Processing of common precursor forms of adrenocorticotropin and endorphins in cultures of mouse pituitary cells and in mouse pituitary.

The initial steps in the processing of the common precursor to adrenocorticotropin (ACTH) and beta-lipotropin (beta-LPH) in mouse pituitary cells (AtT-20) have been investigated. Three forms of the precursor have been resolved by sodium dodecyl sulfate (NaDodSO4) polyacrylamide gel electrophoresis with apparent molecular weights of 29,000, 32,000 and 34,000 (29K, 32K, and 34K ACTH-endorphin). The three precursor forms have a very similar peptide backbone, but their carbohydrate content differs. In particular, a tryptic glycopeptide has been observed in 32K ACTH-endorphin which is not present in 29K ACTH-endorphin and has been identified as a tryptic peptide containing the alpha(22--39) sequence of ACTH. Pulse chase and continuous-labeling studies with radioactive amino acids and sugars suggest that the 29K form is converted to the 32K and 34K forms of the precursor by the addition of carbohydrate. The glycopeptide and pulse chase studies suggest that 29K ACTH-endorphin can either be converted to 4.5K ACTH by proteolytic processing or to 32K ACTH-endorphin by the further addition of carbohydrate.