Reactivity of an immobilized anti-progesterone antiserum with homologous and heterologous progesterone-horseradish peroxidase conjugates

The binding and selectivity features of an immobilised anti-progesterone an tiserum were studied by the use of four different enzyme tracers: progester one 11 alpha-hemisuccinate-horseradish peroxidase (P-11 alpha-HS-HRP), prog esterone 11 alpha-carboxymethyl ether-horseradish peroxidase (P-11 alpha-CM E-HRP), progesterone 11 beta-carboxymethyl ether-horseradish peroxidase (P- 11 beta-CME-HRP) and progesterone 3-(O-carboxymethyl)oxime-horseradish pero xidase (P-3-CMO-HRP). The antiserum-tracer affinities generally showed a re markable reduction in respect to the affinity of the analyte because of the steric hindrance of the enzyme and, among the four tracers, the higher aff inity value was evaluated for the P-11 alpha-HS-HRP (homologous to the immu nogen molecule). The concentration of antibody binding sites interacting wi th the tracers showed the presence of different classes of antibodies able to react with variable affinity with tracers and analyte, as confirmed by t he cross-reactivity values measured towards different progesterone derivati ves. The assays performed with the tracers showed that an increase of sensi tivity can be obtained using enzyme tracers provided with heterologous stru cture features with respect to the immunogen molecule.