Purification and characterization of a novel thiol protease involved in processing the enkephalin precursor

Proteolytic processing enzymes are required to convert the enkephalin precursor to active opioid peptides. In this study, a novel 33-kDa thiol protease that cleaves complete precursor in the form of [35S]methionine preproenkephalin was purified from bovine adrenal medullary chromaffin granules. Chromatography on concanavalin A-Sepharose and Sephacryl S-200, chromatofocusing, and chromatography on thiopropyl-Sepharose resulted in an 88,000-fold purification with a recovery of 35% of enzyme activity. The thiol protease is a glycoprotein with a pI of 6.0. It cleaves [35S]methionine preproenkephalin with a pH optimum of 5.5, indicating that it is functional at the intragranular pH of 5.5-6.0. Interestingly, production of trichloroacetic acid-soluble products was optimal at pH 4.0, suggesting that processing of initial precursor and intermediates may require slightly different pH conditions. The protease requires dithiothreitol for activity and is inhibited by the thiol protease inhibitors iodoacetate, p-hydroxymercuribenzoate, mercuric chloride, and cystatin. These properties distinguish it from other thiol proteases (cathepsins B, H, L, N, and S), indicating that a unique thiol protease has been identified. The enzyme converted [35S]cysteine preproenkephalin (possessing [35S]cysteine residues specifically within the precursor's NH2-terminal segment) to 22.1-, 21.6-, 17.7-, 17.3-, and 15.0-kDa intermediates that contain the precursor's NH2-terminal segment; proenkephalin in vivo is converted to similar intermediates. The enzyme cleaves peptide F at Lys-Arg and Lys-Lys dibasic amino acid sites to generate methionine enkephalin and intermediates. The appropriate vesicular localization, pH optimum, proteolytic products, and cleavage site specificity suggest that this thiol protease may be involved in enkephalin precursor processing. Most interestingly, [35S]methionine beta-preprotachykinin, a precursor of substance P, is minimally cleaved, suggesting that the thiol protease may possess some selectivity for the enkephalin precursor.