Gel chromatographic characterization of immunoreactive adrenocorticotropin in patients with ACTH hypersecretion

We investigated the molecular size of circulating immunoreactive
ACTH by gel chromatography in patients with ACTH hypersecretion due
to various disorders of the hypothalamic-pituitary-adrenal axis. 4
patients with Addison's disease, 2 with Nelson's syndrome, 4 with
Cushing's disease, 6 with the ectopic ACTH syndrome (2 bronchial
carcinoma, 1 medullary carcinoma, 1 metastatic islett cell
carcinoma, 1 benign bronchial carcinoid and 1 patient with occult
ectopic Cushing's syndrome) and 1 patient with hypersecretion of
ACTH from a clinically nonfunctioning pituitary adenoma were
studied. Analysis of the molecular size of immunoreactive ACTH was
performed by gel chromatography on a Sephadex G-75 column
(superfine, 100×1.5 cm) equilibrated with 1% formic acid. 2 ml
fractions were collected and evaporated to dryness. The ACTH
content of the recovered samples was determined by RIA. In
Addison's disease, Nelson's syndrome and Cushing's disease the
plasma showed a single peak of ACTH immunoreactivity at the
expected position of 1–39 ACTH. In the ectopic ACTH syndrome the
plasma of 4 patients revealed at chromatography at least one other
peak eluting between the void volume and 1–39 ACTH suggestive of a
high molecular weight form of ACTH whereas plasma of 2 patients
showed only a single ACTH peak at the position of labeled 1–39
ACTH. The patient with a clinically non-functioning pituitary
adenoma revealed a gel filtration pattern similar to the patients
with ectopic ACTH syndrom and secretion of high molecular weight
ACTH. We conclude that secretion of high molecular weight forms of
ACTH is not a unique feature of the ectopic ACTH syndrome. It may
therefore not serve as a marker of the ectopic Cushing's syndrome
in the differential diagnosis of the ACTH dependent Cushing's
syndrome. Vice versa, lack of high molecular weight ACTH does not
exclude an ectopic source of ACTH secretion as cause of Cushing's
syndrome.