Widespread presence of large molecular weight adrenocorticotropin-like substances in normal rat extrapituitary tissues.

Abstract

ACTH activity in glacial acetic acid extracts of normal rat tissues was studied by both ACTH RIA and a sensitive in vitro bioassay. ACTH immunoactivity was found in all tissues: brain, 278 +/- 54 (mean +/- SE; picograms per mg protein); stomach, 59 +/- 4; kidney, 47 +/- 3; colon, 40 +/- 4; small intestine, 37 +/- 4; liver, 18 +/- 2; and heart, 16 +/- 3. Tissue ACTH showed parallelism with ACTH standard in the RIA. No correlation existed between tissue ACTH and plasma ACTH in normal rats. Dexamethasone treatment (0.4 mg/day for 5 days) suppressed plasma ACTH, but did not affect tissue ACTH levels. When tissue extracts were passed through Sephadex G-75-SF columns, ACTH immunoactivity was exclusively eluted in the portion of bigger molecular weight than ACTH standard, except in the brain. Based on this column chromatography, the molecular weight of the main peak of activity was estimated as 26,000. Tissue ACTH-like material contained no detectable biological activity (less than 2 pg/100 ng tissue). However, biological activity was easily detectable after exposure of the tissue extracts to trypsin. When studied by immunoassay and bioassay, this 26,000 mol wt ACTH was digested and cleaved to 4,500 mol wt and biologically active ACTH with trypsin treatment. Tissue ACTH immunoactivity does not seem to be the result of artifacts: 1) extracts were adjusted to pH 8.0 and a common osmolality (150 mosmol/liter) before assay; 2) protein contents in RIA tubes were only 0.1-1.6 mg; 3) tissue extracts incubated with [125I]iodo-ACTH for 48 h produced less than 5% damage of labeled hormone, as assessed by moderate excess of antibody binding; 4) enzyme inhibitors did not modify tissue ACTH levels; and 5) ACTH immunoactivity of tissue extracts was absorbed by anti-ACTH immunocolumns. We conclude that high molecular weight ACTH-like materials are widespread in normal rat extrapituitary tissues and are probably a precursor form of 4500 mol wt ACTH.