ACTH (1-39)

Adrenocorticotropic hormone (ACTH) is a 39-amino acid peptide hormone secreted by the anterior pituitary gland in response to corticotropin-releasing hormone (CRH) from the hypothalamus. ACTH is the primary physiological regulator of adrenal glucocorticoid synthesis and secretion, playing a central role in the hypothalamic-pituitary-adrenal (HPA) stress response axis.

ACTH binds to the MC2R receptor on adrenal cortical cells, activating adenylate cyclase and stimulating cortisol synthesis through increased expression of steroidogenic enzymes. The N-terminal portion of ACTH (residues 1-24) contains the full biological activity required for adrenal stimulation, while the C-terminal region modulates binding and specificity.

Clinically, synthetic ACTH (typically the 1-24 fragment, cosyntropin) is used in stimulation tests to assess adrenal reserve and diagnose adrenal insufficiency. A normal response demonstrates adequate adrenal cortisol-producing capacity, while a blunted response indicates primary adrenal disease.

Beyond adrenal effects, ACTH demonstrates melanocortin activity at MC1R due to its N-terminal sequence homology with alpha-MSH. This accounts for the hyperpigmentation observed in conditions with chronically elevated ACTH, such as Addison's disease. Research applications include HPA axis physiology, stress biology, and steroidogenesis mechanism studies.