Serpins represent a diverse class of endogenous protease inhibitors that regulate important biological functions. In consideration of the importance of regulated proteolysis within secretory vesicles for the production of peptide hormones and neurotransmitters, this study revealed the molecular identity of a novel serpin, endopin 1, that is localized to neurosecretory vesicles of neuropeptide-containing chromaffin cells (chromaffin granules). Endopin 1 of 68-70 kDa was present within isolated chromaffin granules. Stimulated cosecretion of endopin 1 with chromaffin granule components, [Met]enkephalin and a cysteine protease known as "prohormone thiol protease," demonstrated localization of endopin 1 to functional secretory vesicles. Punctate, discrete immunofluorescence cellular localization of endopin 1 in chromaffin cells was consistent with its secretory vesicle localization. Endopin 1 contains a unique reactive site loop with Arg as the predicted P1 residue, suggesting inhibition of basic residue-cleaving proteases; indeed, trypsin was potently inhibited (K(i(app)) of 5 nM), and plasmin was moderately inhibited. Although endopin 1 possesses homology with alpha(1)-antichymotrypsin, chymotrypsin was not inhibited. Moreover, endopin 1 inhibited the chromaffin granule prohormone thiol protease (involved in proenkephalin processing). These results suggest a role for the novel serpin, endopin 1, in regulating basic residue-cleaving proteases within neurosecretory vesicles of chromaffin cells.