Biosynthesis and characterization of adrenocorticotropic hormone, alpha-melanocyte-stimulating hormone, and an NH2-terminal fragment of the adrenocorticotropic hormone/beta-lipotropin precursor from rat pars intermedia.

Isolated intermediate lobe cells from 40 rat pituitaries were incubated for 3 h with [35S]methionine + [3H]-phenylalanine, [35S]methionine, [3H]valine, and [3H]leucine. The cell extracts were purified by carboxymethyl-cellulose chromatography (CMC) and the fraction eluting with ovine adrenocorticotropic hormone (ACTH) was further purified either by another CMC under the same conditions or by high performance liquid chromatography (HPLC). Microsequencing of the product from the second CMC allowed the identification of a peptide containing methionine 4 and phenylalanine 7, as expected for the NH2 terminus of ACTH. Purification by HPLC of a similar peptide obtained from the three other incubations gave three main raoactive peaks which were further characterized by their migration rates on polyacrylamide gels, molecular weight, and microsequencing. Results indicated that intact ACTH (residues 1-39) is present in extracts of rat intermediate lobe, but in very small quantities (less than 1% of the beta-endorphin content). ACTH is probably broken down into smaller fragments, e.g. alpha-melanocyte-stimulating hormone (alpha-MSH) (ACTH, 1-13) and corticotropin-like intermediate lobe peptide (CLIP) (ACTH, 18-39). These studies also revealed with existence of a peptide having identical sequence with the (N-1) terminus of the ACTH/lipotropin (LPH) precursor.