The "soluble sandwich" approach for immunoassays: methodological and instrumental implications

The simultaneous presence of homogeneous and heterogeneous reactions at different binding sites of a multiepitope antigen makes the description of the kinetic parameters of the so called "one step" solid phase immunometric assays complex. The authors extended the "one step" approach to the concept of the "soluble sandwich" methodology which differs from the former by the delayed solid phase capture of the biotinylated immunocomplex to a streptavidin coated solid support. Using prolactin monoclonal IEMAs as a model, the equilibria involved in the reactions have been studied on a thermodynamic basis through a description of the kinetics of the interactions between biotinylated Mabs and solid phase streptavidin both in presence and/or in absence of the antigen and HRP-conjugated antibody. A comparative evaluation of models in which the biotinylated antibody was previously insolubilized on the streptavidin solid phase has been performed as well. The experimental work was carried out by using 125I labelled McBiot and Prolactin to trace individual interactions and peroxidase/H2O2/TMB systems to develop the enzymatic analytical signals. A new instrument/data reducing system was also optimized to expand the OD reading range provided by conventional, single wavelength colorimeters. The greater flexibility theoretically expected for the "soluble sandwich" approach and the possibility to extend the analyte working range without detrimental effects on the readability of low doses responses have been experimentally confirmed.