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

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Abstract
Five peptides derived from pro-ACTH/endorphin, the pituitary corticotroph cell prohormone, were bioassayed with isolated rat adrenocortical cells: .alpha.- and .beta.-melanotropin, .beta.-lipotropin, .beta.-endorphin and the amino-terminal region of pro-ACTH/endorphin known as 16k [16,000 dalton] 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 s 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 in serum corticosterone concentration results. A portion of 16k fragment molecule may play a hormonal role in the control of adrenocortical steroidogenesis.

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