Regulated Secretion of Pro‐Opiomelanocortin Converting Enzyme and an Aminopeptidase B‐Like Enzyme from Dispersed Bovine Intermediate Lobe Pituitary Cells

Publisher Website
Google Scholar
Abstract
Coordinate secretion of two prohormone/proneuropeptide processing enzymes [pro‐opiomelanocortin converting enzyme (PCE) and an aminopeptidase B‐like enzyme (APBE)] and α‐melanotropin (α‐MSH) from bovine intermediate lobe pituitary cells was studied. Stimulation of secretion with 8‐bromo‐cyclic AMP produced significant increases in levels of immunoreactive α‐MSH, PCE, and APBE. Treatment of cells with the dopaminergic agonist 2‐bromo‐α‐ergocryptine resulted in significant decreases in secretion of α‐MSH, PCE, and APBE. In neither case were there significant changes in levels of cytosolic lactic dehydrogenase or lysosomal β‐glucuronidase in the medium. The secreted PCE activity was shown to process frog and mouse pro‐opiomelanocortin primarily to 23,000‐Mr corticotropin (ACTH), 13,000‐Mr ACTH, β‐lipotropin, a β‐endorphin‐like peptide, and β‐endorphin, products comparable to those synthesized by the mouse and frog intermediate lobe in situ. The secreted enzymatic activity had a pH optimum between 4.0 and 5.0, was strongly inhibited by pepstatin A, and had an inhibitor profile similar to the purified bovine intermediate lobe PCE. The secreted APBE activity cleaved Arg0‐[Met]‐enkephalin to [Met]‐enkephalin and had a pH optimum and inhibitor profile similar to that previously reported for an activity from purified secretory vesicle fractions of bovine intermediate and neural lobes. The coordinate regulated secretion of α‐MSH and enzyme activities (PCE and APBE) strongly indicates their colocalization in the same secretory vesicle compartment within the cell. The characteristics of the two enzymes secreted in the medium paralleled those seen in the tissue and further support their role in pro‐opiomelanocortin processing in vivo.