Aggregation behaviour of human serum albumin-carrying latex particles and binding constants of divalent cations to the latex particles

Abstract

Aggregation of antigen (human serum albumin, HSA)-carrying latex particles caused by cations (Na +, K +, Ca 2+ and Mg 2+) was studied at different cation concentrations in the medium by a spectrophotometric method. HSA was fixed on the surface of carboxylated latex particles through chemical bonding. When HSA-carrying latex (abbreviated hereafter to HSA-latex) particles were mixed with the buffer solution containing cations, aggregation of the latex particles occurred in some cases and it was monitored by measuring the change of light absorption with time. HSA-latex particles were not aggregated by Na + and K + while bare latex particles tended to aggregate in the presence of these monovalent cations. However, Ca 2+ and Mg 2+ produced aggregation of bare latex particles at low concentrations, whereas higher concentrations of these divalent cations were needed to cause aggregation of HSA-latex particles. Thus, bare latex particles became more stable to cations after binding HSA on their surface. Further, the binding constants of the divalent cations to HSA-latex particles were determined from the concentrations of these cations at which the fixed-charge density became zero. As a result, the binding constants to HSA-latex particles were found to be 5.2 M −1 with Ca 2+ and 4.7 M −1 with Mg 2+.