Two-Site Immunoradiometric Assay for Adrenocorticotrophin: A Cautionary Study about the Reactivity to Its Precursor Molecules.

Abstract

The specificity of a "two-site" immunoradiometric assay (IRMA) has been reevaluated by examining its ability to detect heterogeneous adrenocorticotrophin-like immunoreactivity (ACTH-LI) separated by gel column chromatography. Plasma samples from patients with Addison's disease, Nelson's syndrome and ectopic ACTH syndrome and tissue extract of human anterior pituitary were subjected to ACTH-IRMA and the levels of ACTH-LI were compared with those measured by conventional ACTH-radioimmunoassay (RIA). The level of ACTH-LI measured by IRMA was considerably lower than that measured by RIA in the plasma of a case of ectopic ACTH syndrome and the ACTH-LI did not show a dilution curve parallel with that of the standard. Gel exclusion chromatography revealed that the plasma contained a relatively large quantity of "big ACTH" which was found to be poorly detected by the IRMA. In the plasma of Addison's disease or the extract of pituitary gland in which "big ACTH" constituted a small portion, whole ACTH-LI was apparently diluted in parallel with the ACTH standard, although the "big ACTH" also did not show full parallelism with the ACTH standard in the IRMA. These data suggest that "big ACTH" derived not only from an ectopic ACTH-producing tumour but also from a normal human pituitary gland cannot be detected as well as authentic ACTH by the ACTH-IRMA system. Therefore, samples which contain a relatively large proportion of "big ACTH" in the total ACTH-LI should be carefully evaluated by ACTH-IRMA.

POMC: The Physiological Power of Hormone Processing

“…Hodgkinson et al (171) was found not to detect the "big ACTH" produced by tumors (139). Similarly, ACTH fragments not detected in the immunoradiometric assays can cause problems (317).…”

Leukemia inhibitory factor induces differentiation of pituitary corticotroph function: an immuno-neuroendocrine phenotypic switch.

“…LIF acts through a specific receptor subunit forming a heterodimeric complex with gpl3O (9)(10), a signal transduction molecule shared with oncostatin M (OSM) and interleukin 6. Corticotrophin-releasing hormone (CRH) is an important regulator of the hypothalamic-pituitary-adrenal axis during development and also in response to stress (11)(12). This hypothalamic peptide induces adenylate cyclase activity and stimulates pro-opiomelanocortin (POMC) gene transcription (13) and adrenocorticotrophin (ACTH) release (14), and may be mitogenic for pituitary corticotroph cells in vivo (15) and in vitro (16).…”