Characterization of pro-opiocortin-converting activity in purified secretory granules from rat pituitary neurointermediate lobe.

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Abstract
Lysates of secretory granules from rat pituitary neurointermediate lobes were incubated with [3H]arginine- or [3H]phenylalanine-labeled toad pro-opiocortin. The processed products formed were identified by immunoprecipitation with ACTH and endorphin antisera and by migration behavior on acid/urea/polyacrylamide gels. Pro-opiocortin was cleaved by the proteolytic activity in the secretory granule fraction to .apprxeq. 21,000 MW ACTH, .apprxeq. 13,000 MW ACTH, .alpha.-melanotropin, 16,000 MW NH2-terminal glycopeptide, .beta.-lipotropin and an endorphin-related peptide. Characterization of this pro-opiocortin-converting activity shows that it is present in membrane and soluble fractions of the granule lysates, has a pH optimum of 5.0, appears to cleave at pairs of basic amino acid residues in the precursor, and is inhibited by leupeptin, pepstatin A, and p-chloromercuribenzoate but not diisopropyl fluorophosphate, N.alpha.-p-tosyl-L-lysine chloromethyl ketone hydrochloride, chloroquine, or EDTA. The converting-enzyme activity apparently is due to an acid thiol, arginyl protease, distinct from any known cathepsin B-like activity.