Design of (omega-N-(O-acyl)hydroxy amid) aminodicarboxylic acid pyrrolidides as potent inhibitors of proline-specific peptidases

Dipeptidyl peptidase-IV inhibitory action of Calebin A: An in silico and in vitro analysis

BACKGROUND

Dipeptidyl peptidase-IV (DPP-IV) inhibitors, the enhancers of incretin are used for the treatment of diabetes. The non-glycaemic actions of these drugs (under developmental stage) also proved that repurposing of these molecules may be advantageous for other few complicated disorders like cardiovascular diseases, Parkinson's disease, Alzheimer's disease, etc. OBJECTIVE: The present study was aimed to investigate the DPP-IV inhibitory potential of Calebin-A, one of the constituents of Curcuma longa.

MATERIAL AND METHODS

The phytoconstituent was subjected for various in silico studies (using Schrödinger Suite) like, Docking analysis, molecular mechanics combined with generalized Born model and solvent accessibility method (MMGBSA) and Induced fit docking (IFD) after validating the protein using Ramachandran plot. Further, the protein-ligand complex was subjected to molecular dynamic simulation studies for 50 nanoseconds. And finally, the results were confirmed through enzyme inhibition study.

RESULTS

Insilico results revealed possible inhibitory binding interactions in the catalytic pocket (importantly Glu205, Glu206 and Tyr 662 etc.) and binding affinity in terms of glide g-score and MMGBSA dG bind values were found to be -6.2 kcal/mol and -98.721 kcal/mol. Further, the inhibitory action towards the enzyme was confirmed by an enzyme inhibition assay, in which it showed dose-dependent inhibition, with maximum % inhibition of 55.9 at 26.3 μM. From molecular dynamic studies (50 nanoseconds), it was understood that Calebin A was found to be stable for about 30 nanoseconds in maintaining inhibitory interactions.

CONCLUSION

From the in silico and in vitro analysis, the current research emphasizes the consideration of Calebin A to be as a promising or lead compound for the treatment of several ailments where DPP-IV action is culprit.

Neuropeptides in hyperthermia

Brain damage as a result of hyperthermia or heat-stress has been the focus of attention in many areas of neuroscience in recent years. Heat-induced alterations in structural components of the central nervous system (CNS) will obviously also influence the relevant transmitter systems, which may be involved in a variety of different behaviors. Indeed, many studies have indicated that excitatory amino acids, and monoaminergic and peptidergic systems are affected during hyperthermia. This chapter will address past and current research on various neuropeptides that have been implicated in the consequences of hyperthermia and various other heat disorders. However, considering the large and even increasing number of identified neuroactive peptides, it is necessary to limit this chapter to a few peptides or peptide systems, which have received particular attention in relation to hyperthermia. Among these are the opioid peptides, the tachykinins, calcitonin gene-related peptide (CGRP), and peptides belonging to the angiotensin system. Most of these neuropeptides are not only affected by hyperthermia and abnormal alterations in the body temperature but also are involved in the endogenous mechanisms of regulating body temperature. This review does not endeavor to fully cover the field but it does aim to give the reader an idea of how various neuropeptides may be involved in the control of body heat and how peptidergic systems are affected during various thermal changes, including both immediate and long-term consequences.

Mechanism of Gly-Pro-pNA cleavage catalyzed by dipeptidyl peptidase-IV and its inhibition by saxagliptin (BMS-477118)

Dipeptidyl peptidase-IV (DPP-IV) is a serine protease with a signature Asp-His-Ser motif at the active site. Our pH data suggest that Gly-Pro-pNA cleavage catalyzed by DPP-IV is facilitated by an ionization of a residue with a pK of 7.2 +/- 0.1. By analogy to other serine proteases this pK is suggestive of His-Asp assisted Ser addition to the P1 carbonyl carbon of the substrate to form a tetrahedral intermediate. Solvent kinetic isotope effect studies yielded a D2Okcat/Km=2.9+/-0.2 and a D2Okcat=1.7+/-0.2 suggesting that kinetically significant proton transfers contribute to rate limitation during acyl intermediate formation (leaving group release) and hydrolysis. A "burst" of product release during pre steady-state Gly-Pro-pNA cleavage indicated rate limitation in the deacylation half-reaction. Nevertheless, the amplitude of the burst exceeded the enzyme concentration significantly (approximately 15-fold), which is consistent with a branching deacylation step. All of these data allowed us to better understand DPP-IV inhibition by saxagliptin (BMS-477118). We propose a two-step inhibition mechanism wherein an initial encounter complex is followed by covalent intermediate formation. Final inhibitory complex assembly (kon) depends upon the ionization of an enzyme residue with a pK of 6.2 +/- 0.1, and we assigned it to the catalytic His-Asp pair which enhances Ser nucleophilicity for covalent addition. An ionization with a pK of 7.9 +/- 0.2 likely reflects the P2 terminal amine of the inhibitor hydrogen bonding to Glu205/Glu206 in the enzyme active site. The formation of the covalent enzyme-inhibitor complex was reversible and dissociated with a koff of (5.5 +/- 0.4) x 10(-5) s(-1), thus yielding a Ki* (as koff/kon) of 0.35 nM, which is in good agreement with the value of 0.6 nM obtained from steady-state inhibition studies. Proton NMR spectra of DPP-IV showed a downfield resonance at 16.1 ppm. Two additional peaks in the 1H NMR spectra at 17.4 and 14.1 ppm were observed upon mixing the enzyme with saxagliptin. Fractionation factors (phi) of 0.6 and 0.5 for the 17.4 and 14.1 ppm peaks, respectively, are suggestive of short strong hydrogen bonds in the enzyme-inhibitor complex.

Pharmacogenetics in model systems: defining a common mechanism of action for mood stabilisers

Defining the underlying causes of psychiatric disorders has provided an ongoing and intractable problem. The analysis of the genetic basis of manic depression, in particular, has been impeded by the absence of a suitable model system and by the lack of candidate causative genes. One recent approach to overcome these problems has involved identifying those genes which control the sensitivity to anti-manic drugs in a model organism. Characterisation of the role of these genes and their encoded proteins in this model has allowed the analysis of their mammalian homologues to elucidate the therapeutic role of these drugs and the possible aetiology of manic depression. This approach has been used successfully with the cellular slime mould, Dictyostelium discoideum. This article introduces the use of model systems for pharmacogenetics research. It describes the identification of prolyl oligopeptidase in D. discoideum as a modulator of inositol phosphate signalling, and the subsequent identification of a common mechanism of action of three anti-manic drugs in mammalian neurons. The use of pharmacogenetics in model systems will provide a powerful tool for the ongoing analysis of both the treatment and cause of psychiatric disorders.

A rapid in vitro screening for delivery of peptide-derived peptidase inhibitors as potential drug candidates via epithelial peptide transporters

Targeting drugs or prodrugs to a specific enzyme by simultaneously targeting cell membrane carriers for efficient transport should provide the highest bioavailability along with specificity at the site of action. The peptide transporters PEPT1 and PEPT2 are expressed in a variety of tissues, including the brush-border membranes of epithelial cells of the small intestine and kidney. The transporters accept a wide range of substrates and are therefore good targets for a transporter-mediated drug delivery. Here, we report a screening procedure for peptidomimetic drug candidates combining two independent expression systems: 1) a competition assay in transgenic Pichia pastoris yeast cells expressing either mammalian PEPT1 or PEPT2 for identifying substrate interaction with the transporter binding site; and 2) a Xenopus laevis-based oocyte expression of the peptide transporter for assessing electrogenic transport of drug candidates. Based on the known oral availability and in vivo efficacy of the dipeptidyl peptidase IV (DPIV) inhibitor isoleucine-thiazolidide and its peptide-like structure, we first tested whether this compound is a substrate of epithelial peptide transporters. Additionally, a series of structurally related inhibitors were analyzed for transport. We identified various compounds that serve as substrates of the intestinal peptide transporter PEPT1. In contrast, none of these DPIV inhibitors showed electrogenic transport by PEPT2, although a variety of the compounds displayed good affinities for competition in peptide uptake in PEPT2-expressing cells, suggesting that they may serve as efficient inhibitors. In conclusion, we have applied an in vitro screening system that predicts efficient intestinal absorption of peptide-derived peptidase inhibitors via PEPT1 in vivo.

Two peptidase activities decrease in treated bipolar disorder not schizophrenic patients

BACKGROUND

Inhibition of prolyl oligopeptidase (PO) in primary neuronal cultures has been shown to reverse the effect of the common mood-stabilizers lithium, valproic acid and carbamazepine. In clinical studies, abnormal plasma PO activity has been associated with bipolar disorder (BD) and schizophrenia. However, this association is complicated by the discovery in bovine plasma of a Z-Pro-prolinal-insensitive peptidase (ZIP), a novel enzyme that cleaves the same substrate as PO.

METHODS

We developed an assay to distinguish between ZIP and PO and measured both activities in plasma from 48 BD and 50 schizophrenic patients undergoing treatment and compared them with 50 control subjects.

RESULTS

ZIP activity is restricted to blood plasma, whereas PO activity is present in the cytosol of lymphocytes, but can also be detected in blood plasma. Significant decreases in their plasma activities were found between treated BD (p = 0.007 and 0.03 respectively) but not schizophrenic (p > 0.05) patients and controls.

CONCLUSIONS

We have found that the enzyme activity previously reported as plasma PO actually comprises two enzymes, PO and ZIP. This study shows a statistically significant decrease of both enzymes in BD patients undergoing lithium treatment. No statistically significant change in PO or ZIP activity is observed in schizophrenic patients.

Cloning and characterization of dipeptidyl peptidase 10, a new member of an emerging subgroup of serine proteases

Two dipeptidyl peptidase IV (DPPIV, DPP4)-related proteins, DPP8 and DPP9, have been identified recently [Abbott, Yu, Woollatt, Sutherland, McCaughan, and Gorrell (2000) Eur. J. Biochem. 267, 6140-6150; Olsen and Wagtmann (2002) Gene 299, 185-193; Qi, Akinsanya, Riviere, and Junien (2002) Patent application WO0231134]. In the present study, we describe the cloning of DPP10, a novel 796-amino-acid protein, with significant sequence identity to DPP4 (32%) and DPP6 (51%) respectively. We propose that DPP10 is a new member of the S9B serine proteases subfamily. The DPP10 gene is located on the long arm of chromosome 2 (2q12.3-2q14.2), close to the DPP4 (2q24.3) and FAP (2q23) genes. The active-site serine residue is replaced by a glycine residue in DPP10, resulting in the loss of DPP activity. The serine residue is also replaced in DPP6, which lacks peptidase activity. DPP8 and DPP9 share an identical active site with DPP4 (Gly-Trp-Ser-Tyr-Gly). In contrast with the previous results suggesting that DPP9 is inactive, we show that DPP9 is a DPP, hydrolysing Ala-Pro-(7-amino-4-methyl-coumarin) with similar pH-specificity and protease-inhibitor-sensitivity to those of DPP4 and DPP8. Northern-blot analysis shows that whereas DPP8 and DPP9 are widely expressed, DPP10 is expressed mainly in the brain and pancreas. DPP6, which has the highest amino acid identity with DPP10, has been shown previously [Nadal, Ozaita, Amarillo, de Miera, Ma, Mo, Goldberg, Misumi, Ikehara, Neubert et al. (2003) Neuron 37, 449-461] to associate with A-type K(+) channel subunits, modulating their transport and function in somatodendritic compartments of neurons. It is possible that DPP10 is involved in similar functions in the brain. Elucidation of the physiological or pathophysiological role of DPP8, DPP9 and DPP10 and characterization of their structure-function relationships will add impetus to the development of inhibitor molecules for pharmacological or therapeutic use.

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of the incretins. DPP IV inhibitors improve glucose tolerance and pancreatic islet cell function in animal models of type 2 diabetes and in diabetic patients. The role of DPP IV/ CD26 within the immune system is a combination of its exopeptidase activity and its interactions with different molecules. This enables DPP IV/CD26 to serve as a co-stimulatory molecule to influence T cell activity and to modulate chemotaxis. DPP IV is also implicated in HIV-1 entry, malignant transformation, and tumor invasion.

Modulation of inositol 1,4,5-triphosphate concentration by prolyl endopeptidase inhibition

Prolyl endopeptidase (PEP) is a proline-specific oligopeptidase with a reported effect on learning and memory in different rat model systems. Using the astroglioma cell line U343, PEP expression was reduced by an antisense technique. Measuring different second-messenger concentrations revealed an inverse correlation between inositol 1,4,5-triphosphate [Ins(1,4,5)P3] concentration and PEP expression in the generated antisense cell lines. However, no effect on cAMP generation was observed. In addition, complete suppression of PEP activity by the specific inhibitor, Fmoc-Ala-Pyrr-CN (5 micro m) induced in U343 and other cell lines an enhanced, but delayed, increase in Ins(1,4,5)P3 concentration. This indicates that the proteolytic activity of PEP is responsible for the observed effect. Furthermore, the reduced PEP activity was found to amplify Substance P-mediated stimulation of Ins(1,4,5)P3. The effect of reduced PEP activity on second-messenger concentration indicates a novel intracellular function of this peptidase, which may have an impact on the reported cognitive enhancements due to PEP inhibition.

A common mechanism of action for three mood-stabilizing drugs

Lithium, carbamazepine and valproic acid are effective mood-stabilizing treatments for bipolar affective disorder. The molecular mechanisms underlying the actions of these drugs and the illness itself are unknown. Berridge and colleagues suggested that inositol depletion may be the way that lithium works in bipolar affective disorder, but others have suggested that glycogen synthase kinase (GSK3) may be the relevant target. The action of valproic acid has been linked to both inositol depletion and to inhibition of histone deacetylase (HDAC). We show here that all three drugs inhibit the collapse of sensory neuron growth cones and increase growth cone area. These effects do not depend on GSK3 or HDAC inhibition. Inositol, however, reverses the effects of the drugs on growth cones, thus implicating inositol depletion in their action. Moreover, the development of Dictyostelium is sensitive to lithium and to valproic acid, but resistance to both is conferred by deletion of the gene that codes for prolyl oligopeptidase, which also regulates inositol metabolism. Inhibitors of prolyl oligopeptidase reverse the effects of all three drugs on sensory neuron growth cone area and collapse. These results suggest a molecular basis for both bipolar affective disorder and its treatment.

Eating disorders: a role for dipeptidyl peptidase IV in nutritional control

Dipeptidyl peptidase IV (DPP IV), a serine protease with broad tissue distribution and known activity in serum, has been postulated to modulate nutrition control by modification or inactivation of peptide hormones operating in the enteroinsular axis. We hypothesized that changes of DPP IV activity in serum are related to the nutrition status of patients with eating disorders. Serum DPP IV activity was measured in 52 patients (28 with anorexia nervosa and 24 with bulimia nervosa) in four consecutive weekly analyses. Simultaneously, the number of CD26 (DPP IV)-positive peripheral blood lymphocytes was counted. The same analyses were carried out in 28 healthy female volunteers. In week 1 and throughout the observation period, DPP IV activity in the sera of patients with anorexia nervosa and, to a lesser extent, those with bulimia nervosa was elevated in comparison to that of healthy controls (week 1: means = 92.8 U/L for anorexia-nervosa patients and 89.3 U/L for bulimia-nervosa patients versus 74.7 U/L for healthy control subjects, P = 0.014; weeks 1-4: 91.8 U/L for anorexia-nervosa patients and 86.2 U/L for bulimia-nervosa patients versus 77.6 U/L for healthy controls, P < 0.001). We assume that the increase in DPP IV serum activity will increase the turnover of distinct peptide hormones with known effects on nutrition control and susceptibility to degradation by DPP IV. The potential impact of an increase in DPP IV activity in serum on satiety and nutrition control contributes to previously reported implications for immune function.

Synthesis and activity of pyrrolidinyl- and thiazolidinyl-dipeptide derivatives as inhibitors of the Tc80 prolyl oligopeptidase from Trypanosoma cruzi

Pyrrolidinyl- and thiazolidinyl- dipeptide derivatives, featuring either a vinyl sulfone-, a 2-ketobenzothiazole-, a nitrile-, or a benzimidazole group at the C-terminus, were designed and synthesized as potential inhibitors of the prolyl-specific Tc80 proteinase from Trypanosoma cruzi, the agent of Chagas' disease. These compounds were evaluated in vitro towards the target enzyme which was classified as a serine protease belonging to the prolyl oligopeptidase family (EC 3.4.21.26). A peptidyl nitrile and two peptidyl alpha-ketobenzothiazoles were shown to be potent reversible and competitive inhibitors of Tc 80 proteinase, with K(i) values in the range 38-219 nM, and compared advantageously with some known mammalian prolyl oligopeptidase inhibitors.

Loss of a prolyl oligopeptidase confers resistance to lithium by elevation of inositol (1,4,5) trisphosphate

The therapeutic properties of lithium ions (Li+) are well known; however, the mechanism of their action remains unclear. To investigate this problem, we have isolated Li+-resistant mutants from Dictyostelium. Here, we describe the analysis of one of these mutants. This mutant lacks the Dictyostelium prolyl oligopeptidase gene (dpoA). We have examined the relationship between dpoA and the two major biological targets of lithium: glycogen synthase kinase 3 (GSK-3) and signal transduction via inositol (1,4,5) trisphosphate (IP3). We find no evidence for an interaction with GSK-3, but instead find that loss of dpoA causes an increased concentration of IP3. The same increase in IP3 is induced in wild-type cells by a prolyl oligopeptidase (POase) inhibitor. IP3 concentrations increase via an unconventional mechanism that involves enhanced dephosphorylation of inositol (1,3,4,5,6) pentakisphosphate. Loss of DpoA activity therefore counteracts the reduction in IP3 concentration caused by Li+ treatment. Abnormal POase activity is associated with both unipolar and bipolar depression; however, the function of POase in these conditions is unclear. Our results offer a novel mechanism that links POase activity to IP3 signalling and provides further clues for the action of Li+ in the treatment of depression.

Nonpeptide small-molecular inhibitors of dipeptidyl peptidase IV: N-phenylphthalimide analogs

A novel series of nonpeptide small-molecular dipeptidyl peptidase IV (DPP-IV) inhibitors with an N-phenylphthalimide skeleton has been developed. Some of the compounds, including 4-amino-(2,6-dimethylphenyl)phthalimides (7), 4- and 5-hydroxy-(2,6-diethylphenyl)phthalimide (11 and 14), 4-hydroxy-(2,6-diisopropylphenyl)phthalimide (12), and thiocarbonyl analogs of (2,6-diisopropylphenyl)phthalimide and their 4,5,6,7-tetrafluorinated derivative (18, 19 and 20), were more potent than the well-known DPP-IV-specific inhibitor, Pro-boroPro (PBP). Among them, 18 was revealed to be a DPP-IV-specific inhibitor, while the others also showed inhibitory activity toward another peptidase, aminopeptidase N (APN).

A study of prolyl endopeptidase in bovine serum and its relevance to the tissue enzyme

Prolyl endopeptidase (PE) belongs to a group of enzymes that specifically recognise the imino acid proline. The characterisation of bovine serum PE was undertaken so that its relationship to its tissue counterparts could be considered. Using various chromatographic methods, PE was partially purified from bovine serum. This preparation was deemed to be enzymatically pure, based on its failure to hydrolyse a wide range of fluorimetric substrates. A native molecular mass of 69.7 kDa was estimated for the enzyme. PE was optimally active at pH 8.0-8.5, demonstrated a preference for phosphate buffer and remained stable over a pH range of 5.0-9.0. A narrowly focused optimal assay temperature of 37 degrees C was evident. Functional reagent studies indicated that this enzyme was a serine protease with a cysteine residue located near or at the active site. The enzyme was also sensitive to heavy metal inhibition. Substrate specificity investigations revealed that the bioactive peptides angiotensin II, bradykinin, luliberin and substance P were hydrolysed by the enzyme preparation, but lower specificities were evident towards these peptides in comparison with the enzyme's tissue counterparts. Specific inhibitor studies, using a range of compounds previously untested against a single PE source, indicated that alpha-ketobenzothiazole was the most effective PE inhibitor, with an IC50 value of 41 pM. In conclusion, the results presented in this paper indicate that bovine serum PE shares many of the characteristics associated with its tissue counterparts, with the exception of its specificity towards certain bioactive peptides.

Proline specific peptidases

Proline is unique among the 20 amino acids due to its cyclic structure. This specific conformation imposes many restrictions on the structural aspects of peptides and proteins and confers particular biological properties upon a wide range of physiologically important biomolecules. In order to adequately deal with such peptides, nature has developed a group of enzymes that recognise this residue specifically. These peptidases cover practically all situations where a proline residue might occur in a potential substrate. In this paper we endeavour to discuss these enzymes, particularly those responsible for peptide or protein hydrolysis at proline sites. We have detailed their discovery, biochemical attributes and substrate specificities and have provided information as to the methodology used to detect and manipulate their activities. We have also described the roles, or potential roles that these enzymes may play physiologically and the consequences of their dysfunction in varied disease states.

Dipeptide-derived diphenyl phosphonate esters: mechanism-based inhibitors of dipeptidyl peptidase IV

A number of dipeptide diphenyl phosphonate esters were studied as inhibitors of dipeptidyl peptidase IV, focusing on the role of the P2 residue in the inactivation process. The active compounds were slow irreversible inhibitors of the catalytic activity of the enzyme. With proline (or alanine) in the P1 position, the rate constants of inactivation correlated with the acylation rate constants reported for homologous dipeptide derived substrates. The kinetic data indicate that the mechanism of inhibition consists of the formation of a fairly weak initial complex, followed by a slow irreversible inactivation step. This indicates that, as in the case of trypsin-like proteinases, dipeptide diphenyl phosphonate esters form a covalent adduct with the catalytic site of DPP IV, even though this enzyme belongs to a completely distinct class of serine peptidases. Enantioselectivity and secondary specificity further support the evidence that diphenyl phosphonate esters are mechanism-based inhibitors. The dipeptide diphenyl phosphonate esters had a half-life of 3-10 h at 37 degrees C in Tris buffer. The inhibitors were degraded in human plasma, depending on the type of amino-terminal amino acid. The compound with proline in the P2 position was the most resistant to degradation in plasma. Due to their stability and the irreversible nature of the inhibition, the diphenyl phosphonate esters promise to be useful tools in the continuing investigation of the physiological function of dipeptidyl peptidase IV.

Substrates containing phosphorylated residues adjacent to proline decrease the cleavage by proline-specific peptidases

Thirteen dipeptide rho-nitroanilides of the common structure H-Xaa-Pro-4-NA (Xaa = serine, threonine and tyrosine) and seven tripeptide rho-nitroanilides of the common structure H-Gly-Xaa-Pro-4-NA (Xaa = serine or threonine) were prepared and analyzed as substrates of the proline-specific peptidases dipeptidyl peptidase IV and prolyl endopeptidase, respectively. The side chains of the hydroxy amino acids were synthetically modified by various acyl-, benzyl- and phosphate residues. The presence of aliphatic or aromatic residues attached to the side chain of the P2-hydroxy amino acids resulted in no significant change of the specificity constants of the enzyme-catalyzed substrate hydrolysis. In some cases, however, substrate inhibition was observed. In contrast, the reactivity of dipeptidyl peptidase IV and prolyl endopeptidase decreases more than two orders of magnitude towards the phosphorylated di- and tripeptide substrates compared to the hydrolysis of unmodified substrates. The kinetic data obtained with the model compounds suggest that side-chain modification of proline-containing peptide substrates may influence their resistance towards the hydrolytic activity of proline-specific hydrolases. Additionally, the results support that structural changes of the substrate during enzyme-hydrolysis may be involved in the mechanism of action of proline-specific serine peptidases. From this result we speculate that posttranslational phosphorylation of peptide sequences found in protein kinase recognition motifs such as -Xaa-Ser/Thr-Pro-Yaa- and -Xaa-Pro-Ser/Thr-Yaa- may serve as structural determinants that modulate their proteolytic stability.

Contribution of proteasome-mediated proteolysis to the hierarchy of epitopes presented by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTL) recognize peptide epitopes of protein antigens in a hierarchical fashion. We investigated whether proteolytic cleavage, in particular by proteasomes, is important in determining epitope hierarchy. Using highly purified 20S proteasomes, we find preferred cleavage sites directly adjacent to the N- and C-terminal ends of the immunodominant epitope of chicken ovalbumin, Ova257-264, while most of the subdominant epitope, Ova55-62, is destroyed by a major cleavage site located within this epitope. Moreover, we show that variations in amino acid sequences flanking these epitopes influence proteasomal cleavage patterns in parallel with the efficacy of their presentation. The results suggest that proteasomal cleavage within and adjacent to class I-restricted epitopes contributes to their level of presentation.