The Binding of Human Serum Albumin by Monodisperse Polystyrene Latex Particles

The binding of human serum albumin by monodisperse latex particles was studied applying the radioisotope labelling technique. The radiochemically active substance was 131I‐labelled albumin and the latices were monodisperse samples ranging in particle size from 200 to 940 nm. All experiments were performed in buffered solutions at pH 8.0 and the ionic strength 0.05. In one type of binding experiments human serum albumin was added in great excess leading to a saturation of latex particle surface. The excess albumin was removed by centrifugation and washing of the sediments by pure buffer. For two latex samples the bindng isotherms were determined. The results of the experiments with excess albumin suggest that a nearly constant amount is bound per unit paricle surface irrespective of the latex sample and particle size. One washing was shown to take away approximately some 10% of bound albumin from the particle surface demostrating fairly strong binding. From the binding isotherms the binding constants K and the standard free energies of binding ΔGo were estimated. Values of K about 6μM−1 and ΔGo about –9 kcal/mol indicate high affinity of human serum albumin to the latex particle surface. The results of the present experiments are discussed from the point of view of recent findings on protein interactions with detergent molecules. The results indicate a possible expansion of the tertiary structure of the human serum albumin molecule in its part attached to the particle surface whereas the part of the molecule oriented to the solution remains in its native conformation as is seen from the preserved immunochemical activity against specific antibodies.