Dissociation of the Disappearance of Bioactive and Radioimmunoreactive ACTH from Plasma in Man

Abstract

The disappearance of ACTH from human blood in vivo and in vitro has been measured simultaneously by bioassay and by radioimmunoassays using 2 antisera, one directed against the N-terminal steroidogenic portion of the ACTH molecule, the other directed against the C-terminal portion. Bioactive ACTH disappeared from the circulation faster than immunoreactive ACTH in normal subjects given either single injections or 6-hr infusions of ACTH. A similar dissociation between bioactivity and immunoreactivity was seen in vitro when ACTH was incubated in blood, plasma or plasmin solution. Disappearance of added ACTH was faster from whole blood and from frozenthawed plasma than from fresh plasma. The enzyme inhibitor Trasylol slowed the disappearance of both bioactive and immunoreactive ACTH. C-terminal ACTH immunoreactivity disappeared more slowly than N-terminal ACTH immunoreactivity. High-speed centrifugation prior to freezing or heat inactivation after freezing, addition of sodium edetate, or incubation at 4 C all slowed the disappearance of ACTH from thawed plasma. It is suggested that the rapid loss in bioactivity results in part from enzyme action on the N-terminal portion of the ACTH molecule, the products of which retain significant immunoreactivity but little or no bioactivity. Further enzyme action results in loss of immunoreactivity. The measurement of plasma ACTH concentration by radioimmunoassay in general accurately reflects the physiological status of the subject. It should be recognized, however, that the radioimmunoassay may in some circumstances give absolute values different from those determined by bioassay. Thus, caution should be exercised in equating immunoreactive ACTH concentration with true biological ACTH activity.