Corticotropin (ACTH), synthesized by the anterior pituitary gland, stimulates the adrenal cortex. Human ACTH has a molecular weight of 4,541 and contains 39 amino acids (Lee et al., 1961).

ACTH has structural similarities to melanotropin (melanocyte-stimulating hormone) (alpha-MSH). Human beta-melanotropin (beta-MSH) has 22 amino acid residues and a molecular weight of 2,661.

Work on the structure of the ACTH gene by restriction enzyme techniques showed that 6 hormones are derived from one gene: ACTH, lipotropin, alpha-MSH, beta-MSH, endorphin, and one other.

Thus, extensive amino acid differences between these hormones were not adequate evidence for their being distinct.

ACTH and beta-lipotropin (beta-LPH) are derived from a large precursor peptide. Each of these hormones is known to include smaller peptides having distinct biologic activities: alpha-melanotropin (alpha-MSH) and corticotropin-like intermediate lobe peptide (CLIP) are formed from ACTH; gamma-LPH and beta-endorphin are peptide components of beta-LPH. Beta-MSH is contained within gamma-LPH. The precursor peptide was called proopiomelanocortin (POMC) in 1979.