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Characteristics of "Big ACTH" in Human Plasma and Pituitary Extracts

ROSALYN S. YALOW and SOLOMON A. BERSON

Nuclear Medicine Service, Veterans Administration Hospital Bronx, New York
Department of Medicine, Mt. Sinai School of Medicine, The City University of New York New York, New York

In this study, "Big ACTH," previously identified in plasma and in pituitary and tumor extracts, has been partly characterized by its behavior on Sephadex gel filtration and on starch gel electrophoresis and by its immunochemical reactivity and susceptibility to tryptic digestion. "Big ACTH" is a more acidic molecule than ACTH and has an elution volume between human growth hormone and serum albumin on Sephadex gel filtration. The immunoreactivity of "Big ACTH" with ACTH antibodies is indistinguishable from that of ACTH. On treatment with trypsin, "Big ACTH" rapidly releases a component that resembles ACTH in its behavior on starch gel electrophoresis and Sephadex gel filtration. The immunoreactive ACTH-like component released by tryptic digestion and authentic ACTH are degraded by trypsin at the same rates. During treatment by trypsin to effect conversion of "Big ACTH" to the ACTH-like component there is no significant change in total immunoreactivity until the ACTH-like component is itself degraded. These results suggest that "Big ACTH" contains within it ACTH covalently linked to the carboxyl group of a basic amino acid of a larger, more acidic, peptide.

^* Deceased April 11, 1972.

Received August 1, 1972.

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