Structural alterations in the peptide backbone of beta-amyloid core protein may account for its deposition and stability in Alzheimer's disease

Metallothioneins in brain--the role in physiology and pathology

A symposium on the role of brain metallothioneins (MTs) in physiology and pathology was held at the 1996 Annual Society of Toxicology Meeting in Anaheim, California. The objectives of this symposium were to: (1) review the physiologic function of MTs, (2) examine the distribution of brain MTs with particular emphasis on cell-specific localization (neurons vs neuroglia), (3) discuss MT gene responsiveness upon toxic insult with metals, and (4) discuss the potential role of MTs in the etiology of neurodegenerative disorders. Dr. Cherian discussed the biochemical properties of the MTs, emphasizing structural similarities and differences between the MTs. Dr. Klaassen addressed the expression and distribution of the MTs in brains with special reference to the cell-specific localization of MTs. Dr. Aschner provided data illustrating a potential role for MTs in attenuating the cytotoxicity caused by methylmercury (MeHg) in cultured neonatal astrocytes. Dr. Palmiter discussed the properties of MT-III and the increased sensitivity of MT-III knockout mice to kainate-induced seizures. Cerebral zinc metabolism, its relationship to MT homeostasis, and its pathogenic potential in Alzheimer's disease was addressed by Dr. Bush.

Aspartate residue 7 in amyloid beta-protein is critical for classical complement pathway activation: implications for Alzheimer's disease pathogenesis

Fibrillar amyloid beta-protein has been implicated in the pathogenesis of Alzheimer's disease because of its neurotoxicity and its ability to activate complement. Reactive microglia, astrocytes and complement (C') components (reviewed in ref. 6) are associated with senile plaques, the fibrillar, beta-sheet assemblies of amyloid beta-peptide found predominantly in brain from individuals with AD (ref. 7). These indications of inflammatory events are not prevalent in the nonfibrillar "diffuse" plaques often seen in age-matched control cases without dementia. Clinical studies over the past several years have correlated the use of anti-inflammatory drugs with a decrease in the incidence and progression of AD dementia and/or dysfunction, supporting a role for gliosis and inflammation in AD pathogenesis (reviewed in ref. 6). C5a, a product of C' activation, is chemotactic for microglia. Thus, complement activation provides a specific mechanism for recruiting reactive glial cells to the site of the fibrillar amyloid beta-protein plaque, which could lead to inflammatory events, neuronal dysfunction and degeneration. With the use of truncated amyloid beta-peptides, the region of amyloid beta-protein limited by residues 4 and 11 has been identified as critical in the interaction between amyloid beta-protein and C1q, the recognition component of the classical complement pathway (CCP), which results in the activation of C'. Furthermore, substitution of an isoaspartic acid for aspartic acid at amyloid beta-protein residue 7 resulted in the complete elimination of CCP-activating activity. A molecular model of this interaction has been generated that should be useful in the design of candidate therapeutic inhibitors of CCP activation by amyloid beta-protein.

The "nonamyloidogenic" p3 fragment (amyloid beta17-42) is a major constituent of Down's syndrome cerebellar preamyloid

Down's syndrome (DS) patients show accelerated Alzheimer's disease (AD) neuropathology, which consists of preamyloid lesions followed by the development of neuritic plaques and neurofibrillary tangles. The major constituents of preamyloid and neuritic plaques are amyloid beta (Abeta) peptides. Preamyloid lesions are defined as being Abeta immunoreactive lesions, which unlike neuritic plaque amyloid are Congo red-negative and largely nonfibrillar ultrastructurally. DS patients can develop extensive preamyloid deposits in the cerebellum, without neuritic plaques; hence, DS cerebellums are a source of relatively pure preamyloid. We biochemically characterized the composition of DS preamyloid and compared it to amyloid in the neuritic plaques and leptomeninges in the same patients. We found that Abeta17-42 or p3 is a major Abeta peptide of DS cerebellar preamyloid. This 26-residue peptide is also present in low quantities in neuritic plaques. We suggest that preamyloid can now be defined biochemically as lesions in which a major Abeta peptide is p3.

The profile of soluble amyloid beta protein in cultured cell media. Detection and quantification of amyloid beta protein and variants by immunoprecipitation-mass spectrometry

To study the metabolism of amyloid beta protein (Abeta) in Alzheimer's disease, we have developed a new approach for analyzing the profile of soluble Abeta and its variants. In the present method, Abeta and its variants are immuno-isolated with Abeta-specific monoclonal antibodies. The identities of the Abeta variants are determined by measuring their molecular masses using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The levels of Abeta variants are determined by their relative peak intensities in mass spectrometric measurements by comparison with internal standards of known identities and concentrations. We used this method to examine the Abeta species in conditioned media of mouse neuroblastoma cells transfected with cDNAs encoding wild type or mutant human amyloid precursor protein. In addition to human Abeta-(1-40) and Abeta-(1-42), more than 40 different human Abeta variants were identified. Endogenous murine Abeta and its variants were also identified by this approach. The present approach is a new and sensitive method to characterize the profile of soluble Abeta in conditioned media and biological fluids. Furthermore, it allows direct measurement of each individual peptide in a peptide mixture and provides comprehensive information on the identity and concentration of Abeta and Abeta variants.

The length of amyloid-beta in hereditary cerebral hemorrhage with amyloidosis, Dutch type. Implications for the role of amyloid-beta 1-42 in Alzheimer's disease

In hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D), a genetic variant (E22Q) of amyloid beta (Abeta) accumulates predominantly in the small vessels of leptomeninges and cerebral cortex, leading to fatal strokes in the fifth or sixth decade of life. Abeta deposition in the neuropil occurs mainly in the form of preamyloid, Congo red negative deposits, while mature neuritic plaques and neurofibrillary tangles, hallmark lesions in Alzheimer's disease (AD), are characteristically absent. A recent hypothesis regarding the pathogenesis of AD states that Abeta extending to residues 42-43 (as opposed to shorter species) can seed amyloid formation and trigger the development of neuritic plaques followed by neuronal damage in AD. We characterized biochemically and immunohistochemically Abeta from three cases of HCHWA-D to determine its length in vascular and parenchymal deposits. Mass spectrometry of formic acid-soluble amyloid, purified by size-exclusion gel chromatography, showed that Abeta 1-40 and its carboxyl-terminal truncated derivatives were the predominant forms in leptomeningeal and cortical vessels. Abeta 1-42 was a minor component in these amyloid extracts. Immunohistochemistry with antibodies S40 and S42, specific for Abeta ending at Val-40 or Ala-42, respectively, were consistent with the biochemical data from vascular amyloid. In addition, parenchymal preamyloid lesions were specifically stained with S42 and were not labeled by S40, in agreement with the pattern reported for AD, Down's syndrome, and aged dogs. Our results suggest that in HCHWA-D the carboxyl-terminal Abeta heterogeneity is due to limited proteolysis in vivo. Moreover, they suggest that Abeta species ending at Ala-42 may not be critical for the seeding of amyloid formation and the development of AD-like neuritic changes.

The carboxyl termini of beta-amyloid peptides 1-40 and 1-42 are generated by distinct gamma-secretase activities

We have studied the effects of peptide aldehyde protease inhibitors on the secretion of beta-amyloid peptide 1-40 (Abeta(1-40)) and Abeta(1-42) by HEK 293 and COS-1 cells expressing beta-amyloid precursor protein with the Swedish double mutation. A multiphasic SDS-polyacrylamide gel electrophoresis system was used for the discrimination of Abeta(1-40) and Abeta(1-42). Calpain inhibitor I, carbobenzoxyl-Leu-Leu-leucinal, and calpeptin were found to reduce the amount of Abeta(1-40) released into the medium in a dose-dependent manner. The reduction of Abeta(1-40) after treatment with 50 microM calpain inhibitor I or 5 microM carbobenzoxyl-Leu-Leu-leucinal was accompanied by a slight increase of Abeta(1-42) released into the medium. These observations suggest that the cleavages at residues 40 and 42 are accomplished by different enzyme activities.

Analytical chemistry and biochemistry of D-amino acids

The methodologies for the analysis of D-amino acids in biological materials have been reviewed, including the use of enzymes, gas and liquid chromatography with chiral stationary phases and diastereomer derivatization with chiral reagents followed by GC or HPLC separation. The distribution of D-amino acids in the body, their origin, metabolism and possible roles in human diseases are discussed.

Familial Alzheimer's disease-linked presenilin 1 variants elevate Abeta1-42/1-40 ratio in vitro and in vivo

Mutations in the presenilin 1 (PS1) and presenilin 2 genes cosegregate with the majority of early-onset familial Alzheimer's disease (FAD) pedigrees. We now document that the Abeta1-42(43)/Abeta1-40 ratio in the conditioned media of independent N2a cell lines expressing three FAD-linked PS1 variants is uniformly elevated relative to cells expressing similar levels of wild-type PS1. Similarly, the Abeta1-42(43)/Abeta1-40 ratio is elevated in the brains of young transgenic animals coexpressing a chimeric amyloid precursor protein (APP) and an FAD-linked PS1 variant compared with brains of transgenic mice expressing APP alone or transgenic mice coexpressing wild-type human PS1 and APP. These studies provide compelling support for the view that one mechanism by which these mutant PS1 cause AD is by increasing the extracellular concentration of Abeta peptides terminating at 42(43), species that foster Abeta deposition.

Serum levels of the iron binding protein p97 are elevated in Alzheimer's disease

Alzheimer's disease is a progressive and incurable disease whose prevalence increases dramatically with age. A biochemical marker for monitoring the onset and progression of the disease would be a valuable tool for disease management. In addition, such a marker might be used as an end point in clinical intervention protocols. Here we provide evidence that the soluble form of the iron binding protein p97 is found in elevated amounts in the serum of Alzheimer's patients compared with healthy controls. This biochemical marker has the potential for identifying subjects afflicted with the disease and possibly for monitoring the onset and longitudinal progression of the disease.

Local and distant histopathological effects of unilateral amyloid-beta 25-35 injections into the amygdala of young F344 rats

To determine if amyloid-beta (A beta) induces tau-immunoreactivity (IR) and reactive astrocytosis in vivo, we injected A beta 25-35 (5.0 nmol) into the right amygdala of rats. At 8 days postinjection, the peptide induced tau-2 IR in neuronal cell bodies and processes ipsilaterally in the amygdala, cingulate cortex, and hippocampus. At 32 days postinjection, the intensity of tau-2 IR was greater than at 8 days in the amygdala and hippocampus, but not in the cingulate cortex. Induction of Alz-50 IR also was progressive but the morphology and distribution was different from tau-2 IR. Beaded fibers with occasional neuronal perikarya were visualized with Alz-50, and the IR was primarily observed in the ipsilateral amygdala. In addition, amygdaloid injections of A beta 25-35 induced reactive astrocytosis, particularly in the ipsilateral hippocampus at 32 days postoperatively. To our knowledge, this is the first study to show that in vivo injections of A beta 25-35 induce progressive transsynaptic cytoskeletal and astrogliotic reactions, that gradually spread from the area of injection to brain regions that have prominent efferent connections with that area. These findings also suggest a direct association between plaque and tangle formation in Alzheimer's disease.

High mass methyl-accepting protein (HMAP), a highly effective endogenous substrate for protein L-isoaspartyl methyltransferase in mammalian brain

A previously unidentified endogenous substrate for protein L-isoaspartyl methyltransferase in mammalian brain has been characterized and partially purified. This high mass methyl-accepting protein (HMAP) is concentrated in rat brain cytosol and is not detectable in rat liver, heart, lung, kidney, or skeletal muscle. HMAP is acidic and heterogeneous in size, with an average mass, as judged by size-exclusion high performance liquid chromatography, greater than 700 kDa. After partial purification from cow brain by anion-exchange chromatography, ammonium sulfate fractionation, and gel filtration, HMAP could accept 12.1 nmol of methyl groups per mg of protein, suggesting that it contains a level of isoaspartate at least 50 times greater than that of the average protein in brain cytosol. Partially purified HMAP is degraded by trypsin, verifying that it is composed, at least in part, of protein. Additional studies on this unusual macromolecule may shed important new light on mechanisms of isoaspartate formation in cells and the molecular pathology of brain aging.

Amino- and carboxyl-terminal heterogeneity of beta-amyloid peptides deposited in human brain

We aimed to determine quantitatively the fine amino- and carboxyl-terminal structure of A beta peptides deposited in human brain using a set of 12 anti-A beta antibodies that distinguish between terminal modifications including isomerization, stereoisomerization, limited proteolysis, and cyclization. Immunochemical examination of cortical blocks from aged subjects distinguished by their total plaque load and from a young Down's syndrome patient identified the major invariantly deposited species as A beta x-42 (X = 1(D-aspartate) and 3(pyroglutamate) and/or 11(pyroglutamate)). These molecular forms, presumably representing by-products of metabolic intermediates toward degradation, are similar in being resistant to major aminopeptidases. A beta 17-42 ("p3' fragment), a major secreted form of truncated A beta with high insolubility, was found to be a minor one. A possible interpretation for these observations would be that proteolysis of A beta from its amino terminus may limit the rate of A beta catabolism.

Analysis of heterogeneous A4 peptides in human cerebrospinal fluid and blood by a newly developed sensitive Western blot assay

The betaA4 peptide, a major component of senile plaques in Alzheimer's disease (AD) brain, has been found in cerebrospinal fluid (CSF) and blood of both AD patients and normal subjects. Although betaA4 1-40 is the major form produced by cell metabolism and found in CSF, recent observations suggest that the long-tailed betaA4 1-42 plays a more crucial role in AD pathogenesis. Here, we established new monoclonal antibodies against the C-terminal end of betaA4 1-40 and 1-42, and used them for the specific Western blot detection. After optimizing the assay conditions, these antibodies detected low picogram amount of betaA4, and both betaA4 1-40 and 1-42 levels in CSF could be determined by direct loading of the samples. Blood levels of betaA4 1-40 and 1-42 were also determined by specific immunoprecipitation followed by Western blot detection. We found that CSF betaA4 1-42 level is lower in AD patients compared with non-demented controls, although there was a significant overlap between the groups. The level of betaA4 1-40 in CSF, and of betaA4 1-40 as well as betaA4 1-42 in plasma, were not different between AD patients and controls. Besides the 4-kDa full-length betaA4 band, we could also detect several N-terminal variants of betaA4 in CSF and plasma of both AD patients and controls. Two N-terminally truncated betaA4 species migrating at the position of 3.3 and 3.7 kDa were found in CSF, while 3.7- and 5-kDa forms were found in plasma. The relative abundance of these various species were considerably different in the CSF and plasma, suggesting that the cellular source and/or clearance of betaA4 is different in these two compartments.

Glycation and microglial reaction in lesions of Alzheimer's disease

Single, double, and triple immunostaining of cryostat sections of elderly normal and Alzheimer disease (AD) brain was performed with monoclonal and polyclonal antibodies to advanced glycation end products (AGE). The sections were counterstained with thioflavin-S or with immunocytochemistry for A beta and also stained with markers for microglia. AGE-immunoreactivity was detected in senile plaques and neurofibrillary tangles (NFT). AGE immunoreactivity was most intense in dense or reticular amyloid deposits and extracellular NFT, while intracellular NFT and diffuse amyloid had less AGE immunoreactivity. This pattern of immunoreactivity was similar to that noted in previous studies with antibodies to apolipoprotein-E (apo-E). Therefore, double labeling with antibodies to apo-E and AGE was performed. AGE immunoreactivity colocalized to a very high degree with apo-E immunoreactivity, except that relatively more intense apo-E immunoreactivity was detected in amyloid deposits and more intense AGE immunoreactivity in NFT. The lesions that were immunostained with antibodies to AGE and apo-E were often, but not always, associated with a local microglial reaction. The results raise the possibility that apo-E or a fragment of apo-E may be glycated. Biochemical studies are needed to determine the extent of possible apo-E glycation in AD. The present results raise the possibility that glycation may serve as one of the signals for activation of microglia associated with amyloid deposits and extracellular NFT.

Morphology and toxicity of Abeta-(1-42) dimer derived from neuritic and vascular amyloid deposits of Alzheimer's disease

In the course of analyzing the chemical composition of Alzheimer's disease neuritic and vascular amyloid, we have purified stable dimeric and trimeric components of Abeta peptides. These peptides (molecular mass 9.0 and 13.5 kDa) were separated by size exclusion chromatography in the presence of 80% formic acid or 5 guanidine thiocyanate, pH 7.4. The average ratio of monomers, dimers, and trimers was 55:30:15, respectively. Similar structures were produced over time upon incubation of synthetic Abeta-(1-42) at pH 7.4. The stability of these oligomeric forms was also demonstrated by Western blot and mass spectrometry. Atomic force microscopy and electron microscopy rotary shadowing revealed that the monomers polymerized into 8-10-nm filaments, whereas the dimers generated prolate ellipsoids measuring 3-4 nm in diameter. The pathogenic effects of the dimeric Abeta-(1-40/42) were tested in cultures of rat hippocampal neuron glia cells. Only in the presence of microglia did the dimer elicit neuronal killing. It is possible that these potentially pathogenic Abeta-(1-40/42) dimers and trimers from Alzheimer's disease amyloid represent the soluble oligomers of Abeta recently described in Alzheimer's disease brains (Kuo, Y.-M., Emmerling, M. R., Vigo-Pelfrey, C., Kasunic, T. C., Kirkpatrick, J. B., Murdoch, G. H., Ball, M. J., and Roher, A. E. (1996) J. Biol. Chem., 271, 4077-4081).

Comparable amyloid beta-protein (A beta) 42(43) and A beta 40 deposition in the aged monkey brain

Two distinct species of amyloid beta-protein (A beta), A beta 42(43) and A beta 40, are deposited in the brains of patients with Alzheimer's disease and normal aged individuals. A beta 42(43), the long tailed A beta, is the initially and predominantly deposited species in senile plaques. Deposition of A beta is also observed in the aged monkey brains. We investigated the A beta species in the aged monkey brains immunocytochemically using monoclonal antibodies that discriminate between the C-termini of A beta 42(43) and A beta 40. We report here that A beta 40 as well as A beta 42(43) deposit in various types of senile plaques, including diffuse plaques of the aged monkey brain and that the ratio of A beta 40 to A beta 42(43) is higher compared with that in human brain.

RAGE and amyloid-beta peptide neurotoxicity in Alzheimer's disease

Amyloid-beta peptide is central to the pathology of Alzheimer's disease, because it is neurotoxic--directly by inducing oxidant stress, and indirectly by activating microglia. A specific cell-surface acceptor site that could focus its effects on target cells has been postulated but not identified. Here we present evidence that the 'receptor for advanced glycation end products' (RAGE) is such a receptor, and that it mediates effects of the peptide on neurons and microglia. Increased expressing of RAGE in Alzheimer's disease brain indicates that it is relevant to the pathogenesis of neuronal dysfunction and death.

Kinetic study of racemization of aspartyl residues in model peptides of alpha A-crystallin

We have reported that two aspartyl (Asp-151 and Asp-58) residues in alpha A-crystallin in human eye lens were inverted to the D-isomer and isomerized to beta-aspartyl residues with age. We report here the kinetics of the Asp racemization of three model peptides corresponding to fragments of alpha A-crystallin: IQTGLD151ATHAER (T18 peptide), TVLD58SGISEVR (T6 peptide) and HFSPED84LTVK (T10 peptide, as a control). The rate constants of the racemization of Asp residues in these peptides were measured at pH 7.0, at five temperatures: 50, 60, 70, 80 and 90 degrees C. From the Arrhenius equation, we estimated the activation energy (E) of racemization and the time required for the Asp D/L ratio to approximate to 1.0 (D/L ratio of Asp = 0.99) at body temperature. For the peptide T18, E = 21.4 kcal/mol and t = 13.5 yr. For the peptide T6, E = 26.8 kcal/mol and r = 49.5 yr. For the control peptide T10, E = 28.3 kcal/mol and t = 78.1 yr. The racemization rate of Asp in these three peptides is parallel to that of Asp residues in alpha A-crystallin. The racemization rate of Asp in the T18 peptide was very rapid compared to that in the other peptides. This result also reflects the racemization rate in native alpha A-crystallin.

Secreted amyloid beta-protein similar to that in the senile plaques of Alzheimer's disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease

To determine whether the presenilin 1 (PS1), presenilin 2 (PS2) and amyloid beta-protein precursor (APP) mutations linked to familial Alzheimer's disease (FAD) increase the extracellular concentration of amyloid beta-protein (A beta) ending at A beta 42(43) in vivo, we performed a blinded comparison of plasma A beta levels in carriers of these mutations and controls. A beta 1-42(43) was elevated in plasma from subjects with FAD-linked PS1 (P < 0.0001), PS2N1411 (P = 0.009), APPK670N,M671L (P < 0.0001), and APPV7171 (one subject) mutations. A beta ending at A beta 42(43) was also significantly elevated in fibroblast media from subjects with PS1 (P < 0.0001) or PS2 (P = 0.03) mutations. These findings indicate that the FAD-linked mutations may all cause Alzhelmer's disease by increasing the extracellular concentration of A beta 42(43), thereby fostering cerebral deposition of this highly amyloidogenic peptide.

beta-Amyloid protein-dependent nitric oxide production from microglial cells and neurotoxicity

beta-Amyloid protein (A beta) is the major component of the senile plaques in Alzheimer's disease (AD), and microglial cells have been shown to be closely associated with these plaques. However, the roles of A beta and microglial cells in pathogenesis of AD remain unclear. Incubation of rat microglial cells with A beta(1-40) caused a significant increase in nitrite, a stable metabolite of nitric oxide (NO), in culture media, while there was no detectable increase in nitrite in astrocyte-rich glial cells or cortical neurons after incubation with A beta(1-40). Nitrite production by microglial cells was also induced by A beta(1-42), but not A beta(25-35). An inhibitor of NO synthase, NG-monomethyl-L-arginine (NMMA), as well as dexamethasone and actinomycin D, dose-dependently inhibited this nitrite production. Among the various cytokines investigated such as interleukin-1, interleukin-6, tumor necrosis factor-alpha and interferon-gamma (IFN-gamma), only IFN-gamma markedly enhanced A beta-dependent nitrite production. Cultured cortical neurons were injured by microglial cells stimulated with A beta in a dose-dependent manner in the presence of IFN-gamma. Neurotoxicity caused by the A beta plus IFN-gamma-stimulated microglial cells was significantly attenuated by NMMA. Thus, although further investigations into the effect of A beta on human microglial cells are needed, it is likely that A beta-induced NO production by microglial cells is one mechanism of the neuronal death in AD.

Oxidative metabolism deficiencies in brains of patients with Alzheimer's disease

Glucose metabolism in the brain has an important influence on many normal cellular processes. It contributes to the synthesis of acetylcholine, glutamate, aspartate, gamma-aminobutyric acid, glycine, and ATP production (the driving force behind almost all cellular and molecular activity). Neuronal glucose metabolism is controlled antagonistically by insulin and cortisol. Desensitization of the neuronal insulin receptor causes abnormalities in oxidative energy metabolism. During normal aging, the cerebral energy pool is slightly diminished, but its level increases after stressful events. In age-related sporadic late-onset dementia of the Alzheimer type (SDAT), glucose metabolism and formation of cellular energy are severely reduced. Desensitization of the neuronal insulin receptor seems to be an early event in the pathogenesis or even etiology of SDAT causing disturbances in oxidative glucose metabolism and energy failure in insulin-sensitive brain structures. These abnormalities appear to induce a cascade of disturbances that leads to abnormal APP processing and amyloid formation, membrane damage, and neuronal death.

The effects of aspartic acid-bond isomerization on in vitro properties of the amyloid beta-peptide as modeled with N-terminal decapeptide fragments

The 42-amino acid A beta, the major constituent of the senile plaque deposits of the brains of Alzheimer's disease patients, exhibits a high degree of heterogeneity at its N-terminus. Isomerization of aspartic acid bonds at residues 1 and 7 renders A beta more prone to aggregate and form extended structure as it was shown by in vivo and in vitro studies. We recently demonstrated the ability of mid-chain aspartic acid-bond isomerization to break the dominant helical structure of the N-terminal decapeptide fragment by CD. In the current study we use molecular modeling to show that insertion of the extra -CH2-group into the decapeptide backbone results in the formation of stable reverse-turns and destabilizes the helical conformer that competes with the extended structure at the full-sized peptide level. The molecular modeling also reveals a limited propensity of the diisomerized peptide to form extended structure directly. Anti-A beta pAb 2332 is more sensitive for the non-isomerized status of the decapeptide than that of the full-sized peptide. mAb 6E10, raised against unmodified A beta recognizes only the unmodified decapeptide or the peptide isomerized at the first aspartic acid in a conformation-dependent manner, but does not recognize the mid-chain isomerized or diisomerized decapeptide in any circumstance. The diisomerized decapeptide was used as immunogen to generate polyclonal antibody 14943 that is not selective for the isomerized status of either the full-size peptide or the decapeptide, but recognizes the isomerized peptides preferentially when the peptide antigen structures are conserved during the enzyme-linked immunoassay procedure. Owing to the aberrant behavior of the full-sized A beta peptide during standard RP-HPLC, serum stability studies that indicate extracellular stability can be more effectively performed on the decapeptide fragments. Remarkably, the diisomerized peptide exhibits a significantly increased stability towards serum peptidases compared with the unmodified or monoisomerized peptides, suggesting a possible mechanism of the retention of the isomerized A beta peptide in the affected brains.

Brain amyloid--a physicochemical perspective

The ability to form stable cross-beta fibrils is an intrinsic physicochemical characteristic of the human beta-amyloid peptide (A beta), which forms the brain amyloid of Alzheimer's disease (AD). The high amyloidogenicity and low solubility of this hydrophobic approximately 40-mer have been barriers to its study in the past, but the availability of synthetic peptide and new physical methods has enabled many novel approaches in recent years. Model systems for A beta aggregation (relevant to initial nidus formation) and A beta deposition (relevant to plaque growth and maturation) in vitro have allowed structure/activity relationships and kinetics to be explored quantitatively, and established that these processes are biochemically distinct. Different forms of the peptide, with different physiochemical characteristics, are found in vascular and parenchymal amyloid. Various spectroscopic methods have been used to explore the three-dimensional conformation of A beta both in solution and in solid phase, and demonstrated that the peptide adopts a different configuration in each state. A significant conformational transition is essential to the transformation of A beta from solution to fibril. These observations suggest new therapeutic targets for the treatment of AD.

Accumulation of beta-amyloid fibrils in pancreas of transgenic mice

Some forms of familial Alzheimer's disease are caused by mutations in the amyloid beta protein precursor (beta APP), and there is excellent evidence that these mutations foster amyloid deposition by increasing secretion of total amyloid beta protein (A beta) or the highly amyloidogenic A beta 1-42 form. These observations provide a powerful rationale for developing an animal model of AD by generating transgenic mice in which cerebral amyloid deposition is induced by A beta overproduction. To produce substantial A beta in vivo, we generated mice expressing the transgene of signal peptide and 99 residues of carboxyl-terminal fragment (CTF) of beta APP under control of the cytomegalovirus enhancer/chicken beta-actin promoter. The transgenic mRNA was detected in many tissues of these mice, but the levels of transgenic mRNA, CTF, and A beta did not correlate well indicating that tissue-specific posttranslational processing may play an important role in determining the amount of A beta that accumulates in various tissues. A beta was detected biochemically in brain, kidney, and pancreas with the largest amount present in pancreas. In transgenic plasma, there was a marked accumulation of human A beta 1-40 and A beta 1-42(43) to levels over 30-times those observed in normal human plasma. Thus, the transgenic mice produce and secrete considerable A beta. Despite this increase in A beta secretion and the elevated A beta in brain, immunohistochemistry revealed no consistent cerebral A beta deposition. In pancreas, however, intracellular A beta deposits were detected immunohistochemically in acinar cells and interstitial macrophages, some of which showed severe degeneration. In addition, examination of these cells by immunoelectron microscopy revealed many putative amyloid fibrils (7-12 nm) that were stained by anti-A beta antibodies. Overall, our findings indicate that tissue-specific posttranslational processing may play a pivotal role in A beta production and amyloid fibril formation in vivo. By carefully analyzing the changes that occur in the transgenic mice described here as compared to the transgenic line that has recently been shown to form extracellular amyloid plaques in brain, it may be possible to gain considerable insight into the factors that determine the location and amount of A beta that accumulates as amyloid.

Carboxy terminal of beta-amyloid deposits in aged human, canine, and polar bear brains

Immunocytochemistry, using antibodies specific for different carboxy termini of beta-amyloid. A beta 40 and A beta 42(43), was used to compare beta-amyloid deposits in aged animal models to nondemented and demented Alzheimer's disease human cases. Aged beagle dogs exhibit diffuse plaques in the absence of neurofibrillary pathology and the aged polar bear brains contain diffuse plaques and PHF-1-positive neurofibrillary tangles. The brains of nondemented human subjects displayed abundant diffuse plaques, whereas the AD cases had both diffuse and mature (cored) neuritic plaques. Diffuse plaques were positively immunostained with an antibody against A beta 42(43) in all examined species, whereas A beta 40 immunopositive mature plaques were observed only in the human brain. Anti-A beta 40 strongly immunolabeled cerebrovascular beta-amyloid deposits in each of the species examined, although some deposits in the polar bear brain were preferentially labeled with anti-A beta 42(43). beta-amyloid deposition was evident in the outer molecular layer of the dentate gyrus in the aged dog, polar bear, and human. Within this layer, A beta 42 was present as diffuse deposits, although these deposits were morphologically distinct in each of the examined animal models. In dogs, A beta 42 was cloud-like in nature; the polar bear demonstrated a more aggregated type of deposition, and the nondemented human displayed well-defined deposits. Alzheimer's disease cases were most frequently marked by neuritic plaques in this region. Taken together, the data indicate that beta-amyloid deposition in aged mammals is similar to the earliest stages observed in human brain. In each species, A beta 42(43) is the initially deposited isoform in diffuse plaques.

Water-soluble Abeta (N-40, N-42) oligomers in normal and Alzheimer disease brains

Ultracentrifugation and graded molecular sieving, as well as a sensitive sandwich enzyme-linked immunosorbent assay were used to isolate and quantitate the amounts of water-soluble oligomers of beta amyloid (Abeta) peptides N-40 and N-42 in cerebral cortex of normal and Alzheimer disease (AD) brains. AD brains contained 6-fold more water-soluble Abeta (wsAbeta) than control brains. The majority of water-soluble peptides in most AD cases was A beta N-42, representing 12 times the amount found in control brains. The wsAbeta was present in the form of monomers and oligomers ranging from less than 10 kDa to greater than 100 kDa. The amount of wsAbeta N-42 in AD brains is about 50 times greater than the level of soluble Abeta N-42 found in the CSF of AD patients. This disparity may be due to the rapid association of wsAbeta N-42 into fibrillar deposits and/or to the integrity of the anatomical barriers which separate the two extracellular spaces. In this paper, we consider soluble any form of Abeta which has not yet polymerized into its insoluble, filamentous form. This includes both the newly synthesized forms of Abeta and those peptides which may be loosely attached to insoluble filaments but which can, nevertheless, still be considered soluble. It has been previously shown that, once it has aggregated into its filamentous form, the Abeta peptides are resistant to disaggregation and degradation by a number of denaturing agents and aqueous buffers containing proteolytic enzymes. Therefore, it is likely that the water-soluble Abeta peptides we quantified are precursors to its insoluble, filamentous form. Consequently, reducing the levels of soluble Abeta in AD brains could have profound effects on AD pathophysiology.

Effect on protein phosphatase activity of peptide backbone modification and truncation of the autoinhibitory domain peptide of calcineurin

Solid-phase synthesis of the autoinhibitory domain of calcineurin, CaN A467-491, also produced [aspartimide477]CaN A467-491 and [iso-Asp477]CaN467-491 when Boc-based chemistry was employed. In addition, the truncated peptide CaN A467-488 was obtained when Fmoc-based chemistry was employed. All four peptides proved to be effective inhibitors of protein phosphatase activity of calcineurin. The full-length peptide and the C-terminally truncated peptide (CaN467-488) were indistinguishable, with Ki values of 28 +/- 3 and 31 +/- 5 mu M, respectively. The internally modified peptides, [iso-Asp477]CaN A467-491 and [aspartimide477]-CaN A467-491, possessed lower inhibitory potencies (Ki values of 87 +/- 10 and 55 +/- 3 mu M, respectively).

Amyloid beta protein in rat soleus muscle in chloroquine-induced myopathy using end-specific antibodies for A beta 40 and A beta 42: immunohistochemical evidence for amyloid beta protein

Previous immunohistochemical studies from this laboratory demonstrated that monoclonal antibodies raised against various regions of amyloid precursor protein (APP) (i.e., N-terminus, amyloid beta protein (A beta), and C-terminus) strongly labeled vacuoles in chloroquine-induced myopathy-affected muscle in rats. In this study, we used antibodies end specific for the A beta 40 and A beta 42 species, and a monoclonal antibody to A beta 1-9 which reacts with APP and A beta. Most vacuoles clearly reacted with anti-A beta 1-9, while about half reacted with anti-A beta 42, and only a few reacted with anti-A beta 40. These results demonstrate that vacuoles in chloroquine-induced myopathy-affected muscle contain cleaved A beta, and that distribution of the two major A beta species is similar to what is observed in A beta deposition in Alzheimer's disease (AD)-affected brain. This provides further evidence that chloroquine-induced myopathy in rats provides a suitable model to understand APP processing into A beta, and the role of APP in terms of the pathogenesis of AD.

beta-Amyloid 25-35 and/or quinolinic acid injections into the basal forebrain of young male Fischer-344 rats: behavioral, neurochemical and histological effects

beta-Amyloid peptides have been shown to potentiate the neurotoxic effect of excitatory amino acids in vitro. In order to determine if this occurs in vivo, four experiments were performed. We injected beta-amyloid 25-35 (beta A 25-35) and/or quinolinic acid (QA) bilaterally into the ventral pallidum/substantia innominata (VP/SI) of rats. Control rats received vehicle infusions. A high dose of QA (75.0 nmol/3 microliters) increased open field activity and impaired spatial learning in the Morris water maze, but did not affect the acquisition of a one-way conditioned avoidance response. These changes were associated with histological evidence of neurotoxicity and a reduction in amygdaloid but not frontal cortical or hippocampal choline acetyltransferase (ChAT) activity. A lower dose of QA (37.5 nmol/3 microliters) produced no behavioral effects. It reduced amygdaloid ChAT activity to a lesser extent than the higher dose (15% vs. 29-37%), and caused less histological damage. beta A 25-35 (1.0 or 8.0 nmol/3 microliters) failed to produce behavioral, histological or neurochemical signs of toxicity. Neither dose of beta A 25-35 potentiated the effects of QA (37.5 nmol) on behavior or amygdaloid ChAT activity, and did not appear to increase the histological damage caused by QA. These results suggest that in vivo beta A 25-35 is not neurotoxic and does not potentiate the neurotoxicity of QA in the VP/SI. Further, the histological effects of a high dose of beta A 25-35 (8.0 nmol/3 microliters; a cavitation containing a Congo red positive proteinaceous material) are quite distinct from those produced by a high dose of QA (75.0 nmol/3 microliters; widespread neuronal loss and gliosis).

Amyloid beta-protein ending at Thr43 is a minor component of some diffuse plaques in the Alzheimer's disease brain, but is not found in cerebrovascular amyloid

The distribution of amyloid beta-protein ending at 40 (A beta 40), 42 (A beta 42), and 43 (A beta 43) were immunocytochemically examined in Alzheimer's brains. A large number of diffuse plaques were stained with A beta 42 antibody (S42), but some were weakly labeled with A beta 43 antibody (S43). Immature and mature plaques were labeled with A beta 40 antibody (S40), S42, and S43. Thus, A beta 42 is the major component of diffuse plaques, the initial phase of plaque formation, and A beta 43 is present as a minor component. Only A beta 40 is present in cerebrovascular amyloid.

Carboxyl end-specific monoclonal antibodies to amyloid beta protein (A beta) subtypes (A beta 40 and A beta 42(43)) differentiate A beta in senile plaques and amyloid angiopathy in brains of aged cynomolgus monkeys

Senile plaques (SPs) and cerebral amyloid angiopathy (CAA) in the brains of five aged (20-26 years old) cynomolgus monkeys were investigated immunohistochemically using two monoclonal antibodies (anti-A beta 40 (BA27) and anti-A beta 42(43) (BC05)) that can differentiate the carboxyl termini of amyloid beta protein (A beta) subtypes. In four of five animals, all types of SPs (i.e. diffuse, primitive, and classical plaques; DPs, PPs, and CPs, respectively) were identified by BC05. However, BA27 did not label DPs and stained only about one third of PPs and CPs, mainly labeling granular structures and cored portions, respectively. In CAA, lesions of cortical capillaries reacted to BC05 in four of five cases, but rarely and weakly to BA27 in two of five cases. On the other hand, lesions of parenchymal and meningeal arterioles were stained by both BA27 and BC05. These staining profiles of SPs in cynomolgus monkeys correspond well to those in humans, although there are two remarkable features in cynomolgus monkeys. First, BA27 stained PPs associated with granular structures. Secondly, capillary A beta reacted intensely to BC05 but only slightly to BA27. Despite these unique features, the results suggest that aged cynomolgus monkeys can be used to investigate the pathogenesis of A beta deposition in SPs and CAA.

Beta-amyloid precursor protein (APP) and APP-RNA are rapidly affected by glutamate in cultured neurons: selective increase of mRNAs encoding a Kunitz protease inhibitor domain

Alternative splicing of beta-amyloid precursor protein (APP) RNA generates APP isoforms with or without a Kunitz protease inhibitor (KPI) domain. Previously, we showed that KPI (+) APP RNA, but not KPI (-) APP RNA, is upregulated in response to experimental lesions in which neurotoxicity is dependent on NMDA receptor activation and in Alzheimer's disease hippocampus. Recent studies by Mucke et al. (1995) showed that neuronal expression of human KPI (+) APP, but not KPI (-) APP, in transgenic mice is neuroprotective against experimental lesions. In this study we examined the direct effects of the excitotoxic amino acid Glu on alternatively, spliced APP RNAs and the corresponding protein isoforms in cultured rat cortical neurons. Glu treatment rapidly induced (4.5 h) KPI (+) APP RNA but not KPI (-) APP RNA. Induction of KPI (+) RNA preceded Glu-induced neuronal cell death and was partially blocked by an NMDA-receptor antagonist. In contrast to the RNA, cellular levels of KPI (+) APP were not changed by 4.5 h of Glu treatment. Instead, the cellular full-length form of the protein KPI (-) APP was reduced by approximately 50% after 2 h of Glu treatment and remained depleted after 24 h of treatment. Cellular levels of KPI (+) forms of amyloid precursor-like protein 2 (APLP2) were not changed by Glu treatment. Our data are consistent with the hypothesis that sustained NMDA-receptor activation can regulate alternative splicing of the APP pre-mRNA in neurons.

Interaction of synthetic Alzheimer beta-protein-derived analogs with aqueous aluminum: a low-field 27Al NMR investigation

Synthetic peptides corresponding to the soluble Alzheimer beta-protein, i.e., beta 1-40 and beta 6-25, were utilized to investigate the association of aluminum using low-field 27Al nuclear magnetic resonance (NMR) spectroscopy and reversed-phase high-performance liquid chromatography (RP-HPLC). Addition of beta 1-40 or beta 6-25 to aqueous Al3+ gives rise to a 27Al NMR signal corresponding to the association of Al3+ with the peptides; this effect is not easily reversed by EDTA. Based on the relative intensity of the Al(3+)-peptide signal between pH 4 and 6, there are at least 4 Al3+ ions associated with each peptide molecule. Microheterogeneity is observed with RP-HPLC on incubating solutions of Al3+ with beta 1-40 and beta 6-25. The 27Al NMR spectra of chromatographically pure fractions of beta 1-40 and beta 6-25 indicate that the peptide-associated Al3+ is released below pH 3.5. We propose that soluble beta 1-40 provides an anchor for Al3+ to bind, eventually leading to an increased deposition of amyloid in the Alzheimer brain.

Amino-terminal deletions enhance aggregation of beta-amyloid peptides in vitro

beta-Amyloid protein, which assembles into pathological aggregates deposited in Alzheimer's disease brain tissue, exhibits N-terminal heterogeneity both in vitro and in vivo. To investigate the effects of this N-terminal heterogeneity on the assembly characteristics and biophysical properties of beta-amyloid, we synthesized a series of peptides with progressively shortened N termini (initial residues at positions beta 1, beta 4, beta 8, beta 12, and beta 17) and C termini extending to residue beta 40 or beta 42. We report that peptides with N-terminal deletions exhibit enhanced peptide aggregation relative to full-length species, as quantitatively assessed by sedimentation analyses. Overall, sedimentation levels were greater for peptides terminating at residue beta 42 than for those terminating at residue beta 40. To determine if established biophysical features of the full-length protein were maintained in the truncated peptides, structural and bioactive properties of these peptides were examined and compared. Full-length and truncated peptides exhibiting aggregation showed circular dichroism spectra consistent with predominant beta-sheet conformation, fibrillar morphology under transmission electron microscopy, and significant toxicity in cultures of rat hippocampal neurons. These data demonstrate that N-terminal deletions enhance aggregation of beta-amyloid into neurotoxic, beta-sheet fibrils and suggest that such peptides may initiate and/or nucleate the pathological deposition of beta-amyloid.

Reduction of beta-amyloid peptide42 in the cerebrospinal fluid of patients with Alzheimer's disease

In this clinical study the cerebrospinal fluid (CSF) level of a novel form of the beta-amyloid peptide (A beta) extending to position 42 (A beta 42) was determined in patients with Alzheimer's disease (AD) as well as controls. In addition to measurement of CSF A beta 42 levels, total A beta peptides, microtubule-associated protein tau, and apolipoprotein E (ApoE) genotype were also assessed. It is interesting that CSF A beta 42 levels were found to be significantly lower in AD patients relative to controls, whereas total A beta levels were not. A beta 42 has recently been shown to preferentially deposit in the brain tissue of patients with AD, suggesting that diminished clearance may account for its reduction in CSF. As previously reported, tau levels were increased in AD patients; however, neither A beta 42 nor tau levels were apparently influenced by the ApoE genotype.

GM1 ganglioside-bound amyloid beta-protein (A beta): a possible form of preamyloid in Alzheimer's disease

The earliest event so far known that occurs in the brain affected with Alzheimer's disease (AD) is the deposition and fibril formation of amyloid beta-protein (A beta). A beta is cleaved from a glycosylated membrane protein, called beta-amyloid protein precursor, and normally secreted into the extracellular space. Here we report on the presence of membrane-bound A beta that tightly binds GM1 ganglioside. This suggests that this novel A beta species, rather than secreted A beta, may act as a 'seed' for amyloid and further that intracellular abnormalities in the membrane recycling already exist at the stage of amyloidogenesis.

Soluble multimeric Alzheimer beta(1-40) pre-amyloid complexes in dilute solution

Aqueous solutions of beta(1-40) peptide spontaneously associate to form pentameric/hexameric complexes that can be demonstrated by SDS-PAGE following treatment with glutaraldehyde and borohydride reduction. Under amyloidogenic conditions of pH and high peptide concentration these aggregates can further associate to form pentameric/hexameric complexes that can be demonstrated by SDS-PAGE following treatment with glutaraldehyde and borohydride reduction. Under amyloidogenic conditions of pH and high peptide concentration these aggregates can further associate to form sedimentable and filterable structures with beta-sheet amyloid characteristics of Thioflavine T fluorescence. The presence of such preamyloid structures at low peptide concentration suggests a mechanism by which amyloid plaques can accrete additional material by a cooperative rather than monomeric growth. The existence of a monomer<==>multimer equilibrium may partly explain the divergence of biological consequences with respect to neurotoxicity.

Amyloid beta-proteins 1-40 and 1-42(43) in the soluble fraction of extra- and intracranial blood vessels

To investigate the process of amyloid beta-protein (A beta) accumulation in cerebral amyloid angiopathy (CAA), the levels of A beta were determined in the soluble fraction of extra- and intracranial blood vessels and leptomeninges obtained at autopsy. Two enzyme immunoassays were employed that are known to sensitively and specifically quantify two A beta species, A beta 1-40 and 1-42(43). A beta was detectable in the intracranial blood vessels and leptomeninges with the latter containing the highest levels, while it was undetectable in the extracranial blood vessels. Thus the levels of soluble A beta correlated well with the predilection sites for CAA. Among individuals aged 20 to 90, the A beta levels in the leptomeninges increased sharply in those aged 50 to 70 and thereafter tended to decline. However, only slight degrees of CAA were detected by immunocytochemistry, even when those leptomeninges contained high levels of A beta comparable with those in Alzheimer's disease. The level of A beta 1-42 was almost always severalfold that of A beta 1-40 in the soluble fraction of leptomeninges. This is in good agreement with the immunocytochemical result showing the presence of A beta 40-negative, A beta 42(43)-positive meningeal vessels. These results indicate that A beta 1-42 is the initially deposited species in CAA and that the disruption of A beta homeostasis precedes A beta deposition in the meningeal vessels.

Proteoglycan-mediated inhibition of A beta proteolysis. A potential cause of senile plaque accumulation

Senile plaques of Alzheimer's disease brain contain, in addition to beta amyloid peptide (A beta), multiple proteoglycans. Systemic amyloidotic deposits also routinely contain proteoglycan, suggesting that these glycoconjugates are generally involved in amyloid plaque formation and/or persistence. We demonstrate that heparan sulfate proteoglycan (HSPG) and chondroitin sulfate proteoglycan (CSPG) inhibit the proteolytic degradation of fibrillar, but not non-fibrillar, A beta at physiological pH. In accordance with the proteolysis studies, high affinity binding of proteoglycans to fibrillar A beta(1-40) and A beta(1-42) is observed from pH 4 to 9, whereas appreciable binding of HSPG or CSPG to non-fibrillar peptide is only seen at pH < 6. This differing pH dependence of binding suggests that a lysine residue is involved in proteoglycan association with fibrillar A beta, whereas a protonated histidine appears to be needed for binding of the glycoconjugates to non-fibrillar peptide. Scatchard analysis of fibrillar A beta association with proteoglycans indicates a single affinity interaction, and the binding of both HSPG and CSPG to fibrillar A beta is completely inhibited by free glycosaminoglycan chains. This implies that these sulfated carbohydrate moieties are primarily responsible for proteoglycan.A beta interaction. The ability of proteoglycans to bind fibrillar A beta and inhibit its proteolytic degradation suggests a possible mechanism of senile plaque accumulation and persistence in Alzheimer's disease.

Cerebral beta amyloid deposition in patients with malignant neoplasms: its prevalence with aging and effects of radiation therapy on vascular amyloid

We examined immunohistochemically 123 autopsy brains from patients aged between 30 to 59, who died as a result of malignant neoplasms. Using antiserum to amyloid beta protein (A beta), we found that cerebral A beta deposits began in the subjects' fifth decade; its prevalence was 0%, 9.8% and 21.5% in the fourth, fifth and sixth decades, respectively. The major form of A beta deposition was diffuse-type plaques, although one third of the brains with A beta deposition showed amyloid angiopathy. Subpial A beta deposition is frequently associated with amyloid angiopathy. The prevalence of cerebral A beta deposits was about two times higher in the patients who had received brain radiation therapy (27.8%) compared to non-radiated patients (14.8%). Amyloid angiopathy was much more prominent (P < 0.05) with radiation therapy (22.2%) than without (8.0%). We found that cerebral A beta-deposition is dependent on aging, even in patients with malignant tumors and at beginning in their forties, and that brain radiation therapy is a possible risk factor of A beta deposition, especially in the form of amyloid angiopathy.

Characterization of stable complexes involving apolipoprotein E and the amyloid beta peptide in Alzheimer's disease brain

Genetic evidence suggests a role for apolipoprotein E (apoE) in Alzheimer's disease (AD) amyloidogenesis. Here, amyloid-associated apoE from 32 AD patients was purified and characterized. We found that brain amyloid-associated apoE apparently exists not as free molecules but as complexes with polymers of the amyloid beta peptide (A beta). Brain A beta-apoE complexes were detected irrespective of the apoE genotype, and similar complexes could be mimicked in vitro. The fine structure of purified A beta-apoE complexes was fibrillar, and immunogold labeling revealed apoE immunoreactivity along the fibrils. Thus, we conclude that A beta-apoE complexes are principal components of AD-associated brain amyloid and that the data presented here support a role for apoE in the pathogenesis of AD.

Effect of adjacent histidine and cysteine residues on the spontaneous degradation of asparaginyl- and aspartyl-containing peptides

Aspartate and asparagine residues in polypeptides are subject to nonenzymatic reactions that lead to deamidation, isomerization, peptide bond cleavage and racemization. Much of this reactivity is due to the propensity for the initial formation of a cyclic succinimide intermediate. We have been interested in determining the effect of the side chains of neighboring histidine and cysteine residues in facilitating these reactions, particularly in the possibility that they can act as general acids and bases. In this study, we found little or no effect of histidine residues preceding an asparagine residue in hexapeptides derived from the sequence of adrenocorticotropic hormone, while a histidine residue preceding an aspartic acid residue was found to increase the rate of succinimide formation 8- to 11-fold. The presence of a histidine residue following either an asparagine or aspartic acid residue did not effect the rate of succinimide formation by peptide-bond nitrogen attack, but did increase the rate of the competing side-chain nitrogen attack leading to cleavage in the asparaginyl-containing peptide. We found that the effect of a cysteine residue following an asparagine or aspartic acid residue was in general similar to that of a serine residue, although the cleavage reaction appeared to be enhanced. These results suggest that His-Asp sequences may be particularly labile to spontaneous degradation in proteins and peptides, possibly owing to the ability of the histidine residue to facilitate succinimide formation by protonating the OH- leaving group on the side chain carboxylic acid of the aspartic acid residue.(ABSTRACT TRUNCATED AT 250 WORDS)