Membrane adsorption of beta 2-microglobulin: equilibrium and kinetic characterization

Abstract

Enhanced extracorporeal removal of beta 2-microglobulin (beta 2m) may prevent the development of dialysis-related amyloidosis (DRA). One mechanism of beta 2m removal is membrane adsorption. Therefore, we fundamentally characterized beta 2m adsorption to the highly permeable polyacrylonitrile (PAN) membrane. Porous and nonporous PAN fragments were incubated in buffer containing 125I-beta 2m. Over a concentration range of 8 to 60 mg/liter, the equilibrium adsorption isotherm was linear (r = 0.99) for porous PAN while the isotherm for nonporous PAN suggested either multilayer binding or adsorption of proteins with differing orientations. In kinetic analyses, the approach to equilibrium versus (time)1/2 was evaluated. For both porous and nonporous PAN, this relationship was linear (r = 0.99), consistent with a diffusion-controlled process. Adsorption reversibility was assessed by comparing the amount bound at varying residence times (0 to 4 hr) to the amount remaining adsorbed after a subsequent incubation in buffer. The fractions remaining bound at 60, 120, and 240 minutes (0.34 +/- 0.02, 0.36 +/- 0.06, and 0.44 +/- 0.03; mean +/- SEM) were significantly greater (P < 0.05) than the value at five minutes (0.23 +/- 0.01). This suggests membrane-induced conformational changes in adsorbed beta 2m. This investigation permits the comparison of beta 2m adsorptive properties of PAN to those of other membrane-based and nonmembrane-based therapies designed to prevent DRA.