Evidence for the Proenkephalin Processing Enzyme Prohormone Thiol Protease (PTP) as a Multicatalytic Cysteine Protease Complex:  Activation by Glutathione Localized to Secretory Vesicles

Abstract

The cysteine protease known as "prohormone thiol protease" (PTP) has been identified as a major proenkephalin processing enzyme in secretory vesicles of adrenal medulla (known as chromaffin granules). This study provides the first demonstration that PTP exists as a multicatalytic cysteine protease complex that can be activated by endogenous glutathione present in chromaffin granules. The high molecular mass nature of PTP, of approximately 185 kDa, was demonstrated by elution of a single peak of 35S-enkephalin precursor cleaving activity by Sephacryl S200 gel filtration chromatography and by a single band of 35S-enkephalin precursor cleaving activity detected on radiozymogram gels under native buffer conditions. Importantly, when 0.1% SDS was included in radiozymogram gels, PTP activity was resolved into three bands of proteolytic activity with apparent molecular masses of 88, 81, and 61 kDa. These activities were all cysteine proteases, since they were inhibited by the cysteine protease inhibitor E-64c but not by pepstatin A or EDTA that inhibit aspartyl protease and metalloprotease, respectively. Purification of native PTP by preparative gel electrophoresis indicated that PTP was composed of four polypeptides of 66, 60, 33, and 29 kDa detected on SDS-PAGE gels. These four protein subunits accounted for the three catalytic activities of PTP, as demonstrated on 35S-enkephalin precursor radiozymogram gels. Results also indicated that the electrophoretic mobilities of the four subunits differed under reducing compared to nonreducing conditions. The multicatalytic activities of the PTP complex all require reducing conditions for activity, which can be provided by endogenous reduced glutathione in chromaffin granules. These novel findings provide the first evidence for a role of a multicatalytic cysteine protease complex, PTP, in chromaffin granules that may be involved in the proteolytic processing of proenkephalin and perhaps other precursors into active neuropeptides.

Cathepsin L in secretory vesicles functions as a prohormone-processing enzyme for production of the enkephalin peptide neurotransmitter

“…In addition, cathepsin L is known to exist as a glycoprotein with glycosylation at Asn-222 (21). Although tryptic peptides containing Asn-222 were not detected in this study, evidence for the glycoprotein nature of the secretory vesicle cathepsin L was suggested by Con A-Sepharose as one of the steps for purification of the PE-cleaving activity (10).…”

“…Secretory vesicles were isolated from bovine adrenal medulla by differential sucrose density centrifugation (10) and fixed in 0.2% glutaraldehyde͞2% paraformaldehyde in 0.1 M sodium cacodylate buffer, pH 7.2. Samples were osmicated in 2% osmium tetroxide in 0.1 M cacodylate and embedded in Epon 812.…”

“…The cysteine protease activity termed PTP represents the major PE-cleaving activity in secretory vesicles of chromaffin cells (7)(8)(9)(10)(11). In this study affinity labeling with the activity-based probe DCG-04, combined with peptide sequencing by MS, provided molecular identification of secretory vesicle cathepsin L as the cysteine protease responsible for PE-processing activity.…”

“…The primary PE-cleaving activity in chromaffin granules has been characterized as a cysteine protease complex known as ''prohormone thiol protease'' (PTP) (7)(8)(9)(10). The cysteine protease activity cleaves PE and enkephalin-containing peptide substrates at paired basic residues, as well as at certain monobasic residues, to generate appropriate enkephalin-related peptide products.…”

“…Results identified secretory vesicle cathepsin L as the enzyme responsible for the previously described PTP cysteine protease activity involved in enkephalin and neuropeptide production (7)(8)(9)(10). Cathepsin L generated the active peptide ME by cleaving enkephalin-containing peptide substrates at native dibasic and monobasic sites.…”