Homogeneous enzymoimmunoassay for anticonvulsant drugs: effects of hapten-enzyme bridge length.

The effects of the enzyme-hapten chemical bridge on the quality of the enzymatic tracer in homogeneous enzymoimmunoassay were studied, using as a model the conjugates of glucose-6-phosphate dehydrogenase with different phenobarbital and diphenylhydantoin carboxyalkyl derivatives. For both series of conjugates the maximal immunoinhibition level was found to depend critically on the bridge length, best results being obtained with carboxymethyl derivatives. A tendency to immuno re-activation with increasing degrees of hapten substitution in the enzyme structure, particularly evident for phenobarbital-carboxyethyl and carboxypropyl conjugates, was also observed. Effects of steric hindrance on the antibody interaction, in the case of short bridges, and ineffectiveness of antibody interaction towards the active enzyme site, in the case of long bridges, have been hypothesized to contribute in defining a critical bridge size for immuno inhibition processes. Instead the tendency to immunoactivation has been tentatively interpreted in terms of occurrence of "conformational" antibody interactions involving hapten residues in enzyme regions other than the catalytic site.