Nerve growth factor in mouse and rat serum: correlation between bioassay and radioimmunoassay determinations.

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Abstract
High levels of nerve growth factor (NGF) determined by competition radioimmunoassay do not agree with values obtained by bioassay. This discrepancy was illustrated here with rat and mouse serum as examples in which values up to 1000 ng/ml have been found by using competition radioimmunoassays. An explanation for the discordant results is presented: serum components bind NGF with an intermediate affinity (Kd = 10-7 M) but with a very large capacity (up to 0.5 mg of NGF per ml of rat serum). The binding of 125I-labeled NGF to serum components competed with the binding to the solid-phase antibodies (Kd = 10-9 M) present in limiting amounts, according to the principle of competition radioimmunoassays. Less radioactivity was recovered bound to the antibodies, giving the erroneous impression that NGF is present. To overcome this difficulty, a 2-site radioimmunoassay was developed which utilized nonlimiting numbers of antibody binding sites. This assay provided a reliable determination of NGF levels in serum. In rat and mouse serum (either sex) there is less than 5 ng of NGF per ml, in agreement with the results of the bioassay.