Major rat brain membrane-associated and cytosolic enkephalin-degrading aminopeptidases: comparison studies

Structure of puromycin-sensitive aminopeptidase and polyglutamine binding

Puromycin-sensitive aminopeptidase (E.C. 3.4.11.14, UniProt P55786), a zinc metallopeptidase belonging to the M1 family, degrades a number of bioactive peptides as well as peptides released from the proteasome, including polyglutamine. We report the crystal structure of PSA at 2.3 Ǻ. Overall, the enzyme adopts a V-shaped architecture with four domains characteristic of the M1 family aminopeptidases, but it is in a less compact conformation compared to most M1 enzymes of known structure. A microtubule binding sequence is present in a C-terminal HEAT repeat domain of the enzyme in a position where it might serve to mediate interaction with tubulin. In the catalytic metallopeptidase domain, an elongated active site groove lined with aromatic and hydrophobic residues and a large S1 subsite may play a role in broad substrate recognition. The structure with bound polyglutamine shows a possible interacting mode of this peptide, which is supported by mutation.

Mass-spectrometric analysis of myelin proteolipids reveals new features of this family of palmitoylated membrane proteins

In this study, we have investigated the structure of the native myelin proteolipid protein (PLP), DM-20 protein and several low molecular mass proteolipids by mass spectrometry. The various proteolipid species were isolated from bovine spinal cord by size-exclusion and ion-exchange chromatography in organic solvents. Matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) of PLP and DM-20 revealed molecular masses of 31.6 and 27.2 kDa, respectively, which is consistent with the presence of six and four molecules of thioester-bound fatty acids. Electrospray ionization-MS analysis of the deacylated proteins in organic solvents produced the predicted molecular masses of the apoproteins (29.9 and 26.1 kDa), demonstrating that palmitoylation is the major post-translational modification of PLP, and that the majority of PLP and DM-20 molecules in the CNS are fully acylated. A series of myelin-associated, palmitoylated proteolipids with molecular masses raging between 12 kDa and 18 kDa were also isolated and subjected to amino acid analysis, fatty acid analysis, N- and C-terminal sequencing, tryptic digestion and peptide mapping by MALDI-TOF-MS. The results clearly showed that these polypeptides correspond to the N-terminal region (residues 1-105/112) and C-terminal region (residues 113/131-276) of the major PLP, and they appear to be produced by natural proteolytic cleavage within the 60 amino acid-long cytoplasmic domain. These proteolipids are not postmortem artifacts of PLP and DM-20, and are differentially distributed across the CNS.

Changes in puromycin-sensitive aminopeptidases in postmortem schizophrenic brain regions

We studied the distribution of puromycin-sensitive aminopeptidase (PSA) in well-defined human brain ares by Western immunoblot in an attempt to examine its possible role in schizophrenia. The schizophrenic brains were from suicide victims (n = 13) of either sex, with an age range of 30-60 yr (average 45). The controls were mostly victims of myocardial infarction (n = 12), of either sex and between 32 and 56 yr old (average 44). The brain regions were obtained within 48 h after death. After ultracentrifugation the PSA was quantified by Western blot analysis using a PSA antiserum. The distribution of the two most abundant antigens, MW 100 kDa (PSA-100) and 170 kDA (PSA-170), were compared. PSA-100 had peptidase activity, PSA-170 did not. PSA-100 was found in all of the region studied. In the control brain areas prefrontal cingulate and frontal cortices, thalamus, hippocampus, hypothalamus and outer globus pallidus contained significantly more PSA-100 than the corresponding areas from schizophrenic brain. PSA-170 was mostly found only in areas of schizophrenic brains. In three control brains, in one area of each, it could be detected, but the level in each of these regions was less than 30% of that in the corresponding schizophrenic area. PSA-170 was found in all the schizophrenic brains, in 20 of the 35 regions we studied, with parahippocampal cortex the highest (134 ng/g wet tissue) and frontal inferior cortex the lowest (9.3 ng/g wet tissue). It was not detectable in cerebral or cerebellar white matter. Our data show that the amounts and distribution of PSA-170, a protein of unknown function, is restricted mostly to schizophrenic brain areas. The difference is not due either to neuroleptic treatment of the patient or to the postmortem proteolysis of the brain samples.

Properties of rat brain dipeptidyl aminopeptidases in the presence of detergents

Rat brain dipeptidyl aminopeptidases I to IV were assayed in the soluble and membrane-bound fractions of rat brain, and the effects of the detergents Triton X-100 and sodium deoxycholate on their activities were studied. Dipeptidyl aminopeptidases I and II were significantly inhibited in the presence of sodium deoxycholate, but were not affected by the presence of Triton X-100. However, dipeptidyl aminopeptidase III was not influenced by either detergent, whereas the activity of dipeptidyl aminopeptidase IV was stimulated in the presence of Triton X-100, but remained unaffected by deoxycholate. These effects were partially or totally reversed after detergents were removed from the medium with adsorbent polymeric beads. Although detergents may have different effects on each DAP activity, the behavior of each enzyme activity in the presence of these substances was similar regardless of their subcellular location. These findings suggest that, as with other aminopeptidases, each of these proteins corresponds to the same molecular species in two different cell compartments.

Transmucosal delivery of leucine enkephalin: stabilization in rabbit enzyme extracts and enhancement of permeation through mucosae

Leucine enkephalin (Tyr-Gly-Gly-Phe-Leu; Leu-Enk) is a naturally occurring peptide that has been shown to have pain modulating properties. To evaluate the feasibility of using various absorptive mucosae as a route of systemic delivery, the stability of Leu-Enk and the effect of enzyme inhibitors (e.g., amastatin, EDTA, and thimerosal) on stabilization and permeation of Leu-Enk through rabbit mucosae in the presence of dihydrofusidates were investigated. Enzymes in the nasal, rectal, and vaginal mucosae were extracted and Leu-Enk (50 micrograms/mL) was added to each of the enzyme extracts and incubated to determine the kinetics and mechanism of degradation. The rate of degradation in the extracts in the absence of inhibitors followed the order: rectal > vaginal > nasal. Whereas EDTA had the best stabilizing effect on Leu-Enk, thimerosal was the best stabilizer for the degradation intermediates. A combination of amastatin (50 microM), EDTA (5 mM), and thimerosal (50 microM) had the greatest stabilizing effect on Leu-Enk and its degradation intermediates. For permeation studies, each mucosa was mounted onto a Valia-Chien permeation cell with Leu-Enk (200 micrograms/mL) in isotonic phosphate buffer (as donor solution). The enhancers used for the study were sodium tauro-dihydrofusidate (STDHF), sodium glycodihydrofusidate (SGDHF), and phosphato-dihydrofusidate (PHDHF). The greatest effect was achieved by PHDHF for all the mucosae. STDHF had a significant effect only on the rectal permeation, whereas SGDHF had significant effects on rectal and vaginal mucosae. Mechanisms by which the dihydrofusidates enhance permeating may involve micelle formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Two cytosolic puromycin-sensitive aminopeptidase isozymes in chicken brain: molecular homology to brain-specific 14-3-3 protein

Two puromycin-sensitive aminopeptidase isozymes (PSA-I and PSA-II) were isolated from chicken brain cytosol by ammonium sulfate fractionation followed by column chromatography on Cellex D and AH-Sepharose 4B and separated on Bio-Gel HTP. Each was purified to homogeneity on Sephadex G-200, Arg-Tyr-AH-Sepharose, Bio-Gel HTP, and preparative gel electrophoresis. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, PSA-I appeared to be a monomer with a molecular mass of 105 kDa, and PSA-II to be composed of two subunits of 25 kDa and 100 kDa. The tryptic maps of 100 kDa and 105 kDa protein in HPLC are different in peak frequency, height, and composition. The internal peptide sequence of PSA-I has a considerable homology to PSA-II. Both isozymes have repeated copies of common peptide segments and have no significant sequence homology to other peptidases and proteinases. These thio and Co(2+)-activated isozymes have a neutral pH optimum and are inhibited by puromycin and bestatin. PSA-II is more sensitive to trypsin and heat treatment, has a lower Km to Met-enkephalin, and is more active on Arg BNA and Pro BNA. Our results suggest that PSA-I and PSA-II derive from translation of two RNAs of a new gene family related to the brain-specific 14-3-3 protein.

A new soluble brain-specific protein: identification and partial purification

A new soluble 170-kDa protein (BP170) was found to be present exclusively in the brain of all the vertebrates that we studied by Western immunoblotting. It was not detected in peripheral rat tissues, including heart, kidney, liver, spleen, lung, muscle, adrenal, intestinal mucosa, sciatic nerve, or pituitary. In rat brain, its regional distribution was found to be heterogeneous, with its highest concentration in the cerebrum and its lowest in the hypothalamus, and 89% of it was in the post-microsomal fraction. BP170 constitutes at least 0.05% of the total brain cytosol proteins. Its level increases during development, being the lowest at 5 days and the highest at 90 days postnatal. BP170 is a single-chain polypeptide. It could be partially purified by precipitation with polyethylene glycol followed by column chromatography on Q Sepharose. Although BP170 was identified by an antiserum against puromycin-sensitive aminopeptidase (PSA), the two proteins differ in molecular weight, chromatographic properties, regional and subcellular distribution, developmental changes, immunoreactivity, and enzyme activity. Self-incubation or trypsin treatment of the partially purified BP170 generates no PSA activity, indicating that BP170 is not a PSA precursor. Furthermore, BP170 is neither an inhibitor nor an activator of PSA. Our data suggest that BP170 is a novel brain-specific protein not previously described.

Identification and characterization of aminopeptidases from Aplysia californica

Aminopeptidase activities were identified in extracts of kidney, ovotestis, head ganglia, heart and haemolymph of Aplysia californica. These enzyme preparations hydrolysed [3H][Leu]enkephalin at the Try-1-Gly-2 bond as determined by h.p.l.c. analysis of cleavage products. In all these tissues, enkephalin-degrading aminopeptidase activities were present both in membrane-bound and cytosolic fractions. The bivalent-cation-chelating agent, 1,10-phenanthroline, inhibited kidney membrane aminopeptidase activity with an IC50 of 30 microM, suggesting that this enzyme is a metalloproteinase. The aminopeptidase inhibitor amastatin was the most potent inhibitor of [Leu]enkephalin degradation (IC50 25 nM) by membrane-bound aminopeptidase, and bacitracin, bestatin and puromycin were about 100-1000 times less potent. In contrast with membrane-bound aminopeptidase, the cytosolic form is sensitive to puromycin. Angiotensin-converting enzyme inhibitor had no effect on [Leu]enkephalin degradation by kidney membranes, while the neutral endopeptidase inhibitors were poor inhibitors of the enzymes in this preparation. The Km values of the aminopeptidase in the kidney membranes and cytosolic fractions for the [Leu]enkephalin substrate were 2.4 and 7.4 microM respectively. The aminopeptidase present in the kidney membranes also hydrolysed endogenous Phe-Met-Arg-Phe-amide peptide at the Phe-1-Met-2 bond as well as synthetic alanine p-nitroanilide and leucine p-nitroanilide. When used in a competition assay, these substrates inhibited hydrolysis of [3H][Leu]enkephalin, suggesting that the same enzyme degraded all these substrates. Taken together, these results suggest that Aplysia tissues contain both a membrane-bound aminopeptidase related to the mammalian aminopeptidase N and a cytosolic puromycin-sensitive aminopeptidase.

A monoclonal antibody to common acute lymphoblastic leukemia antigen (neutral endopeptidase) immunostains senile plaques in the brains of patients with Alzheimer's disease

We immunostained brain tissues of patients with Alzheimer's disease (AD) together with non-demented aged and younger controls with a battery of anti-human hemopoietic cell monoclonal antibodies (OK series, Ortho Diagnostics Co., Ltd. and some others) by the avidin-biotin-peroxidase complex (ABC) method to see if any epitopes are shared with the nervous system or might contribute to the neurodegenerative changes in this disease. One out of 29 monoclonal antibodies, OKBcALLa, which recognizes common acute lymphocytic leukemia antigen (CALLA, CD10), immunostained senile plaques in the brains of patients with AD. The pattern and intensity of this staining, using cryopreserved samples, was almost identical to that obtained with anti beta-protein. Thus, senile plaques in the Alzheimer's brain share an epitope with CALLA.

An enkephalin degrading aminopeptidase of human brain preserved during the vertebrate phylogeny

1. A soluble human brain aminopeptidase which hydrolyses the Tyr-Gly bond of Met-enkephalin and Leu-enkephalin was identified in the brains of the following vertebrates: mammals (Callithrix jacchus and Rattus norvegicus), bird (Gallus domesticus), reptile (Tupinambis teguixin), amphibia (Bufo paracnemis), fish (Sarotherdon niloticus) and elasmobranchy (Galeocerdo cuvieri). 2. The properties of this enzyme are: molecular weight near 100,000 Da, isoelectric point near 4.9, optimum pH near 7.5, activation by dithiothreitol, strong inhibition by Cu2+, Zn2+, Ni2+, puromycin and bacitracin, hydrolysis of enkephalins and basic and neutral aminoacid-beta-naphythylamide substrates. 3. The results indicate the preservation of this human brain aminopeptidase during the course of vertebrate phylogeny.

Cellular localization of puromycin-sensitive aminopeptidase isozymes

We developed monoclonal antibodies (mAbs) against two isozymes of a cytosolic puromycin-sensitive aminopeptidase (PSA-I and PSA-II) purified from chicken brain. The isozymes could be distinguished using Ouchterlony double-immunodiffusion and Western immunoblot. Their distribution in neuronal and glial cells as visualized by indirect immunofluorescence with these mAbs was found to differ: PSA-I was confined mostly to glial lysosomes; PSA-II showed fibrillar distribution in both types of nerve cells, but in disparate patterns. These results and our findings of peptide structural differences suggest that the two PSA isozymes are expressed differently in the nervous system.

Prolyl aminopeptidase from rat brain and kidney. Action on peptides and identification as leucyl aminopeptidase

Based on the liberation of proline from ProLeuGlyNH2 (MIF-1, melanostatin) manganese-activated prolyl aminopeptidase activities were purified from rat brain and kidney cytosolic fractions. They were distinguished from other di- and tripeptidases and an arylamidase liberating N-terminal proline. Purified prolyl aminopeptidase from both sources had identical molecular properties (native Mr 300,000, subunit Mr 54,000) and very similar catalytic properties. The action of the purified enzymes was not restricted to proline. Other, in particular lipophilic, amino acids were cleaved from di-, tri- and oligopeptides with even higher velocities. Peptides with N-terminal penultimate proline residues were not degraded. From a comparison of molecular data, action on peptides, influence of pH values, inhibitors and activators, it is concluded that the activity is identical with leucyl aminopeptidase (EC 3.4.11.1) and that a separate prolyl aminopeptidase (EC 3.4.11.5) does not exist in rats.