Abstract
To clearly identify cells and organelles containing the common precursor (31,000 dalton) for both ACTH and .beta.-lipotropin (.beta.-LPH), an immunohistochemical localization of a fragment (16,000 dalton) of the precursor that is not common to .beta.-LPH and ACTH was conducted in rat and human pituitary glands. With the help of specific antibodies that do not crossreact with .beta.-LPH and ACTH, the 16,000 dalton fragment was localized in the cells that also produce ACTH and .beta.-LPH in both the pars distalis and pars intermedia of the rat pituitary. At the EM level, the secretory granules that contain ACTH were also stained for 16,000 dalton fragment. In the human pituitary, the 16,000 dalton fragment was also observed in all secretory granules of lipocorticotrophs. After enzymatic cleavage, fragment(s) of the common precursor and/or the whole common precursor may remain packaged within the secretory granules with peptides of known activity.

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