A novel two-site immunoradiometric assay for beta-endorphin using nitrocellulose as solid phase

Interference of immunoglobulins in the radioimmunoassay of human beta-endorphin

The interference of immunoglobulins in the radioimmunoassay (RIA) of human beta-endorphin was investigated. Human IgM showed no cross-reactivity. Human IgA showed a weak cross-reaction, but the dilution curve of IgA did not show parallelism with the standard curve of beta-endorphin, thus indicating its antigenic difference. The dilution curves of human IgG showed 0.18% displacement with respect to the human beta-endorphin standard curve, with good parallelism. Moreover, five patients with multiple myeloma of the IgG type showed falsely elevated beta-endorphin levels. We investigated the possibility that certain IgGs may be responsible for the displacement of [125I]beta-endorphin in the beta-endorphin kit. The apparent beta-endorphin level of plasma from multiple myeloma patients was markedly decreased after affinity chromatography of the serum on protein A-Sepharose. In another 3 patients with multiple myeloma, we examined IgG interference by measuring the beta-endorphin levels in their lyophilized IgG diluted with saline. The results demonstrated high values of 20.2, 25.5 and 21.2 pmol/l respectively, also showing good parallelism. These immunological parallels to human beta-endorphin verify that a part of the amino acid sequence of human IgG is similar to that of human beta-endorphin. Consequently, in the measurement of beta-endorphin with polyclonal antibody, the results may sometimes be spuriously high due to cross-reaction with IgG, e.g., in patients with IgG myeloma. To avoid IgG interference, a specific monoclonal antibody to synthetic beta-endorphin should be used rather than polyclonal antibodies.

Cerebral beta-endorphin levels in a woman with Prader-Labhart-Willi syndrome

By means of a specific two-site immunoradiometric assay, we explored the beta-endorphin levels in various brain regions of a patient affected by Prader-Labhart-Willi Syndrome. The rank of the beta-endorphin levels of five cerebral zones (hypothalamus, substantia grisea centralis, pons dorsalis, medulla oblongata dorsalis medialis, thalamus medialis) of the patient was homologous to that of subjects without the syndrome, except for the medulla oblongata dorsalis medialis. In patient with the Prader-Labhart-Willi Syndrome this region had a higher ranking level than in subjects without it. However, a functional meaning cannot be attributed to such difference because the patient of this study did not exhibit neurological disturbances relating to elevated beta-endorphin levels in the medullary region investigated.