes must therefore not simply be considered as filters of low-molecular-weight metabolites but should be equally assessed for their capacity to eliminate potentially deleterious low-molecular-weight plasma proteins.