Adrenocortical response to corticotropin is potentiated by part of the amino-terminal region of pro-corticotropin/endorphin

Five peptides derived from pro-corticotropin/endorphin (pro-ACTH/endorphin, the pituitary corticotroph cell prohormone, were bioassayed with isolated rat adrenocortical cells: α- and β-melanotropin, β-lipotropin, β-endorphin, and the amino-terminal region of pro-ACTH/endorphin known as '16k fragment'. The effect of each on steroidogenesis was measured at potentially physiological concentrations (0.01-1 nM) in both the absence and presence of varying concentrations of ACTH-(1-24). of the peptides tested, only 16k fragment, the amino-terminal region of pro-ACTH/endorphin, has a slight but significant potentiating effect on ACTH-(1-24) action. Prior treatment of 16k fragment with trypsin for 30 sec dramatically increases this dose-dependent synergism. Experiments performed in vivo with hypophysectomized female rats indicate that the trypsin digest of 16k fragment stimulates cholesterol ester hydrolase (cholesterol esterase; sterol-ester acylhydrolase, EC 3.1.1.13) activity in the adrenal cortex but fails to activate cholesterol side-chain cleavage. The effect of the trypsinized material can therefore be qualitatively distinguished from that of ACTH-(1-24). When both ACTH-(1-24) and the digest are administered together, a synergistic increase corticosterone concentration results. We propose that a portion of 16k fragment molecule may play a hormonal role in the control of adrenocortical steroidogenesis.