Release of proteins via ion exchange from albumin-heparin microspheres

Albumin-heparin and albumin microspheres were prepared as ion exchange gels for the controlled release of positively charged polypeptides and proteins. The adsorption isotherms of chicken egg and human lysozyme, as model proteins, on microspheres were obtained. An adsorption isotherm of chicken egg lysozyme on albumin-heparin microspheres was linear until saturation was abruptly reached, The adsorption isotherms of human lysozyme at low and high ionic strength were typical of adsorption isotherms of proteins on ion exchange gels. The adsorption of human lysozyme on albumin-heparin and albumin microspheres fit the Freundlich equation suggesting heterogeneous binding sites. This was consistent with the proposed multivalent, electrostatic interactions between human lysozyme and negatively charged microspheres. Scatchard plots of the adsorption processes of human lysozyme on albumin-heparin and albumin microspheres suggested negative cooperativity, while positive cooperativity was observed for chicken egg lysozyme adsorption on albumin-heparin microspheres. Human lysozyme loading of albumin-heparin microspheres was 3 times higher than with albumin microspheres, with long term release occurring via an ion exchange mechanism. Apparent diffusion coefficients of 2.1 × 10-1 and 3.9 × 10-11cm2/sec were obtained for the release of human lysozyme from albumin-heparin and albumin microspheres, respectively. The release was found to be independent of diffusion, since the rate determining step was likely an adsorption/desorption processes. An apparent diffusion coefficient of 4.1 × 10-12 cm2/sec was determined for the release of chicken egg lysozyme from albumin-heparin microspheres. Low release of the lysozymes from albumin-heparin microspheres was observed in deionized water, consistent with the proposed ion exchange release mechanism. Overall, albumin-heparin microspheres demonstrated enhanced ion exchange characteristics over albumin microspheres.