Biosynthesis of phosphorylated forms of corticotropin-related peptides.

Abstract
Phosphorylated forms of ACTH, ACTH (1-39), corticotropin-like intermediary lobe peptide[CLIP], ACTH (18-39) and the common precursor for ACTH and .beta.-lipotropin [.beta.-LPH] were identified in extracts of rat pituitaries. 32P-Labeled inorganic phosphate was successfully incorporated into ACTH (1-39), CLIP and the ACTH/.beta.-LPH precursor in rat neurointermediary lobe explants and into ACTH (1-39) in isolated rat anterior pituitary cells. After peptidase digestion of the labeled CLIP and ACTH, the radioactive phosphate was recoverable as O-phosphoserine. The serine residue at position 31 was the only amino acid phosphorylated in CLIP and ACTH (1-39). The unphosphorylated forms of both peptides were also synthesized. The demonstration of the incorporation of [32P]phosphate into CLIP, ACTH (1-39) and the ACTH/.beta.-LPH precursor is consistent With the hypothesis that, within the rat intermediary lobe, phosphorylated CLIP is derived from a phosphorylated form of the common precursor, with phosphorylated ACTH (1-39) acting as a biosynthetic intermediate.