Nature of protein-plastic interaction

In a recent study (Viscidi et al., 1984) desorbing activity (up to 60%) was linked to certain fractions in the sample (fecal) with a molecular weight of about 25000 having proteolytic activity since this effect could be reversed with protease inhibitors or a large excess of inert protein. 

Up to a certain limit, a constant fraction of the proteins is adsorbed to plastic surfaces. For example, 80% of the IgM but only about 25% of BSA is adsorbed, with a limit of about 1.5 ng/mm for both proteins, indepedent of the input. According to Butler (1981), the protein molecules at this limit become equidistantly distributed on the surface and the failure to exceed this coverage (about 1/3 of the surface) is due to steric hindrance. 

With an excess of protein, more adsorption occurs than dictated by the binding capacity, due to the formation of multiple layers, stacked on the protein monolayer by protein-protein interactions. Such secondary interactions are not very stable and interfere in the EIA. Early studies of Oreskes and Singer (1961) on the interaction of human IgG with polystyrene indicated that adsorption occurred in two steps, characterized by different binding constants, cor- 

An important implication of these studies is that antigen present in a complex mixture binds in a non-competitive manner, up to a limit of about 1.5 ng total protein/mm, above which the binding is no longer representative of the concentration of the various antigens in the sample. 

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