Characteristics of "big ACTH" in human plasma and pituitary extracts.

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...