Adrenocorticotropic Hormone Biotransformation, Clearance, and Catabolism*

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Abstract
The nature of ACTH in circulation has been studied after i.v. injection of C3H3-methylated ACTH into hypophysectomized adult male rats. The tritium labeling of ACTH by reductive methylation led to no discernible loss of biological activity but resulted in some decrease in immunoreactivity. After i.v. injection of C3H3-methylated ACTH, the disappearance of radioactivity, bioactivity and immunoreactivity was determined at various times post-injection. Total radioactivity was cleared from plasma in a multiphasic manner, and the disappearance of immunoreactivity approximately paralleled radioactivity. In contrast, bioactivity decreased at a much more rapid rate. Gel exclusion chromatography (Sephadex G-50 fine) of plasma at various times after injection of C3H3-methylated ACTH revealed 3 components of radioactivity. The major component, up to 30 min postinjection, chromatographed with control C3H3-methylated ACTH; the amount and proportion of this component decreased with time after injection. This component retained essentially full immunoreactivity, but its bioactivity decreased below the assay sensitivity within 10 min. After 30 min, the major radioactive component chromatographed at the column volume (i.e., an elution volume corresponding to low MW materials); Cu-Sephadex chromatography demonstrated this component to solely represent amino acids. A minor component of radioactivity chromatographed at the void volume of the column (i.e., an elution volume corresponding to MW > 30,000). The greatest amount of tissue uptake occurred in the kidneys and liver, although muscle and adipose tissue also accounted for appreciable removal of radioactivity. Of the organs tested, only adrenal uptake was specific in the sense that it was saturable. Urinary radioactivity never exceeded more than 2% of the injected dose up to several h after injection of C3H3-methylated ACTH and seemed to represent mainly oligopeptide fragments with no appreciable immunoreactivity; in contrast, radioactive oligopeptide fragments were not detectable in plasma. Circulating ACTH is removed and catabolized by various tissues. A particularly interesting aspect of its metabolism is the biotransformation which results in a great reduction in biological activity of circulating ACTH with no significant loss of immunoreactivity. As estimated by gel exclusion chromatography, the biotransformed product exhibits a MW close to that of human (.alpha.1-39) ACTH indicating a rather subtle structural alteration probably involving the removal of one or a few N-terminal residues or side-chain modification.