Affinity and specificity of penicillin-antibody interaction determined by an enzyme (E. coli beta-galactosidase)...

An enzyme immunoassay (EIA) for a penicillin derivative is described with a sensitivity at least at the nanogram level. The label, E. coli beta-galactosidase is a macromolecule of 540 000 daltons: the size of the enzyme and the ease of linking penicilloyl residues to it make it an interesting model to study the effect of the degree of haptenic substitution (DS) in the tracer on the parameters of EIA. Our results show that the affinity of the binding reaction between antibody and tracer is proportional to the DS but the sensitivity of inhibition is not affected, at least not between 1 and 10 penicilloyl residues per GZ molecule. The theoretical consequences and practical applications of multivalent tracers in EIA are discussed.