[Sensitive noncompetitive measurement of small molecules by open sandwich immunoassay]

Proteins having multiple epitopes can be usually measured with sandwich ELISA employing two kinds of antibodies, which permits high sensitivity as well as a wide working range of more than three orders of magnitude. On the other hand, so-called monovalent antigens with MW less than 1000 are not susceptible to sandwich assays due to their small size and have almost always been measured in competitive assays. However, while a competitive assay needs only one antibody due to the principle of ratiometric measurement, optimization of the reaction conditions is necessary to attain suitable sensitivity and working range, which are often inferior to those of sandwich assays. As an alternative immunoassay for small antigens, here we propose a noncompetitive "open sandwich" immunoassay, which is based on the principle of stabilization of the antibody variable region Fv upon binding with antigen. Using ELISA to detect labeled V(H) fragments bound to immobilized V(L) in the presence of sample in microplate wells, various small molecules with MW around 200-300 can be measured with a superior detection limit and working range compared with those achieved with the corresponding competitive assays. The results indicate a common antigen recognition mode of anti-hapten antibody and wide applicability of the assay to the sensitive and handy analysis of low molecular-weight substances in areas such as environmental analysis and clinical diagnostics.