Measurement of delta pH and membrane potential in secretory vesicles isolated from bovine pituitary intermediate lobe

Differential Neuropeptidomes of Dense Core Secretory Vesicles (DCSV) Produced at Intravesicular and Extracellular pH Conditions by Proteolytic Processing

Neuropeptides mediate cell–cell signaling in the nervous and endocrine systems. The neuropeptidome is the spectrum of peptides generated from precursors by proteolysis within dense core secretory vesicles (DCSV). DCSV neuropeptides and contents are released to the extracellular environment where further processing for neuropeptide formation may occur. To assess the DCSV proteolytic capacity for production of neuropeptidomes at intravesicular pH 5.5 and extracellular pH 7.2, neuropeptidomics, proteomics, and protease assays were conducted using chromaffin granules (CG) purified from adrenal medulla. CG are an established model of DCSV. The CG neuropeptidome consisted of 1239 unique peptides derived from 15 proneuropeptides that were colocalized with 64 proteases. Distinct CG neuropeptidomes were generated at the internal DCSV pH of 5.5 compared to the extracellular pH of 7.2. Class-specific protease inhibitors differentially regulated neuropeptidome production involving aspartic, cysteine, serine, and metallo proteases. The substrate cleavage properties of CG proteases were assessed by multiplex substrate profiling by mass spectrometry (MSP-MS) that uses a synthetic peptide library containing diverse cleavage sites for endopeptidases and exopeptidases. Parallel inhibitor-sensitive cleavages for neuropeptidome production and peptide library proteolysis led to elucidation of six CG proteases involved in neuropeptidome production, represented by cathepsins A, B, C, D, and L and carboxypeptidase E (CPE). The MSP-MS profiles of these six enzymes represented the majority of CG proteolytic cleavages utilized for neuropeptidome production. These findings provide new insight into the DCSV proteolytic system for production of distinct neuropeptidomes at the internal CG pH of 5.5 and at the extracellular pH of 7.2.

The secreted inhibitor of invasive cell growth CREG1 is negatively regulated by cathepsin proteases

Previous clinical and experimental evidence strongly supports a breast cancer-promoting function of the lysosomal protease cathepsin B. However, the cathepsin B-dependent molecular pathways are not completely understood. Here, we studied the cathepsin-mediated secretome changes in the context of the MMTV-PyMT breast cancer mouse model. Employing the cell-conditioned media from tumor-macrophage co-cultures, as well as tumor interstitial fluid obtained by a novel strategy from PyMT mice with differential cathepsin B expression, we identified an important proteolytic and lysosomal signature, highlighting the importance of this organelle and these enzymes in the tumor micro-environment. The Cellular Repressor of E1A Stimulated Genes 1 (CREG1), a secreted endolysosomal glycoprotein, displayed reduced abundance upon over-expression of cathepsin B as well as increased abundance upon cathepsin B deletion or inhibition. Moreover, it was cleaved by cathepsin B in vitro. CREG1 reportedly could act as tumor suppressor. We show that treatment of PyMT tumor cells with recombinant CREG1 reduced proliferation, migration, and invasion; whereas, the opposite was observed with reduced CREG1 expression. This was further validated in vivo by orthotopic transplantation. Our study highlights CREG1 as a key player in tumor–stroma interaction and suggests that cathepsin B sustains malignant cell behavior by reducing the levels of the growth suppressor CREG1 in the tumor microenvironment.

Molecular Basis and Regulation of Store-Operated Calcium Entry

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous mechanism for Ca²⁺ influx in mammalian cells with important physiological implications. Since the discovery of SOCE more than three decades ago, the mechanism that communicates the information about the amount of Ca²⁺ accumulated in the intracellular Ca²⁺ stores to the plasma membrane channels and the nature of these channels have been matters of intense investigation and debate. The stromal interaction molecule-1 (STIM1) has been identified as the Ca²⁺ sensor of the intracellular Ca²⁺ compartments that activates the store-operated channels. STIM1 regulates two types of store-dependent channels: the Ca²⁺ release-activated Ca²⁺ (CRAC) channels, formed by Orai1 subunits, that conduct the highly Ca²⁺ selective current I _CRAC and the cation permeable store-operated Ca²⁺ (SOC) channels, which consist of Orai1 and TRPC1 proteins and conduct the non-selective current I _SOC. While the crystal structure of Drosophila CRAC channel has already been solved, the architecture of the SOC channels still remains unclear. The dynamic interaction of STIM1 with the store-operated channels is modulated by a number of proteins that either support the formation of the functional STIM1-channel complex or protect the cell against Ca²⁺ overload.

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides

Pro-opiomelanocortin (POMC) is a prohormone that encodes multiple smaller peptide hormones within its structure. These peptide hormones can be generated by cleavage of POMC at basic residue cleavage sites by prohormone-converting enzymes in the regulated secretory pathway (RSP) of POMC-synthesizing endocrine cells and neurons. The peptides are stored inside the cells in dense-core secretory granules until released in a stimulus-dependent manner. The complexity of the regulation of the biosynthesis, trafficking, and secretion of POMC and its peptides reflects an impressive level of control over many factors involved in the ultimate role of POMC-expressing cells, that is, to produce a range of different biologically active peptide hormones ready for action when signaled by the body. From the discovery of POMC as the precursor to adrenocorticotropic hormone (ACTH) and β-lipotropin in the late 1970s to our current knowledge, the understanding of POMC physiology remains a monumental body of work that has provided insight into many aspects of molecular endocrinology. In this article, we describe the intracellular trafficking of POMC in endocrine cells, its sorting into dense-core secretory granules and transport of these granules to the RSP. Additionally, we review the enzymes involved in the maturation of POMC to its various peptides and the mechanisms involved in the differential processing of POMC in different cell types. Finally, we highlight studies pertaining to the regulation of ACTH secretion in the anterior and intermediate pituitary and POMC neurons of the hypothalamus.

Human cathepsin V protease participates in production of enkephalin and NPY neuropeptide neurotransmitters

Proteases are required for processing precursors into active neuropeptides that function as neurotransmitters for cell-cell communication. This study demonstrates the novel function of human cathepsin V protease for producing the neuropeptides enkephalin and neuropeptide Y (NPY). Cathepsin V is a human-specific cysteine protease gene. Findings here show that expression of cathepsin V in neuroendocrine PC12 cells and human neuronal SK-N-MC cells results in production of (Met)enkephalin from proenkephalin. Gene silencing of cathepsin V by siRNA in human SK-N-MC cells results in reduction of (Met)enkephalin by more than 80%, illustrating the prominent role of cathepsin V for neuropeptide production. In vitro processing of proenkephalin by cathepsin V occurs at dibasic residue sites to generate enkephalin-containing peptides and an ∼24-kDa intermediate present in human brain. Cathepsin V is present in human brain cortex and hippocampus where enkephalin and NPY are produced and is present in purified human neuropeptide secretory vesicles. Colocalization of cathepsin V with enkephalin and NPY in secretory vesicles of human neuroblastoma cells was illustrated by confocal microscopy. Furthermore, expression of cathepsin V with proNPY results in NPY production. These findings indicate the unique function of human cathepsin V for producing enkephalin and NPY neuropeptides required for neurotransmission in health and neurological diseases.

Evolution of the voltage sensor domain of the voltage-sensitive phosphoinositide phosphatase VSP/TPTE suggests a role as a proton channel in eutherian mammals

The voltage-sensitive phosphoinositide phosphatases provide a mechanism to couple changes in the transmembrane electrical potential to intracellular signal transduction pathways. These proteins share a domain architecture that is conserved in deuterostomes. However, gene duplication events in primates, including humans, give rise to the paralogs TPTE and TPTE2 that retain protein domain organization but, in the case of TPTE, have lost catalytic activity. Here, we present evidence that these human proteins contain a functional voltage sensor, similar to that in nonmammalian orthologs. However, domains of these human proteins can also generate a noninactivating outward current that is not observed in zebra fish or tunicate orthologs. This outward current has the anticipated characteristics of a voltage-sensitive proton current and is due to the appearance of a single histidine residue in the S4 transmembrane segment of the voltage sensor. Histidine is observed at this position only during the eutherian radiation. Domains from both human paralogs generate proton currents. This apparent gain of proton channel function during the evolution of the TPTE protein family may account for the conservation of voltage sensor domains despite the loss of phosphatase activity in some human paralogs.

Molecular mechanisms of endolysosomal Ca2+ signalling in health and disease

Endosomes, lysosomes and lysosome-related organelles are emerging as important Ca2+ storage cellular compartments with a central role in intracellular Ca2+ signalling. Endocytosis at the plasma membrane forms endosomal vesicles which mature to late endosomes and culminate in lysosomal biogenesis. During this process, acquisition of different ion channels and transporters progressively changes the endolysosomal luminal ionic environment (e.g. pH and Ca2+) to regulate enzyme activities, membrane fusion/fission and organellar ion fluxes, and defects in these can result in disease. In the present review we focus on the physiology of the inter-related transport mechanisms of Ca2+ and H+ across endolysosomal membranes. In particular, we discuss the role of the Ca2+-mobilizing messenger NAADP (nicotinic acid adenine dinucleotide phosphate) as a major regulator of Ca2+ release from endolysosomes, and the recent discovery of an endolysosomal channel family, the TPCs (two-pore channels), as its principal intracellular targets. Recent molecular studies of endolysosomal Ca2+ physiology and its regulation by NAADP-gated TPCs are providing exciting new insights into the mechanisms of Ca2+-signal initiation that control a wide range of cellular processes and play a role in disease. These developments underscore a new central role for the endolysosomal system in cellular Ca2+ regulation and signalling.

Proteomics of dense core secretory vesicles reveal distinct protein categories for secretion of neuroeffectors for cell-cell communication

Regulated secretion of neurotransmitters and neurohumoral factors from dense core secretory vesicles provides essential neuroeffectors for cell-cell communication in the nervous and endocrine systems. This study provides comprehensive proteomic characterization of the categories of proteins in chromaffin dense core secretory vesicles that participate in cell-cell communication from the adrenal medulla. Proteomic studies were conducted by nano-HPLC Chip MS/MS tandem mass spectrometry. Results demonstrate that these secretory vesicles contain proteins of distinct functional categories consisting of neuropeptides and neurohumoral factors, protease systems, neurotransmitter enzymes and transporters, receptors, enzymes for biochemical processes, reduction/oxidation regulation, ATPases, protein folding, lipid biochemistry, signal transduction, exocytosis, calcium regulation, as well as structural and cell adhesion proteins. The secretory vesicle proteomic data identified 371 proteins in the soluble fraction and 384 membrane proteins, for a total of 686 distinct secretory vesicle proteins. Notably, these proteomic analyses illustrate the presence of several neurological disease-related proteins in these secretory vesicles, including huntingtin interacting protein, cystatin C, ataxin 7, and prion protein. Overall, these findings demonstrate that multiple protein categories participate in dense core secretory vesicles for production, storage, and secretion of bioactive neuroeffectors for cell-cell communication in health and disease.

Differential accessibilities of dibasic prohormone processing sites of proenkephalin to the aqueous environment revealed by H-D exchange mass spectrometry

Proenkephalin (PE) is a prohormone containing dibasic sites that are cleaved by proteases to generate peptide neurotransmitters and hormones. Little is known about the conformational features of such protease cleavage sites within prohormone substrates. Therefore, the goal of this study was to investigate the relative accessibilities of multiple dibasic processing sites of PE by peptide amide hydrogen-deuterium exchange mass spectrometry (DXMS). DXMS demonstrated differences in the relative accessibilities of the KR, KK, and RR cleavage sites of PE to the aqueous environment. DXMS assesses relative rates of exchange of hydrogens of the polypeptide backbone of PE with deuterium atoms from D(2)O (heavy water) in solvent. Analyses of peptides spanning each of the 12 dibasic PE cleavage sites illustrated differences in H-D exchange rates that reflect relative solvent accessibility. The mid-domain cleavage sites (dibasic sites 4-8) exhibited greater accessibility to the aqueous solvent compared to regions of the NH(2) and COOH domains (dibasic sites 2, 3, and 9-11, respectively). The NH(2)- and COOH-terminal domains both exhibited relatively high H-D exchange rates. The hydrogen exchange rate profile of PE, as well as its circular dichroism (CD) features for secondary structure, was modified in trifluoroethanol, an organic solvent that represents a more hydrophobic environment. These findings suggest that the dibasic protease cleavage sites of the PE prohormone with differences in accessibility to the aqueous environment undergo proteolytic processing to generate active neuropeptides for cell-cell communication in neuroendocrine systems.

Precise detection of pH inside large unilamellar vesicles using membrane-impermeable dendritic porphyrin-based nanoprobes

Accurate real-time measurements of proton concentration gradients are pivotal to mechanistic studies of proton translocation by membrane-bound enzymes. Here we report a detailed characterization of the pH-sensitive fluorescent nanoprobe Glu(3), which is well suited for pH measurements in microcompartmentalized biological systems. The probe is a polyglutamic porphyrin dendrimer in which multiple carboxylate termini ensure its high water solubility and prevent its diffusion across phospholipid membranes. The probe's pK is in the physiological pH range, and its protonation can be followed ratiometrically by absorbance or fluorescence in the ultraviolet-visible spectral region. The usefulness of the probe was enhanced by using a semiautomatic titration system coupled to a charge-coupled device (CCD) spectrometer, enabling fast and accurate titrations and full spectral coverage of the system at millisecond time resolution. The probe's pK was measured in bulk solutions as well as inside large unilamellar vesicles in the presence of physiologically relevant ions. Glu(3) was found to be completely membrane impermeable, and its distinct spectroscopic features permit pH measurements inside closed membrane vesicles, enabling quantitative mechanistic studies of membrane-spanning proteins. Performance of the probe was demonstrated by monitoring the rate of proton leakage through the phospholipid bilayer in large vesicles with and without the uncoupler gramicidin present. Overall, as a probe for biological proton translocation measurements, Glu(3) was found to be superior to the commercially available pH indicators.

Loss of membrane cholesterol influences lysosomal permeability to potassium ions and protons

Cholesterol is an essential component of lysosomal membranes. In this study, we investigated the effects of membrane cholesterol on the permeability of rat liver lysosomes to K+ and H+, and the organelle stability. Through the measurements of lysosomal beta-hexosaminidase free activity, membrane potential, membrane fluidity, intra-lysosomal pH, and lysosomal proton leakage, we established that methyl-beta-cyclodextrin (MbetaCD)-produced loss of membrane cholesterol could increase the lysosomal permeability to both potassium ions and protons, and fluidize the lysosomal membranes. As a result, potassium ions entered the lysosomes through K+/H+ exchange, which produced osmotic imbalance across the membranes and osmotically destabilized the lysosomes. In addition, treatment of the lysosomes with MbetaCD caused leakage of the lysosomal protons and raised the intra-lysosomal pH. The results indicate that membrane cholesterol plays important roles in the maintenance of the lysosomal limited permeability to K+ and H+. Loss of this membrane sterol is critical for the organelle acidification and stability.

Overview of cell fractionation

This discussion unit describes the most common methods for cell fractionation which provides the essential ingredients for the increasing number of cell-free assays now being used in test-tube reconstructions of complex cellular events involving intercompartmental interactions. Gel filtration separates on the basis of size, centrifugation separates on the basis of size and density, and electrophoresis separates on the basis of surface charge density. Centrifugation is the most widely used procedure in cell fractionation and is the only approach commonly used to separate crude tissue homogenates (often having quite large volumes) into subfractions as starting material for more refined purification procedures. Therefore, this overview focuses primarily on fractionation of organelles by centrifugation.

Purification of organelles from mammalian cells

Organelle purification procedures capitalize on the differences in size, density, and (occasionally) surface charge density of individual types of organelles. Most fractionation procedures that are based on centrifugation involve some combination of procedures that distinguish both size and density. Initially, a homogenate is prepared in isoosmotic (or slightly hyperosmotic) sucrose or some other predominantly nonelectrolyte medium. A wide range of procedures have been used to fractionate tissue homogenates. The protocols in this unit emphasize different fractionation techniques that have been used for rat liver, an abundant tissue that has been a favorite of many investigators and has served as the source of many organelle preparations of excellent purity. For selected procedures, examples have been given using other tissue sources (e.g., glandular tissues that maintain protein storage granules for regulated secretion) or, where particularly favorable, cultured cells.

Aquaporin 1 is important for maintaining secretory granule biogenesis in endocrine cells

Aquaporins (AQPs), a family of water channels expressed in epithelial cells, function to transport water in a bidirectional manner to facilitate transepithelial fluid absorption and secretion. Additionally, AQP1 and AQP5 are found in pancreatic zymogen granules and synaptic vesicles and are involved in vesicle swelling and exocytosis in exocrine cells and neurons. Here, we show AQP1 is in dense-core secretory granule (DCSG) membranes of endocrine tissue: pituitary and adrenal medulla. The need for AQP1 in endocrine cell function was examined by stable transfection of AQP1 antisense RNA into AtT20 cells, a pituitary cell line, to down-regulate AQP1 expression. These AQP1-deficient cells showed more than 60% depletion of DCSGs and significantly decreased DCSG protein levels, including proopiomelanocotin/pro-ATCH and prohormone convertase 1/3, but not non-DCSG proteins. Pulse-chase studies revealed that whereas DCSG protein synthesis was unaffected, approximately 50% of the newly synthesized proopiomelanocortin was degraded within 1 h. Low levels of ACTH were released upon stimulation, indicating that the small number of DCSGs that were made in the presence of the residual AQP1 were functionally competent for exocytosis. Analysis of anterior pituitaries from AQP1 knockout mice showed reduced prohormone convertase 1/3, carboxypeptidase E, and ACTH levels compared to wild-type mice demonstrating that our results observed in AtT20 cells can be extended to the animal model. Thus, AQP1 is important for maintaining DCSG biogenesis and normal levels of hormone secretion in pituitary endocrine cells.

Overview of cell fractionation

Cell fractionation is a useful preparative and analytical method in cell biology. It is essential for analysis of composition and function of cellular compartments and it is used to prepare materials for in vitro reconstitution studies This overview discusses the basic principles of centrifugation, the instruments available, choice of media, evaluation of fractionation, and procedure optimization.

Mechanism of Lysophosphatidylcholine-Induced Lysosome Destabilization

Lysosomal destabilization is critical for the organelle and living cells. Phospholipase A(2 )(PLA(2)) was shown to be able to destabilize lysosomes under some conditions. By what mechanism the enzyme affects lysosomal stability is not fully studied. In this study, we investigated the effects of lysophosphatidylcholine (lysoPC), a PLA(2)-produced lipid metabolite, on lysosomal ion permeability, osmotic sensitivity and stability. By measuring lysosomal beta-hexosaminidase free activity, membrane potential, proton leakage and their enzyme latency loss in hypotonic sucrose medium, we established that lysoPC could increase the lysosomal permeability to both potassium ions and protons and enhance lysosomal osmotic sensitivity. These changes in lysosomal membrane properties promoted entry of potassium ions into lysosomes via K(+)/H(+) exchange. The resultant osmotic imbalance across the membranes led to losses of lysosomal integrity. The enhancement of lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in osmotic shock. These results suggest that lysoPC may play a key role in PLA(2)-induced lysosomal destabilization.

Secretory vesicle aminopeptidase B related to neuropeptide processing: molecular identification and subcellular localization to enkephalin- and NPY-containing chromaffin granules

Biosynthesis of peptide hormones and neurotransmittters involves proteolysis of proprotein precursors by secretory vesicle cathepsin L. Cathepsin L generates peptide intermediates with basic residues at their NH(2)-termini, indicating that Arg/Lys aminopeptidase is needed to generate the smaller biologically active peptide. Therefore, this study identified the Arg/Lys aminopeptidase that is present in secretory vesicles of adrenal medulla and neuroendocrine tissues, achieved by molecular cloning and localization in 'model' neuropeptide-containing secretory vesicles (bovine). Molecular cloning of the bovine aminopeptidase B (AP-B) cDNA defined its primary sequence that allowed selection of antisera for immunolocalization studies. AP-B was present in secretory vesicles that contain cathepsin L with the neuropeptides enkephalin and neuropeptide Y. The AP-B in several neuroendocrine tissues was detected by western blots. Recombinant bovine AP-B showed preference for Arg-methylcoumarinamide substrate. AP-B was inhibited by arphamenine, an inhibitor of aminopeptidases. Bovine AP-B showed similar activities for Arg-(Met)enkephalin (ME) and Lys-ME neuropeptide substrates to generate ME, while rat AP-B preferred Arg-ME. Furthermore, AP-B possesses an acidic pH optimum of 5.5-6.5 that is similar to the internal pH of secretory vesicles. The significant finding of the secretory vesicle localization of AP-B with neuropeptides and cathepsin L suggests a role for this exopeptidase in the biosynthesis of neuropeptides.

Effects of phospholipase A2 on the lysosomal ion permeability and osmotic sensitivity

In this study, we investigated the mechanism of PLA(2)-induced lysosomal destabilization. Through the measurements of lysosomal beta-hexosaminidase free activity, their membrane potential, the intra-lysosomal pH and the lysosomal latency loss in hypotonic sucrose medium, we established that PLA(2) could increase the lysosomal membrane permeability to both potassium ions and protons. The enzyme could also enhance the organelle osmotic sensitivity. The increases in the lysosomal ion permeability promoted influx of potassium ions into the lysosomes via K(+)/H(+) exchange. The resulted osmotic imbalance across the lysosomal membranes osmotically destabilized the lysosomes. In addition, the enhancement of the lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in the osmotic stress. The results explain how PLA(2) destabilized the lysosomes.

Effects of Arachidonic Acid on the Lysosomal Ion Permeability and Osmotic Stability

In this study, we investigated the effects of arachidonic acid, a PLA2-produced lipid metabolite, on the lysosomal permeability, osmotic sensitivity and stability. Through the measurements of lysosomal beta-hexosaminidase free activity, membrane potential, intralysosomal pH, and lysosomal latency loss in hypotonic sucrose medium, we established that arachidonic acid could increase the lysosomal permeability to both potassium ions and protons, and enhance the lysosomal osmotic sensitivity. As a result, the fatty-acid-promoted entry of potassium ions into the lysosomes via K+/H+ exchange, which could produce osmotic imbalance across their membranes and osmotically destabilize the lysosomes. In addition, the enhancement of lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in osmotic shock. The results suggest that arachidonic acid may play a role in the lysosomal destabilization.

Characterization of the functional coupling of bovine brain vacuolar-type H(+)-translocating ATPase. Effect of divalent cations, phospholipids, and subunit H (SFD)

Vacuolar-type H+-translocating ATPases (V-ATPases or V-pumps) are complex proteins containing multiple subunits and are organized into two functional domains: a peripheral catalytic sector V1 and a membranous proton channel V0. The functional coupling of ATP hydrolysis activity to proton transport in V-pumps requires a regulatory component known as subunit H (SFD) as has been shown both in vivo and in vitro (Ho, M. N., Hirata, R., Umemoto, N., Ohya, Y., Takatsuki, A., Stevens, T. H., and Anraku, Y. (1993) J. Biol. Chem. 268, 18286-18292; Xie, X. S., Crider, B. P., Ma, Y. M., and Stone, D. K. (1994) J. Biol. Chem. 269, 25809-25815). Ca2+ is thought to uncouple V-pumps because it is found to support ATP hydrolysis but not proton transport, while Mg2+ supports both activities. The direct effect of phospholipids on the coupling of V-ATPases has not been reported, likely due to the fact that phospholipids are constituents of biological membranes. We now report that Ca2+-induced uncoupling of the bovine brain V-ATPase can be reversed by imposition of a favorable membrane potential. Furthermore we report a simple "membrane-free" assay system using the V0 proton channel-specific inhibitor bafilomycin as a probe to detect the coupling of V-ATPase under certain conditions. With this system, we have characterized the functional effect of subunit H, divalent cations, and phospholipids on bovine brain V-ATPase and have found that each of these three factors plays a critical role in the functional coupling of the V-pump.

Cysteine proteases are the major ?-secretase in the regulated secretory pathway that provides most of the ?-amyloid in Alzheimer's disease: Role of BACE 1 in the constitutive secretory pathway

This article focuses on beta-amyloid (Abeta) peptide production and secretion in the regulated secretory pathway and how this process relates to accumulation of toxic Abeta in Alzheimer's disease. New findings are presented demonstrating that most of the Abeta is produced and secreted, in an activity-dependent manner, through the regulated secretory pathway in neurons. Only a minor portion of cellular Abeta is secreted via the basal, constitutive secretory pathway. Therefore, regulated secretory vesicles contain the primary beta-secretases that are responsible for producing the majority of secreted Abeta. Investigation of beta-secretase activity in regulated secretory vesicles of neuronal chromaffin cells demonstrated that cysteine proteases account for the majority of the beta-secretase activity. BACE 1 is present in regulated secretory vesicles but provides only a small percentage of the beta-secretase activity. Moreover, the cysteine protease activities prefer to cleave the wild-type beta-secretase site, which is relevant to the majority of AD cases. In contrast, BACE 1 prefers to cleave the Swedish mutant beta-secretase site that is expressed in a minor percentage of the AD population. These new findings lead to a unifying hypothesis in which cysteine proteases are the major beta-secretases for the production of Abeta in the major regulated secretory pathway and BACE 1 is the beta-secretase responsible for Abeta production in the minor constitutive secretory pathway. These results indicate that inhibition of multiple proteases may be needed to decrease Abeta production as a therapeutic strategy for Alzheimer's disease.

Disruption of a receptor-mediated mechanism for intracellular sorting of proinsulin in familial hyperproinsulinemia

In familial hyperproinsulinemia, specific mutations in the proinsulin gene are linked with a profound increase in circulating plasma proinsulin levels. However, the molecular and cellular basis for this disease remains uncharacterized. Here we investigated how these mutations may disrupt the sorting signal required to target proinsulin to the secretory granules of the regulated secretory pathway, resulting in the unregulated release of proinsulin. Using a combination of molecular modeling and site-directed mutagenesis, we have identified structural molecular motifs in proinsulin that are necessary for correct sorting into secretory granules of endocrine cells. We show that membrane carboxypeptidase E (CPE), previously identified as a prohormone-sorting receptor, is essential for proinsulin sorting. This was demonstrated through short interfering RNA-mediated depletion of CPE and transfection with a dominant negative mutant of CPE in a beta-cell line. Mutant proinsulins found in familial hyperproinsulinemia failed to bind to CPE and were not sorted efficiently. These findings provide evidence that the elevation of plasma proinsulin levels found in patients with familial hyperproinsulinemia is caused by the disruption of CPE-mediated sorting of mutant proinsulins to the regulated secretory pathway.

Neuroserpin regulates neurite outgrowth in nerve growth factor-treated PC12 cells

Neuroserpin is a serine protease inhibitor widely expressed in the developing and adult nervous systems and implicated in the regulation of proteases involved in processes such as synaptic plasticity, neuronal migration and axogenesis. We have analysed the effect of neuroserpin on growth factor-induced neurite outgrowth in PC12 cells. We show that small changes in neuroserpin expression result in changes to the number of cells extending neurites and total neurite length following NGF treatment. Increased expression of neuroserpin resulted in a decrease in the number of cells extending neurites and a reduction in total free neurite length whereas reduced levels of neuroserpin led to a small increase in the number of neurite extending cells and a significant increase in total free neurite length compared to the parent cell line. Neuroserpin also altered the response of PC12 cells to bFGF and EGF treatment. Neuroserpin was localised to dense cored secretory vesicles in PC12 cells but was unable to complex with its likely enzyme target, tissue plasminogen activator at the acidic pH found in these vesicles. These data suggest that modulation of neuroserpin levels at the extending neurite growth cone may play an important role in regulating axonal growth.

Beta-amyloid peptide in regulated secretory vesicles of chromaffin cells: evidence for multiple cysteine proteolytic activities in distinct pathways for beta-secretase activity in chromaffin vesicles

A key factor in Alzheimer's disease (AD) is the beta-secretase activity that is required for the production of beta-amyloid (Abeta) peptide from its amyloid precursor protein (APP) precursor. In this study, the majority of Abeta secretion from neuronal chromaffin cells was found to occur via the regulated secretory pathway, compared with the constitutive secretory pathway; therefore, beta-secretase activity in the regulated secretory pathway was examined for the production and secretion of Abeta in chromaffin cells obtained from in vivo adrenal medullary tissue. The presence of Abeta(1-40) in APP-containing chromaffin vesicles, which represent regulated secretory vesicles, was demonstrated by radioimmunoassay (RIA) and reverse-phase high-performance liquid chromatography. These vesicles also contain Abeta(1-42), measured by RIA. Significantly, regulated secretion of Abeta(1-40) from chromaffin cells represented the majority of secreted Abeta (> 95% of total secreted Abeta), compared with low levels of constitutively secreted Abeta(1-40). These results indicate the importance of Abeta production and secretion in the regulated secretory pathway as a major source of extracellular Abeta. Beta-secretase activity in isolated chromaffin vesicles was detected with the substrate Z-Val-Lys-Met-/MCA (methylcoumarinamide) that contains the beta-secretase cleavage site. Optimum beta-secretase activity in these vesicles required reducing conditions and acidic pH (pH 5-6), consistent with the in vivo intravesicular environment. Evidence for cysteine protease activity was shown by E64c inhibition of Z-Val-Lys-Met-MCA-cleaving activity, and E64c inhibition of Abeta(1-40) production in isolated chromaffin vesicles. Chromatography resolved the beta-secretase activity into two distinct proteolytic pathways consisting of: (i) direct cleavage of the beta-secretase site at Met-/Asp by two cysteine proteolytic activities represented by peaks Il-A and Il-B, and (ii) an aminopeptidase-dependent pathway represented by peak I cysteine protease activity that cleaves between Lys-/Met, followed by Met-aminopeptidase that would generate the beta-secretase cleavage site. Treatment of chromaffin cells in primary culture with the cysteine protease inhibitor E64d reduced the production of the beta-secretase product, a 12-14 kDa C-terminal APP fragment. In addition, BACE 1 and BACE 2 were detected in chromaffin vesicles; BACE 1 represented a small fraction of total beta-secretase activity in these vesicles. These results illustrate that multiple cysteine proteases, in combination with BACE 1, contribute to beta-secretase activity in the regulated secretory pathway. These results complement earlier findings for BACE 1 as beta3-secretase for Abeta production in the constitutive secretory pathway that provides basal secretion of Abeta into conditioned media. These findings suggest that drug inhibition of several proteases may be required for reducing Abeta levels as a potential therapeutic approach for AD.

Lipid raft association of carboxypeptidase E is necessary for its function as a regulated secretory pathway sorting receptor

Membrane carboxypeptidase E (CPE) is a sorting receptor for targeting prohormones, such as pro-opiomelanocortin, to the regulated secretory pathway in endocrine cells. Its membrane association is necessary for it to bind a prohormone sorting signal at the trans-Golgi network (TGN) to facilitate targeting. In this study, we examined the lipid interaction of CPE in bovine pituitary secretory granule membranes, which are derived from the TGN. We show that CPE is associated with detergent-resistant lipid domains, or rafts, within secretory granule membranes. Lipid analysis revealed that these rafts are enriched in glycosphingolipids and cholesterol. Pulse-chase and subcellular fractionation experiments in AtT-20 cells show that the association of CPE with membrane rafts occurred only after it reached the Golgi. Cholesterol depletion resulted in dissociation of CPE from secretory granule membranes and decreased the binding of prohormones to membranes. In vivo cholesterol depletion using lovastatin resulted in the lack of sorting of CPE and its cargo to the regulated secretory pathway. We propose that the sorting receptor function of CPE necessitates its interaction with glycosphingolipid-cholesterol rafts at the TGN, thereby anchoring it in position to bind to its prohormone cargo.

Oligomerization of pro-opiomelanocortin is independent of pH, calcium and the sorting signal for the regulated secretory pathway

Studies indicate that pro-opiomelanocortin (POMC) is sorted to the regulated secretory pathway by binding to a sorting receptor identified as membrane-bound carboxypeptidase E (CPE) [Cool et al. (1997) Cell 88, 73-83]. The efficiency of this sorting mechanism could be enhanced if POMC molecules were to self-associate to form oligomers, prior or subsequent to binding to CPE. Using cross-linking and gel filtration techniques, we demonstrated that POMC forms oligomers at both neutral and acidic pHs and calcium was not necessary. delta N-POMC, which lacks the N-terminal sorting signal for the regulated secretory pathway, also formed similar oligomers, indicating that the sorting and oligomerization domains are different.

Effects of photoinduced membrane rigidification on the lysosomal permeability to potassium ions

Mechanism for the photoinduced increase in the lysosomal K+ permeability is still unknown. In this study, we investigated the effect of photodamage-induced membrane rigidification on the lysosomal K+ permeability by measuring the membrane potential with bis(3-propyl-5-oxoisoxazol-4-yl)pentamethine oxonol and by monitoring proton leakage with p-nitrophenol. Membrane fluidity was measured by the steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene. Methylene blue-mediated photodamage to lysosomes decreased their membrane fluidity and increased their K+ permeability. The photoinduced increase in the K+ permeability can be reversed by fluidizing the rigidified lysosomal membranes with benzyl alcohol. The results suggest that the membrane rigidification induced by photodamage may increase lysosomal K+ permeability. This conclusion is supported by the observation that rigidifying lysosomal membranes by the treatment with membrane rigidifier cholesteryl hemisuccinate also enhanced the lysosomal K+ permeability.

Hippocampal cholinergic neurostimulating peptides (HCNP)

Neuronal development and differentiation require a variety of cell interactions. Diffusible molecules from target neurons play an important part in mediating such interactions. Our early studies used explant culture technique to examine the factors that enhance the differentiation of septo-hippocampal cholinergic neurons, and they revealed that several components resident in the hippocampus are involved in the differentiation of presynaptic cholinergic neurons in the medial septal nucleus. One of these components, originally purified from young rat hippocampus, is a novel undecapeptide (hippocampal cholinergic neurostimulating peptide; HCNP); this enhances the production of ChAT, but not of AchE. Later experiments revealed that: (1) a specific receptor appears to mediate this effect; (2) NGF and HCNP act cooperatively to regulate cholinergic phenotype development in the medial septal nucleus in culture; and (3) these two molecules differ both in their mechanism of release from the hippocampus and their mechanism of action on cholinergic neurons. The amino acid sequence deduced from base sequence analysis of cloned HCNP-precursor protein cDNA shows that HCNP is located at the N-terminal domain of its precursor protein. The 21 kDa HCNP precursor protein shows homology with other proteins, and it functions not only as an HCNP precursor, but also as a binding protein for ATP, opioids and phosphatidylethanolamine. The distribution and localization of HCNP-related components and the expression of their mRNAs support the notion that the precursor protein is multifunctional. In keeping with its multiple functions, the multiple enhancers and promoters found in the genomic DNA for HCNP precursor protein may be involved in the regulation of its gene in a variety of cells and at different stages of development. Furthermore, several lines of evidence obtained from studies of humans and animal models suggest that certain types of memory and learning disorders are associated with abnormal accumulation and expression of HCNP analogue peptide and/or its precursor protein mRNA in the hippocampus.

Carboxypeptidase E is a sorting receptor for prohormones: binding and kinetic studies

The binding of pro-opiomelanocortin,(POMC), pro-insulin, pro-enkephalin and chromogranin A (CGA) to the regulated secretory pathway sorting receptor, carboxypeptidase E (CPE), in bovine pituitary secretory granule (SG) membranes was investigated. N-POMC1-26, which contains the POMC sorting signal, bound to CPE in the SG membranes with low affinity and the binding was ion independent. Pro-insulin bound CPE with similar kinetics. Pro-enkephalin, but not CGA bound to CPE with similar IC50 as pro-insulin and N-POMC1-26. Crosslinking studies showed that pro-insulin and pro-enkephalin bound specifically to SG membrane CPE, similar to N-POMC1-26 reported previously. CPE was extracted from the SG membranes with NaHCO3 or KSCN, but not Triton X-100/1 M NaCl. The results show that CPE is tightly associated with SG membranes and binds several prohormones, but not CGA, with similar kinetics, providing further evidence that membrane CPE has the characteristics to function as a common sorting receptor for targeting prohormones to the regulated secretory pathway.

Non-opioid dynorphin binding site on secretory vesicles of a pituitary-derived cell line

Accumulating evidence indicates that the endogenous opioid peptides dynorphinA-(1-17) and dynorphinA-(1-13) interact not only with opioid but also with yet poorly characterized non-opioid receptors. The latter have been implicated in a number of the effects of dynorphins including induction of ACTH release in sheep and in AtT 20 cells, a pituitary-derived mouse cell line. AtT 20 cells do not express opioid receptors and therefore are particularly suitable for search of non-opioid dynorphin receptors. We report here that 3H-dynorphinA-(1-13)-NH2 associates specifically with AtT 20 cells, apparently through an uptake process and a binding site. Within the cell, it binds preferentially to fractions containing secretory vesicles, with a Kd of about 100 nM. DynorphinA-(1-17), and several non-opioid fragments of dynorphin, including A-(2-17), A-(2-16) and A-(2-13), compete with 3H-dynorphinA-(1-13)-NH2 for that site with IC50s ranging from 200 nM to 2 microM. ACTH(1-39) also competes with 3H-dynorphinA-(1-13)-NH2 for the site with an IC50 of about 300 nM. DynorphinA-(2-17) at microM concentrations stimulates release of ACTH from the isolated vesicles. The results indicate the presence of a non-opioid dynorphin binding site on the secretory vesicle fractions of AtT20 cells that might be involved in ACTH release. The ability of ACTH itself to compete for the binding sites associated with the vesicles suggest that those sites may be involved in an autocrine loop.

Activation and processing of non-anchored yapsin 1 (Yap3p)

A C-terminally truncated form of yapsin 1 (yeast aspartic protease 3), the first member of the novel sub-class of aspartic proteases with specificity for basic residues (designated the Yapsins), was overexpressed and purified to apparent homogeneity, yielding approximately 1 microg of yapsin 1/g of wet yeast. N-terminal amino acid analysis of the purified protein confirmed that the propeptide was absent and that the mature enzyme began at Ala68. The mature enzyme was shown to be composed of approximately equimolar amounts of two subunits, designated alpha and beta, that were associated to each other by a disulfide bond. C-terminally truncated proyapsin 1 was also expressed in the baculovirus/Sf9 insect cell expression system and secreted as a zymogen that could be activated upon incubation at an acidic pH with an optimum at approximately 4.0. When expressed without its pro-region, it was localized intracellularly and lacked activity, indicating that the pro-region was required for the correct folding of the enzyme. The activation of proyapsin 1 in vitro exhibited linear kinetics and generated an intermediate form of yapsin 1 or pseudo-yapsin 1.

Carboxypeptidase E is a regulated secretory pathway sorting receptor: genetic obliteration leads to endocrine disorders in Cpe(fat) mice

A proposed mechanism for sorting secretory proteins into granules for release via the regulated secretory pathway in endocrine-neuroendocrine cells involves binding the proteins to a sorting receptor at the trans-Golgi network, followed by budding and granule formation. We have identified such a sorting receptor as membrane-associated carboxypeptidase E (CPE) in pituitary Golgi-enriched and secretory granule membranes. CPE specifically bound regulated secretory pathway proteins, including prohormones, but not constitutively secreted proteins. We show that in the Cpe(fat) mutant mouse lacking CPE, the pituitary prohormone, pro-opiomelanocortin, was missorted to the constitutive pathway and secreted in an unregulated manner. Thus, obliteration of CPE, the sorting receptor, leads to multiple endocrine disorders in these genetically defective mice, including hyperproinsulinemia and infertility.

Demonstration and characterization of hippocampal cholinergic neurostimulating peptide (HCNP) processing enzyme activity in rat hippocampus

Hippocampal cholinergic neurostimulating peptide (HCNP) stimulates cholinergic activity of cultured medial septal nuclei explants. It consists of eleven amino acids that are located at the N-terminal region of its precursor protein. This report concerns the demonstration and characterization of an HCNP processing enzyme that cleaves the bioactive undecapeptide from the precursor. The enzyme was purified from the hippocampus of young Wistar rats. A synthetic deacetylated peptide (peptide(1-26)) consisting of the N-terminal 26 amino acids of the HCNP precursor protein served as substrate. The product of the enzyme reaction was identified and quantitated by HPLC using deacetylated HCNP as standard. The amount of undecapeptide generated was directly proportional to the time of incubation of the enzyme reaction mixture. From molecular sieving chromatography it was estimated that the molecular mass of the enzyme is close to 68 kDa. The HCNP processing enzyme has a pH optimum of 6.0 and a K m of 0.50 mM for peptide(1-26). Preincubation at 56 degrees C causes rapid inactivation of the HCNP processing activity. Enzyme activity is enhanced by EDTA and 1,4-dithiothreitol, and inhibited by antipain, chymostatin and E-64. These findings suggest that the enzyme probably has a thiol group in its active site. This novel enzyme of the hippocampus may represent a valuable tool for further studies on the general protein metabolism in the central nervous system, as well as for elucidating the neurochemical aspects of neurodegenerative disorders.

Isolation and characterization of gastric trypsin from the microsomal fraction of porcine gastric antral mucosa

A gastric serine protease(s) was found in porcine gastric antral mucosa and was shown to be distributed in the endoplasmic reticulum (ER)-microsome fraction and also in the vesicle fraction. Two forms of the protease were purified over 6,000-fold from the ER-microsome fraction. Analyses of various molecular and enzymatic characteristics including the N-terminal and partial internal amino acid sequences of both forms revealed that they share the same properties and are indistinguishable from porcine pancreatic trypsin. This is the first time that trypsin or a protease almost identical with trypsin has been found to be present intracellularly in normal tissues. The gastric trypsin activities from the ER-microsome and the vesicle fractions were located in distinct density regions upon density gradient centrifugation, which indicates association of the protease with different organelle membranes. Taken together, these results suggest that there may be a novel function of trypsin in the gastric mucosa; it might function as a specific degrading or processing enzyme as an intracellular protease.

Thiol and aspartyl proteolytic activities in secretory vesicles of bovine pituitary

Thiol and aspartyl proteolytic activities in isolated secretory vesicles of neural (NL) and intermediate (IL) lobes of bovine pituitary were characterized with heterologous enkephalin and tachykinin precursor substrates, 35S-(Met)-preproenkephalin and 35S-(Met)-beta-preprotachykinin. IL and NL secretory vesicles contained thiol-dependent proteolytic activity that cleaved the enkephalin precursor with a pH optimum of 4.5; this activity resembled a novel "prohormone thiol protease' previously purified and characterized from adrenal medulla chromaffin granules. IL and NL vesicles also demonstrated aspartyl proteolytic activity with acidic pH optimum, as shown by pepstatin A inhibition of tachykinin and enkephalin precursor cleaving activity. This activity may be related to a previously characterized chromaffin granule aspartyl protease (CGAP) related to cathepsin D (2), as indicated by the presence of immunoreactive CGAP in NL secretory vesicles by anti-CGAP immunoblots. These results show that pituitary secretory vesicles, like chromaffin granules, may contain similar thiol-dependent and aspartyl proteolytic activities.

Effect of calcium ions on the processing of pro-opiomelanocortin by bovine intermediate lobe pro-opiomelanocortin-converting enzyme

The effect of Ca2+ on the extent and pattern of processing of pro-opiomelanocortin and an N-terminal fragment by a purified pituitary secretory vesicle, soluble aspartic endoprotease, was studied. Ca2+ stimulated the first cleavage of pro-opiomelanocortin by pro-opiomelanocortin-converting enzyme to yield 21-23 kDa adrenocorticotropin and beta-lipotropin, but its effect was minimal. The production of adrenocorticotropin from the 21-23 kDa intermediate was stimulated approximately 2.3-fold in the presence of 10 mM Ca2+, and processing of beta-lipotropin to beta-endorphin was stimulated about 1.3-1.4-fold by 5-10 mM Ca2+. The production of gamma-melanotropin-immunoreactive material from bovine N-pro-opiomelanocortin(1-77) was stimulated approximately 1.3-fold at both 100 microM and 1.5-2.0 mM Ca2+. Further characterization of the gamma-melanotropin-immunoreactive material by HPLC demonstrated that the major products were gamma 3-[Lys]melanotropin and gamma 3-melanotropin at both Ca2+ concentrations. These results indicate that pro-opiomelanocortin-converting enzyme is stimulated by Ca2+.

Characterization of gastrin amidation in the rat and porcine antrum: comparison with the pituitary

The formation of biologically active gastrin from glycine-extended processing intermediates occurs via the action of a peptide alpha-amidating enzyme. The observation that gastrin exists primarily as unamidated precursors in the pituitary but as amidated gastrin in the antrum prompted these studies to examine whether the amidating enzymes in the two organs were different in their characteristics. Furthermore, the amidating enzyme in the stomach has not previously been characterized in extensive detail. Amidating activity was quantified by measuring the conversion of Tyr-Gly-Trp-Met-Asp-Phe-Gly (glycine-extended hexagastrin) to Tyr-Gly-Trp-Met-Asp-Phe-NH2 (amidated hexagastrin) by radioimmunoassay. The activity of the antral enzyme in both the rat and hog had a similar apparent molecular weight (45,000-60,000), cofactor requirements (copper, ascorbic acid, and catalase), pH optima (5.5-8.5), and Km (12 microM) as the pituitary enzyme. These data suggest that antral and pituitary peptide alpha-amidating enzymes are the same enzyme, thus it is unlikely that differences in amidating enzymes can account for the observed differences in the tissue specific processing of gastrin.

A stopped-flow kinetic study of the interaction of potential-sensitive oxonol dyes with lipid vesicles

The interaction of the dyes oxonol V and oxonol VI with unilamellar dioleoylphosphatidylcholine vesicles was investigated using a fluorescence stopped-flow technique. On mixing with the vesicles, both dyes exhibit an increase in their fluorescence, which occurs in two phases. According to the dependence of the reciprocal relaxation time on vesicle concentration, the rapid phase appears to be due to a second-order binding of the dye to the lipid membrane, which is very close to being diffusion-controlled. The slow phase is almost independent of vesicle concentration, and it is suggested that this may be due to a change in dye conformation or position within the membrane, possibly diffusion across the membrane to the internal monolayer. The response times of the dyes to a rapid jump in the membrane potential has also been investigated. Oxonol VI was found to respond to the potential change in less than 1 s, whereas oxonol required several minutes. This has been attributed to lower mobility of oxonol V within the lipid membrane.

Regulated secretion of pro-opiomelanocortin converting enzyme and an aminopeptidase B-like enzyme from dispersed bovine intermediate lobe pituitary cells

Coordinate secretion of two prohormone/proneuropeptide processing enzymes [pro-opiomelanocortin converting enzyme (PCE) and an aminopeptidase B-like enzyme (APBE)] and alpha-melanotropin (alpha-MSH) from bovine intermediate lobe pituitary cells was studied. Stimulation of secretion with 8-bromo-cyclic AMP produced significant increases in levels of immunoreactive alpha-MSH, PCE, and APBE. Treatment of cells with the dopaminergic agonist 2-bromo-alpha-ergocryptine resulted in significant decreases in secretion of alpha-MSH, PCE, and APBE. In neither case were there significant changes in levels of cytosolic lactic dehydrogenase or lysosomal beta-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, beta-lipotropin, a beta-endorphin-like peptide, and beta-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 Argo-[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 alpha-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.