Activities of ACTH-Potentiating Substances Isolated from Rat Serum on Steroidogenes s in Isolated Rat Adrenal Cells

Peptides that potentiate the response of adrenal cells to ACTH1-24 were isolated from rat serum. ACTH-potentiating activity was found in fractions of 9,000-40,000 in molecular weight (APS-Fr) and of smaller molecular weight (SM-Fr) on Sephadex G-100 gel filtration of the serum extract. The peptides were isolated from APS-Fr by preparative acid polyacrylamide gel electrophoresis and named d1, d, d2, e, f and g. Their molecular weights, estimated by Sephadex G-75 gel filtration, were 41,000, 33,000, 24,000, 17,500, 17,500 and 16,000, respectively. All of these peptides were glycopeptides. The isoelectric point of peptide d was 8.45 and some of the others, such as f and g, were more basic. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis revealed that these peptides were decomposed into various fragments. ACTH-potentiating activity was highest in peptide d1 and lowest in peptide e. The maximum activity of peptide d was observed at 3 X 10(-8) M when steroidogenesis was induced by 9 X 10(-12) M ACTH1-24. While these peptides did not show any ACTH-like activity at any stage of isolation, the fractions of these peptides eluted from a Sephadex G-75 column showed more or less ACTH-like activity. These peptides therefore seemed to possess latent ACTH-like activity. The molecular weight of SM-Fr was smaller than ACTH1-24. The ACTH-potentiating activity of SM-Fr was low, and SM-Fr did not show any ACTH-like activity. SM-Fr therefore seems to be the smallest structure which has ACTH-potentiating activity. The similarity of these peptides to proopiomelanocortin-related substances was discussed.