Acetylcholinesterase hydrolyses chromogranin A to yield low molecular weight peptides

Effects of niacin deficiency on the levels of soluble proteins and enzyme activities in various tissues of Japanese quail

The effects of niacin deficiency on the levels of soluble proteins and enzyme activities of Japanese quail have been investigated. SDS-polyacrylamide gel electrophoresis revealed that in the pectoral muscle the soluble proteins with molecular masses corresponding to 181, 128, 93, 76, 72, 62, 56, 43, 41, 28 and 20 kDa were present in lower amounts but those of 60, 50 and 37 kDa were present in higher amounts. In the heart the soluble proteins with a molecular mass of 181 kDa were present in lower amounts and in the brain those of 43 kDa were present in lower amounts but those of 221 kDa were present in higher amounts. In the intestine the soluble proteins with molecular masses corresponding to 181, 102, 83, 74, 72, 44 and 40 kDa were present in lower amounts but those of 41 kDa and 18 kDa were present in higher amounts. There was a marked reduction in the level of NAD and NADPH in the pectoral muscle of niacin deficient quail but not in other tissues. The specific activity of glyceraldehyde-3-phosphate dehydrogenase decreased markedly both in the liver and pectoral muscle of niacin deficient quail whereas that of 6-phosphogluconate dehydrogenase and malic enzyme decreased markedly in the liver or pectoral muscle, respectively. In contrast, the specific activity of acetylcholinesterase and carboxypeptidase increased markedly in the liver or the pectoral muscle, respectively. The results suggest that a severe niacin deficiency exerted specific effects on levels of some soluble proteins particularly in the pectoral muscle and intestine and on activities of certain enzymes in the liver and the pectoral muscle.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Effect of prolonged starvation on the activities of malic enzyme and acetylcholinesterase in tissues of Japanese quail

During starvation muscle protein degradation is increased but the mechanism for this is uncertain. In this study Japanese quail were starved for 5 days and the activities of malic enzyme and acetylcholinesterase were determined in various tissues. SDS-polyacrylamide gel electrophoresis showed that the soluble proteins with molecular weights corresponding to 160, 120, 108, 99 and 38 kDa were absent in the liver of the starved group. In the pectoral muscle the soluble proteins with molecular weights corresponding to 69, 41 and 34 kDa were missing. The activity of malic enzyme in the liver, heart and pectoral muscle of the starved group decreased markedly whereas that of acetylcholinesterase increased markedly in the pectoral muscle (P < 0.005). It is concluded that in prolonged starvation acetylcholinesterase synthesis may be induced in tissues being subjected to protein catabolism and that this enzyme may be involved as a protease in protein degradation.

Expression of chromogranin A in lesions in the central nervous system from patients with neurological diseases

Expression of chromogranin A in various neurological diseases was examined immunohistochemically using purified anti-human chromogranin A antiserum. The antibody stained dystrophic neurites in senile plaques in Alzheimer disease brain, Pick bodies and ballooned neurons in Pick's disease brain, some Lewy bodies in the substantia nigra of Parkinson's disease, and axonal swellings in various neurological conditions including Parkinson's disease, striatonigral degeneration, Shy-Drager syndrome, amyotrophic lateral sclerosis and cerebral infarction. The present study shows that expression of chromogranin A is not an exclusive feature of Alzheimer disease or Pick's disease, and indicates that it could be a useful marker for various neurological diseases.

Ultrastructural cytochemistry of acetylcholinesterase enzyme activity in pancreatic tissue transplants in rats

The distribution of acetylcholinesterase (AChE) at the ultrastructural level was investigated in normal and in pancreatic fragments transplanted for 56 days into the anterior eye chamber of heterologous rats using enzyme cytochemical methods. Acetylcholinesterase reaction products were seen on the basal surface of the acinar cells in normal pancreas. Acetylcholinesterase enzyme activity was also detected on the axolemma of the surviving nerve fibres. This enzyme reaction product forms alternating thick and thin bands on the axolemma. Some of these AChE-positive nerve fibres accompany blood vessels that also survive after transplantation. AChE were seen in cytoplasm adjacent to the surviving alpha and pancreatic polypeptide cells. We conclude that the ability of some neurons and cells to produce and or store acetylcholinesterase is still retained after transplantation of pancreatic tissue into the anterior eye chamber of rats.

Intracellular and extracellular processing of chromogranin A. Determination of cleavage sites

Chromogranins are a family of acidic soluble proteins which exhibit widespread distribution in endocrine cells and neurons. Chromogranin A (CGA), the major soluble component of the secretory granules in chromaffin cells of the adrenal medulla, is a single polypeptide chain of 431 residues with an apparent molecular mass of 70-75 kDa and a pI of 4.5-5. In mature bovine chromaffin granules about 50% of the CGA has been processed. In the present paper, the structural features of the proteolytic degradation mechanism have been characterized with regard to the possible function of CGA as a prohormone, as suggested by recent studies. CGA-derived components present in chromaffin granules were subjected to either two-dimensional gel electrophoresis or HPLC and the N-terminal of each fragment was sequenced. Immunoblotting with antisera to specific sequences within the CGA molecule were used to characterize these fragments further at their C-terminal. In addition, a similar approach was performed to characterize CGA-derived fragments released into the extracellular space from directly depolarized bovine cultured chromaffin cells. Our results identified several proteolytic cleavage sites involved in CGA degradation. Intragranular processing occurs at 12 cleavage sites along the peptide chain located in both N- and C-terminal moieties of the protein; a preferential proteolytic attack in the C-terminal part was noted. We found that CGA processing also occurs in the extracellular space after release, generating new shorter fragments. The proteolytic cleavage sites identified in this study were compared with the cleavage points which are thought to be involved in generating CGA fragments with specific biological activity: pancreastatin, chromostatin and N-terminal vasostatin fragments. In addition, a new 12-amino-acid CGA-derived peptide corresponding to the sequence 65-76 was identified in the soluble core of purified chromaffin granules. This short peptide was released, together with catecholamines, after stimulation of cultured chromaffin cells suggesting its presence within the storage complex of chromaffin granules. The specific biological activity of this CGA-derived fragment remains to be determined.

Cerebrospinal fluid chromogranin A is unchanged in Alzheimer dementia

Several lines of evidence link chromogranin A (CgA), the major soluble protein in catecholamine storage vesicles, with the cholinergic nervous system, abnormalities of which may play a central role in memory deficits in Alzheimer dementia. Because of reported elevations of CgA in Alzheimer brains and its presence in the senile plaque lesions of such brains, we evaluated the concentration of CgA in cerebrospinal fluid of Alzheimer dementia patients and matched controls. CgA was detectable in each sample, but the results in dementia showed substantial overlap with and no significant (p = 0.55) difference from the results in healthy controls. We conclude that measurement of cerebrospinal fluid CgA offers no diagnostic assistance in Alzheimer dementia.

Processing of chromaffin granule proteins: a profusion of proteases?

Evidence suggests that proenkephalin and members of the chromogranin/secretogranin family of proteins are prohormone precursors, giving rise to a variety of peptides with biologic activity. However, the specific proteases responsible for cleaving these proteins in vivo have not been fully established. Several candidate proteases have been described, some of which have been shown to cleave these proteins in vitro. Proteolytic processing of the chromogranins may be particularly complex, occurring in specific tissue-dependent patterns. To account for this level of complexity several protease systems may be operative, either alone or in concert, both within the neurosecretory granule and in the extracellular space. Specific proteases which are available within neurosecretory cells or in the local extracellular environment, and which may cleave these prohormones include PC1 and PC2 (recently described members of the Kex2/furin family of endoproteases), as well as kallikrein, acetylcholinesterase, and, more recently, the plasminogen/plasmin protease system. The potential role of these specific proteases in the processing of proenkephalin and the chromogranins is discussed, in particular, in the context of possible processing clues available from recent analysis of cDNA and genomic intron/exon structure.

Posttranslational processing of proenkephalins and chromogranins/secretogranins

Posttranslational processing of peptide-precursors is nowadays believed to play an important role in the functioning of neurons and endocrine cells. Both proenkephalins and chromogranins/secretogranins are considered as precursor molecules in these tissues, resulting in posttranslationally formed degradation products with potential biological activities. Among the proteins and peptides of neuronal and endocrine secretory granules, the enkephalins and enkephalin-containing peptides have been most extensively studied. The characterization of the post-translationally formed degradation products of the proenkephalins have enabled the understanding of their processing pathway. Chromogranins/secretogranins represent a group of acidic glycoproteins, contained within hormone storage granules. The biochemistry, biogenesis and molecular properties of these proteins have already been studied for 25 years. The chromogranins/secretogranins have a widespread distribution throughout the neuroendocrine system, the adrenal medullary chromaffin granules being the major source of these storage components. Recent data provide evidence for a precursor role for all members of the chromogranins/secretogranins family although also several other functions have been proposed. In this review, some of the methods applied to study proteolytic processing are described. In addition, the posttranslational processing of chromogranins/secretogranins and proenkephalins, especially the biochemical aspects, will be discussed and compared. Recent exciting developments on the generation and identification of potential physiologically active fragments will be covered.

Distribution and coexistence of chromogranin A-, serotonin- and pancreastatin-like immunoreactivity in endocrine-like cells of the human anal canal

The comparative distribution and coexistence of chromogranin A (CGA)-, serotonin (5-hydroxytryptamine; 5-HT)- and pancreastatin (PST)-like immunoreactivity in endocrine-like cells of the human anal canal was investigated by light-microscopic immunocytochemistry. The largest population of colorectal endocrine-like cells consisted of CGA-immunoreactive (ir) cells, followed by the 5-HT-ir and PST-ir cell population. In the anal transitional zone (ATZ), CGA- and 5-HT-immunoreactivity was equally distributed; ir-PST was confined to a smaller endocrine-like cell population. In the squamous zone and the perianal skin, Merkel cells in the basal layer of the epidermis and hair follicles exhibited ir-CGA and ir-PST but no ir-5-HT. Double immunofluorescence on identical sections revealed distinct coexistence patterns. In the colorectal zone, about 2/3 of the CGA-ir endocrine-like cells also stained for 5-HT, whereas in the ATZ epithelium, CGA- and 5-HT-immunoreactivity completely overlapped. No 5-HT-immunoreactivity could be detected in CGA-ir Merkel cells of the squamous zone of the anal canal and the perianal skin. PST-immunoreactivity was present in about 1/3 of the CGA-ir colorectal and anal transitional endocrine-like cells and in about 1/4 of the Merkel-cell population staining for CGA. These chemically heterogeneous phenotypes of the anal endocrine-like and Merkel cells may reflect a specific regulatory role of these cells in the various epithelial linings of the human anal canal and the perianal skin.

Hydrolysis of substance P in the rabbit retina: I. Involvement of acetylcholine and acetylcholinesterase. An in vivo study

The laminar patterns of acetylcholinesterase (AChE) activity and substance P (SP) immunoreactivity within the inner plexiform layer (IPL) of the rabbit retina show striking similarities. Discrete bands of SP-immunoreactivity were seen at 1-7%, 40-48% and 85-95% depth of IPL. AChE activity was present throughout the entire thickness of the IPL with moderately stained bands in each sublamina (3-24% in sublamina a and 62-89% in sublamina b depth IPL). These bands were bordered on both sides by bands of even greater density (in sublamina a 0-3% and 24-34% and in sublamina b 55-62% and 89-100% depth IPL). Cell processes staining for choline acetyltransferase (ChAT) have previously been shown to ramify at 19-24% and 63-79% depth levels. Thus, SP- and ChAT-immunoreactive bands are located in both sublaminae, positioned within regions of moderate AChE activity and flanked by bands with greater AChE activity. This strong morphological correspondence and reported interactions between acetylcholine (ACh), AChE and SP in vitro provide the basis for the present study to determine whether such interactions can be demonstrated in vivo. Retinas infused with ACh showed a 60% average increase in SP-IR as compared with untreated retinas from the same animals. Treatment with diisopropylfluorophosphate (DFP) also resulted in a 56% increase in SP-IR. The ability of ACh to induce increased levels of SP was not inhibited by CoCl2, atropine or mecamylamine, ruling out the possibilities of polysynaptic transmission or involvement of muscarinic or nicotinic receptors. Infusion of ACh did not increase the levels of preprotachykinin-mRNA indicating that the increase in SP-IR is not due to de novo synthesis but rather to inhibition of the enzyme(s) responsible for SP degradation. Whether AChE functions alone or in concert with other enzymes to hydrolyze SP cannot be determined from these experiments but is addressed in a separate study.

CSF chromogranin A-like immunoreactivity in schizophrenia. Assessment of clinical and biochemical relationships

Chromogranin A (CgA) is co-released with catecholamines and peptides and has a wide distribution in the brain. Chromogranin A provides a measure of tonic arousal. CSF CgA-like immunoreactivity (CgA-LI) was studied in 42 drug-free male schizophrenic patients. 33 of these patients were first studied during chronic haloperidol maintenance treatment. Withdrawal from haloperidol maintenance treatment was associated with a significant increase in CSF CgA-LI, particularly in the patients who did not relapse. Contrary to expectation CSF CgA-LI was higher in drug-free patients who slept longer the night before the lumbar puncture. Significant relationships were observed between CSF CgA-LI and CSF homovanillic acid, acetylcholinesterase, neuropeptide Y-LI and 5-hydroxy-indole acetic acid, but not with CSF norepinephrine or 3-methoxy-4-hydroxyphenylglycol. Ventricular brain ratios correlated negatively with CSF CgA-LI levels.

Dual presence of chromogranin A-like immunoreactivity in a population of endocrine-like cells and in nerve fibers in the human anal canal

Light microscopic immunohistochemistry was used to investigate the presence and distribution of chromogranin A-like immunoreactivity in the human anal canal. In the anal transitional zone (ATZ), anal duct and anal gland epithelium, a varied number of mostly elongated cells strongly stained for CGA, using an antibody directed to a highly species-conserved region of the CGA molecule or the monoclonal antibody LK2H10. The density of CGA-immunoreactive (ir) cells strikingly increased from the ATZ epithelium towards the anal gland epithelium. CGA-ir cells possessed single processes running perpendicularly to reach the epithelial surface and exhibited basal ramifications that extended parallel to the basal lamina. The number of CGA-ir cells in anal glands exceeded CGA-ir cells in the crypt-bearing colorectal-type mucosa. The abundant population of CGA-ir cells in the anal canal most likely represents a population of specialized endocrine or paracrine cells. CGA-like immunoreactivity was also present in anocutaneous Merkel cells. A sparse number of vascular and non-vascular CGA-ir varicose nerve fibers was present throughout the layers and rostrocaudal divisions of the anal canal and in the perianal skin. Proposed functions of CGA in neuroendocrine cells and nerves of the anal canal include calcium binding and regulation, secretory granule matrix formation, and generation of bioactive peptides.

Subcellular distribution of acetylcholinesterase forms in chromaffin cells. Do chromaffin granules contain a specific secretory acetylcholinesterase?

The presence of acetylcholinesterase (AChE) in chromaffin granules has been controversial for a long time. We therefore undertook a study of AChE molecular forms in chromaffin cells and of their distribution during subcellular fractionation. We characterized four main AChE forms, three amphiphilic forms (Ga1, Ga2 and Ga4), and one non-amphiphilic form (Gna4). Each form shows the same molecular characteristics (sedimentation, electrophoretic migration, lectin interactions) in the different subcellular fractions. All forms are glycosylated and seem to possess both N-linked and O-linked carbohydrate chains. There are differences in the structure of the glycans carried by the different forms, as indicated by their interaction with some lectins. Glycophosphatidylinositol-specific phospholipases C converted the Ga2 form, but not the other amphiphilic forms, into non-amphiphilic derivatives. The distinct patterns of AChE molecular forms observed in various subcellular compartments indicate the existence of an active sorting process. Gna4 was concentrated in fractions of high density, containing chromaffin granules. We obtained evidence for the existence of a lighter fraction also containing chromogranin A, tetrabenazine-binding sites and Gna4 AChE, which may correspond to immature, incompletely loaded granules or to partially emptied granules. The distribution of Gna4 during subcellular fractionation suggested that this form is largely, but not exclusively, contained in chromaffin granules, the membranes of which may contain low levels of the three amphiphilic forms.

The distribution of chromogranin A-like immunoreactivity in the human hippocampus coincides with the pattern of resistance to epilepsy-induced neuronal damage

The distribution of chromogranin A-like immunoreactivity in the hippocampus of adult humans who were free of neurological disease was examined by immunohistochemical methods. Immunoreactivity was restricted to the cytoplasm of certain neuronal populations, most notably the mossy fibers of denate granule cells (and a subset of their perikarya), and the perikarya of pyramidal cells of the cornu Ammonis 2 (CA2) sector. Additionally, staining was observed in neurons in the stratum oriens, a population of neurons at the periphery of the CA4 sector, scattered, probably short-axon perikarya in the CA1 sector, and fibers in the perforant path and the molecular layer of the dentate gyrus. Pyramidal neurons in the CA1 and CA3 sectors were not immunoreactive. The two prominently immunoreactive neuronal populations, CA2 pyramids and dentate granule cells, are those spared in human and experimental epileptic brain damage, whereas CA1 and CA3 pyramids, lacking chromogranin, are characteristically destroyed in this condition. The known activities of chromogranin in the periphery as a calcium-binding protein and as a precursor of active peptides (autocrine inhibitory modulators) suggest that its distribution in the hippocampus may help to explain the observed pattern of resistance to epileptic brain damage.

Proteolytic processing of chromogranin A in cultured chromaffin cells

The prohormone chromogranin A is the major soluble component of secretory granules in chromaffin cells of adrenal medulla and in many other different endocrine cell types. The proteolytic processing of chromogranin A was studied in cultured bovine chromaffin cells using [35S]methionine to label proteins and a specific antibody to immunoprecipitate the native protein and its breakdown products. In resting cells, it was found that the degradation of chromogranin A is a slow process, since no degradation was observed after a 40 h incubation with radiolabelled methionine. Stimulation of cells with a single pulse or with successive pulses of nicotine did not significantly enhance the degree of proteolytic processing of chromogranin A. As it has recently been shown (Simon, J.P., Bader, M.F. and Aunis, D. Biochem. J. (1989) 260, 915-922) that protein kinase C may be involved in the regulation of chromogranin A synthesis, the possibility that prohormone processing may also be controlled by protein kinase C was examined using the activator of protein kinase C, 12-O-tetradecanoylphorbol 13-acetate (TPA). However, incubation of cells with TPA did not significantly modify chromogranin A processing, indicating that biosynthesis and proteolytic processing of chromogranin A are two distinctly regulated mechanisms. Glucocorticoids are known to exert regulatory control of chromaffin cell metabolism; however, incubation of cells with dexamethasone did not alter slow chromogranin A processing. Stimulation of labelled cells rapidly released newly synthesized chromogranin A into external medium. In addition, released chromogranin A was found to be actively processed into its 60 kDa and 43 kDa breakdown products. This extracellular proteolytic degradation mechanism may be of importance with regard to the function of chromogranin A as a prohormone.

Chromogranin A-like immunoreactivity in the human brain: distribution in bulbar medulla and cerebral cortex

Chromogranin A is a glycoprotein stored in secretory granules of a variety of neuroendocrine cells. Among other functions, chromogranin A is a calcium-binding protein, and a precursor of modulatory peptides. Although known to be expressed in mammalian CNS neurons, it was previously believed that antibodies directed against human chromogranin A did not label neurons. A method is reported for the immunohistochemical demonstration of chromogranin A-like immunoreactivity in adult human post-mortem brain, utilizing the previously characterized monoclonal antibody LK2H10. Chromogranin A-like immunoreactivity of human brain could be absorbed with heat-stable protein extract from adrenal medulla, but not from liver, and a similar preparation of human cerebral cortex eliminated the labeling of adrenal medulla by LK2H10. However, unlike adrenal medullary chromogranin A-like immunoreactivity, cerebral chromogranin A-like immunoreactivity was destroyed by embedding the tissue in paraffin or treating frozen sections with methanol during the endogenous peroxidase blocking step, suggesting differences in post-translational processing of these two forms of chromogranin A. In both cerebral regions studied, bulbar medulla and parietal cortex, chromogranin A-like immunoreactivity was widespread in neuronal perikarya, dendrites, and axonic terminals, but restricted to certain neuronal populations. Among other findings it is reported that the main olivary neurons are immunoreactive for chromogranin A; this implies a new co-localization of chromogranin A, with corticotropin-releasing factor. The cerebral neocortex showed a laminar pattern of staining of perikarya in layers III-VI, and of the neuropil in the supragranular layers. In conjunction with the evidence of neuromodulatory action in the periphery, these results raise the possibility of a major neurotransmitter role for chromogranin A in the human brain.

Fragmentation of bovine chromogranin A by plasma kallikrein

Chromogranin A has been reported to be processed in vivo by an as yet undefined proteinase(s) suggesting that it is a precursor of biologically active peptides such as pancreastatin. In this study, plasma kallikrein was used as a model proteinase to identify the cleavage sites exposed in bovine parathyroid chromogranin A. Purified bovine parathyroid chromogranin A was digested with human plasma kallikrein. The proteolytic fragments produced were isolated by HPLC and chemically characterized by amino acid composition and sequence analysis. The combined results indicate that the enzyme has preference for specific single Arg residues, cutting C-terminal to this amino acid, although certain pairs of basic sites were also cleaved. The characterized fragments were released in a selective manner from the whole molecule with rapid production of the fragments covering positions 1-247 and 352-358.

Neurochemical and behavioral correlates of unilateral striatal acetylcholinesterase inhibition by fasciculin in rats

Fasciculin 2 (FAS) an anticholinesterase peptide isolated from the venom of the Green mamba (Dendroaspis angusticeps) was injected into the right striatum of albino rats (1.5 micrograms total amount). The inhibition of acetylcholinesterase (AChE) activity was 86 and 60% 24 h and 7 days after FAS injection, respectively. The treatment with apomorphine (APO) (2 mg/kg s.c.) 24 h after FAS provoked a moderate circling towards the lesioned side that was reverted by atropine (30 mg/kg i.p.). The same dose of APO 7 days after FAS, provoked an inconstant contralateral circling. Neither dopamine nor serotonin nor their metabolites were significantly affected 24 h or 7 days after FAS injection. Radioligand binding assays of dopamine, muscarinic and benzodiazepine receptors only showed a decrease of the density of the muscarinic ones 7 days after FAS. These results are interpreted as showing that the changes provoked by FAS would be compensated but the system would remain in an unsteady state only demonstrable after pharmacological challenge. The chronic down-regulation of muscarinic receptors would compensate the increased cholinergic activity and would therefore block its behavioral expression.

Biochemistry of the chromogranin A protein family

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Coexistence of acetylcholinesterase-, human growth hormone-releasing factor(1-37)-, alpha-melanotropin- and melanin-concentrating hormone-like immunoreactivities in neurons of the rat hypothalamus: a light and electron microscope study

Using an antiserum (AS) raised against rat cerebral acetylcholinesterase (AChE), we revealed a neuron population in lateral and dorsal areas of the posterior rat hypothalamus. These neurons were previously described using antibodies to human growth hormone-releasing factor(1-37) (GRF-37), alpha-melanotropin (alpha-MSH) and melanin-concentrating hormone (MCH). Different intracytoplasmic distributions of the immunodeposits were observed depending on the used serum. Ultrastructural investigations demonstrated that AChE-AS labeled rough endoplasmic reticulum and nuclear envelope in control rats. MCH-AS stained Golgi apparatus in control animals and secretory granules in colchicine-injected rats. GRF-37-AS always revealed secretory granules, and alpha-MSH-AS gave the same staining only after colchicine injection.

Effects of local inhibition of locus coeruleus acetylcholinesterase by fasciculin in rats

The inhibition of locus coeruleus (LC) acetylcholinesterase (AChE) by Fasciculin II (FAS), a novel anticholinesterase peptide from the green mamba (Dendroaspis angusticeps) venom, was studied in rats. FAS was stereotaxically injected (0.5 microliters of a 1 mg/ml solution) in the right LC. The left LC was taken as control. A group of rats received only saline injected with the same procedure. An inhibition of 80% of LC AChE activity was observed 24 h later. Monoamine and metabolite levels were assessed by high-performance liquid chromatography (HPLC) with electrochemical detection. A significant increase of noradrenaline (NA) levels was found in the injected side when compared with controls 24 h after injection. Neither dopamine, serotonin nor their metabolites or the NA metabolite 4-methoxyhydroxyphenylglycol showed any change after FAS injection. Atropine (30 mg/kg, i.p.) did not prevent the NA increase.

Acetylcholinesterase in the alimentary canal of fish: light and electron microscopic detection, quantitative distribution in different segments of the gut

The acetylcholinesterase activity was measured and histochemically localized in the alimentary tract of 2 fish species, carp (Cyprinus carpio) and tench (Tinca tinca). A comparison was made of the activities in the different gut segments. Light and electron microscopic histochemistry revealed acetylcholinesterase-positive cell bodies along the entire length of the alimentary canal in both species, between the muscular layers. Acetylcholinesterase-positive, cholinergic motor endplates were frequent in the esophagus of both carp and tench, and they were also present in the striated muscular layers of the tench stomach and midgut. The enzyme activity detected by the method of Ellman et al. (1961) was highest (16.3 U/mg protein) in the tench foregut and midgut, while it was at the same lower level (9.5 U/mg protein) in each segment of the carp gut and in the tench hindgut. The morphological findings and the higher acetylcholinesterase activity in the tench foregut and midgut suggest that the enteric striated musculature is endowed with denser cholinergic innervation than the enteric smooth musculature.

Serum acetylcholinesterase possesses trypsin-like and carboxypeptidase B-like activity

Acetylcholinesterase (AChE, EC 3.1.1.7) purified from the electric organ of eel possesses a protease activity resembling that of a neuropeptide processing enzyme. To examine whether any mammalian AChEs possess a similar protease activity, the enzyme was purified, 110,000-fold from foetal bovine serum. Purified serum AChE cleaved 2 synthetic peptide substrates in a manner resembling the combined actions of trypsin-like and carboxypeptidase B-like enzymes. A synthetic fragment of preproenkephalin A (residues 97-107) containing a complete methionine-enkephalin sequence was cleaved by serum AChE to yield free methionine-enkephalin. The carboxypeptidase action of AChE was weakly stimulated by the presence of 100 microM CoCl2 suggesting the requirement of a metal ion for complete activity. The results support the hypothesis that in many tissues AChE may act as a neuropeptide processing enzyme.

Acetylcholinesterase converts Met5-enkephalin-containing peptides to Met5-enkephalin

Acetylcholinesterase (AChE; E.C. 3.1.1.7) was incubated with a number of enkephalin-containing neuropeptides found in the bovine adrenal medulla. Met5-enkephalin and Leu5-enkephalin were the most stable of the peptides studied, while precursors of Met5-enkephalin were converted to Met5-enkephalin. AChE is therefore capable of limited peptidase activity on Met5-enkephalin precursors. The enzyme hydrolysed the Met5-enkephalin precursor BAM-12P on the C-terminal side of the pair of basic amino acid residues, and cleaved basic amino acids from the carboxy-terminal of Met5-enkephalin-Arg6 and Met5-enkephalin-Arg6-Arg7. These results indicate that AChE, acting alone, is capable of the same pattern of enkephalin processing as that observed in the adrenal medulla.

Identification of a trypsin-like site associated with acetylcholinesterase by affinity labelling with [3H]diisopropyl fluorophosphate

In addition to its ability to hydrolyze acetylcholine, purified eel acetylcholinesterase possesses a trypsin-like endopeptidase activity. The tryptic activity is associated with a serine residue at a site that is distinct from the esteratic site. To label both the esteratic and tryptic sites, the enzyme was incubated with the serine hydrolase inhibitor [3H]diisopropyl fluorophosphate. This compound labelled the protein in a biphasic manner, with both slow and rapid labelling kinetics. The time course of the rapid phase was similar to the time course of inactivation of the esteratic activity. The time course of the slow phase was similar to the time course of inactivation of the tryptic activity. Labelling of the nonesteratic site was inhibited by the trypsin inhibitor N alpha-p-tosyl-L-lysine chloromethyl ketone. The total number of sites labelled by [3H]diisopropyl fluorophosphate on eel acetylcholinesterase was 2.6 mol/280,000 g protein, whereas the number of tryptic sites was less (0.52 mol/280,000 g). The results suggest that a subpopulation of acetylcholinesterase molecules may possess tryptic activity. Extensive chromatography of the purified enzyme by ion-exchange and gel filtration failed to separate the labelled tryptic component from acetylcholinesterase. On sodium dodecyl sulfate-polyacrylamide gels, the labelled tryptic component comigrated with a polypeptide of 50,000 molecular weight, which is a major proteolytic digestion product derived from the intact acetylcholinesterase monomer. Because of its localization in many noncholinergic peptide-containing cells, acetylcholinesterase could act as a neuropeptide processing enzyme in these cells.

Acetylcholinesterase undergoes autolysis to generate trypsin-like activity

Acetylcholinesterase (AChE) is one of the most highly studied enzymes, although its function in many tissues has remained obscure. AChE purified from eel or foetal bovine serum possesses proteolytic activity in addition to esterase activity. The presence of trypsin-like and metallocarboxypeptidase-like activities associated with AChE accounts for its ability to convert enkephalin peptide precursors into enkephalins. Several lines of evidence indicate that AChE's trypsin-like activity is an integral component of the molecule and that it is activated by autolysis. Incubation of affinity-purified eel AChE generated several fragments of low relative molecular mass (Mr). One of these low Mr fragments (Mr = 25,000 Da, 25K) cleaved from the 70K form of AChE, possessed considerable sequence similarity to the N-terminal sequence of pancreatic trypsin. Autolysis of eel AChE may give rise to a neuropeptide processing enzyme.

Secretion from chromaffin cells is controlled by chromogranin A-derived peptides

Chromogranin A (CGA) is the major protein of the secretory granule from chromaffin cells and also is found in a variety of endocrine cells. Although the sequence of this acidic glycoprotein has been elucidated recently, its biological function is unknown. Here we have purified CGA from chromaffin granules; the final preparation contained the 74-kDa native CGA together with two degradation products--three bands near 60 kDa and a single band of 43 kDa. This preparation was found to inhibit (a maximum inhibition of 60% at 1 microM) the nicotine-induced, but not the high K+-evoked, catecholamine secretion from bovine chromaffin cells maintained in primary culture. Spontaneous release was also affected in the nanomolar CGA protein concentration range. The observation that the inhibitory effect is strictly dependent on a preincubation step together with the modification of the CGA protein profile during this preincubation step suggests that the degradation peptide(s) rather than the 74-kDa native CGA--the approximately equal to 60-kDa bands or the 43-kDa singlet band--is actually involved in secretory cell activity. This was demonstrated by using trypsin-generated peptides that were inhibitory without the preincubation period. The finding that unprocessed CGA is not active on chromaffin cell secretion suggests that this molecule is a precursor of a peptide(s) that is able to regulate catecholamine secretion. Thus, the present data suggest that a CGA-derived peptide(s) could exert a feedback control on chromaffin cell secretory activity--a mechanism that might be of importance during stress situations.

Acetylcholinesterase exhibits trypsin-like and metalloexopeptidase-like activity in cleaving a model peptide

Acetylcholinesterase (EC 3.1.1.7) has been shown to possess an intrinsic peptidase activity. [Chubb et al. (1983), Neuroscience 10, 1369-1383]. To examine this activity further, the breakdown of a model hexapeptide (leu-trp-met-arg-phe-ala) LWMRFA was studied. Affinity-purified eel acetylcholinesterase rapidly cleaved the hexapeptide in a trypsin-like manner to produce two peptides (LWMR and FA). Acetylcholinesterase more slowly cleaved the C-terminal alanine residue from the peptide to yield LWMRF. Although the enzyme showed preference for cleaving the hexapeptide at its C-terminal, it was also able to cleave the N-terminal leucine residue form the tryptic product LWMR. Hydrolysis of the peptide at the tryptic site (arg4-phe5) was strongly inhibited by the trypsin inhibitor diisopropylfluorophosphate. Cleavage of the C-terminal alanine was only poorly inhibited by diisopropylfluorophosphate, but more strongly inhibited by metal-ion chelating agents, and it was increased in the presence of Zn2+ and Co2+. The pH optimum for cleavage at the tryptic site was 6, while that for the carboxypeptidase site was 8-9. These results show that acetylcholinesterase can hydrolyse peptides like a trypsin-like endopeptidase and a Zn2+- or Co2+-dependent exopeptidase, and they suggest that these two peptidase activities are associated with two separate active sites on the acetylcholinesterase molecule. As both peptidase activities eluted with acetylcholinesterase from a TSK 4000SW column when it was chromatographed by high-performance liquid chromatography, it is unlikely that the presence of either peptidase activity could be attributable to a contaminant in the acetylcholinesterase preparation. We suggest that acetylcholinesterase may be involved in the breakdown of bioactive peptides or their precursors in neuroendocrine cells.