The development of an enzyme immunometric assay for LH and the effects of the methods on the immunoreactivity of the conjugates

Using an affinity purified sheep anti-human luteinizing hormone IgG-horseradish peroxidase conjugate (sheep anti-hLH IgG-HRP) and rabbit anti-human luteinizing hormone antiserum (rabbit anti-hLH) directed against different antigenic determinants, a solid-phase "sandwich" enzyme immunometric assay (EIMA) for human luteinizing hormone (hLH) was developed. The assay was validated and compared with a liquid phase "two site" immunoradiometric assay (IRMA) for hLH which uses same two antibodies. The sheep anti-hLH IgG, which had been affinity purified by eluting at pH 3.5 from a hLH-sepharose 4 B column, was labelled with 125I. The IRMA is based on simultaneous addition of two antibodies to standards and samples. After overnight incubation, separation was achieved by addition of Sheep anti-Rabbit Fc (SARFc) antiserum. In EIMA; partially denaturated (at pH 2.5) Sheep anti-Rabbit FcIgG (SARFcIgG) coated polystyrene tubes or microtitre plates were employed as solid-phase second antibody. The substrate was N,N'-o-phenylene diamine (2 mg/ml) and H2O2 (O.02%). Two methods, modified NaIO4 and 4-(N-maleimido methyl)-cyclohexane-1-carboxylic acid N-hydroxy succinimide ester (SMCC), were employed in the preparation of sheep anti-hLH IgG-HRP conjugate. The immunoreactivity and peroxidase activity of conjugate prepared with NaIO4 method was impared to various extends. Both EIMA and IRMA had good specificity, were not susceptible to interference from serum components and exhibited very low non-specific binding. The values determined by EIMA were independent of the serum volume employed. Standard added to serum samples was accurately determined and the results obtained from the analysis of serum samples correlated closely with those obtained by IRMA.