Red blood cells contain a pathway for the degradation of oxidant-damaged hemoglobin that does not require ATP or ubiquitin

Proteostasis and metabolic dysfunction in a distinct subset of storage-induced senescent erythrocytes targeted for clearance

Although refrigerated storage slows the metabolism of volunteer donor RBCs, cellular aging still occurs throughout this in vitro process, which is essential in transfusion medicine. Storage-induced microerythrocytes (SMEs) are morphologically-altered senescent RBCs that accumulate during storage and which are cleared from circulation following transfusion. However, the molecular and cellular alterations that trigger clearance of this RBC subset remain to be identified. Using a staining protocol that sorts long-stored SMEs (i.e., CFSE high ) and morphologically-normal RBCs (CFSE low ), these in vitro aged cells were characterized. Metabolomics analysis identified depletion of energy, lipid-repair, and antioxidant metabolites in CFSE high RBCs. By redox proteomics, irreversible protein oxidation primarily affected CFSE high RBCs. By proteomics, 96 proteins, mostly in the proteostasis family, had relocated to CFSE high RBC membranes. CFSE high RBCs exhibited decreased proteasome activity and deformability; increased phosphatidylserine exposure, osmotic fragility, and endothelial cell adherence; and were cleared from the circulation during human spleen ex vivo perfusion. Conversely, molecular, cellular, and circulatory properties of long-stored CFSE low RBCs resembled those of short-stored RBCs. CFSE high RBCs are morphologically and metabolically altered, have irreversibly oxidized and membrane-relocated proteins, and exhibit decreased proteasome activity. In vitro aging during storage selectively alters metabolism and proteostasis in SMEs, targeting these senescent cells for clearance.

Early Perturbations in Red Blood Cells in Response to Murine Malarial Parasite Infection: Proof-of-Concept 1H NMR Metabolomic Study

BACKGROUND

The major focus of metabolomics research has been confined to the readily available biofluids-urine and blood serum. However, red blood cells (RBCs) are also readily available, and may be a source of a wealth of information on vertebrates. However, the comprehensive metabolomic characterization of RBCs is minimal although they exhibit perturbations in various physiological states. RBCs act as the host of malarial parasites during the symptomatic stage. Thus, understanding the changes in RBC metabolism during infection is crucial for a better understanding of disease progression.

METHODS

The metabolome of normal RBCs obtained from Swiss mice was investigated using 1H NMR spectroscopy. Several 1 and 2-dimensional 1H NMR experiments were employed for this purpose. The information from this study was used to investigate the changes in the RBC metabolome during the early stage of infection (~1% infected RBCs) by Plasmodium bergheii ANKA.

RESULTS

We identified over 40 metabolites in RBCs. Several of these metabolites were quantitated using 1H NMR spectroscopy. The results indicate changes in the choline/membrane components and other metabolites during the early stage of malaria.

CONCLUSIONS

The paper reports the comprehensive characterization of the metabolome of mouse RBCs. Changes during the early stage of malarial infection suggest significant metabolic alteration, even at low parasite content (~1%).

GENERAL SIGNIFICANCE

This study should be of use in maximizing the amount of information available from metabolomic experiments on the cellular components of blood. The technique can be directly applied to real-time investigation of infectious diseases that target RBCs.

Oxidative Modification of Proteins: From Damage to Catalysis, Signaling, and Beyond

Significance: The systematic investigation of oxidative modification of proteins by reactive oxygen species started in 1980. Later, it was shown that reactive nitrogen species could also modify proteins. Some protein oxidative modifications promote loss of protein function, cleavage or aggregation, and some result in proteo-toxicity and cellular homeostasis disruption. Recent Advances: Previously, protein oxidation was associated exclusively to damage. However, not all oxidative modifications are necessarily associated with damage, as with Met and Cys protein residue oxidation. In these cases, redox state changes can alter protein structure, catalytic function, and signaling processes in response to metabolic and/or environmental alterations. This review aims to integrate the present knowledge on redox modifications of proteins with their fate and role in redox signaling and human pathological conditions. Critical Issues: It is hypothesized that protein oxidation participates in the development and progression of many pathological conditions. However, no quantitative data have been correlated with specific oxidized proteins or the progression or severity of pathological conditions. Hence, the comprehension of the mechanisms underlying these modifications, their importance in human pathologies, and the fate of the modified proteins is of clinical relevance. Future Directions: We discuss new tools to cope with protein oxidation and suggest new approaches for integrating knowledge about protein oxidation and redox processes with human pathophysiological conditions. Antioxid. Redox Signal. 35, 1016-1080.

Inter-study and time-dependent variability of metabolite abundance in cultured red blood cells

BACKGROUND

Cultured human red blood cells (RBCs) provide a powerful ex vivo assay platform to study blood-stage malaria infection and propagation. In recent years, high-resolution metabolomic methods have quantified hundreds of metabolites from parasite-infected RBC cultures under a variety of perturbations. In this context, the corresponding control samples of the uninfected culture systems can also be used to examine the effects of these perturbations on RBC metabolism itself and their dependence on blood donors (inter-study variations).

METHODS

Time-course datasets from five independent studies were generated and analysed, maintaining uninfected RBCs (uRBC) at 2% haematocrit for 48 h under conditions originally designed for parasite cultures. Using identical experimental protocols, quadruplicate samples were collected at six time points, and global metabolomics were employed on the pellet fraction of the uRBC cultures. In total, ~ 500 metabolites were examined across each dataset to quantify inter-study variability in RBC metabolism, and metabolic network modelling augmented the analyses to characterize the metabolic state and fluxes of the RBCs.

RESULTS

To minimize inter-study variations unrelated to RBC metabolism, an internal standard metabolite (phosphatidylethanolamine C18:0/20:4) was identified with minimal variation in abundance over time and across all the samples of each dataset to normalize the data. Although the bulk of the normalized data showed a high degree of inter-study consistency, changes and variations in metabolite levels from individual donors were noted. Thus, a total of 24 metabolites were associated with significant variation in the 48-h culture time window, with the largest variations involving metabolites in glycolysis and synthesis of glutathione. Metabolic network analysis was used to identify the production of superoxide radicals in cultured RBCs as countered by the activity of glutathione oxidoreductase and synthesis of reducing equivalents via the pentose phosphate pathway. Peptide degradation occurred at a rate that is comparable with central carbon fluxes, consistent with active degradation of methaemoglobin, processes also commonly associated with storage lesions in RBCs.

CONCLUSIONS

The bulk of the data showed high inter-study consistency. The collected data, quantification of an expected abundance variation of RBC metabolites, and characterization of a subset of highly variable metabolites in the RBCs will help in identifying non-specific changes in metabolic abundances that may obscure accurate metabolomic profiling of Plasmodium falciparum and other blood-borne pathogens.

The physiological role of the free 20S proteasome in protein degradation: A critical review

BACKGROUND

It has been almost three decades since the removal of oxidized proteins by the free 20S catalytic unit of the proteasome (20SPT) was proposed. Since then, experimental evidence suggesting a physiological role of proteolysis mediated by the free 20SPT has being gathered.

SCOPE OF REVIEW

Experimental data that favors the hypothesis of free 20SPT as playing a role in proteolysis are critically reviewed.

MAJOR CONCLUSIONS

Protein degradation by the proteasome may proceed through multiple proteasome complexes with different requirements though the unequivocal role of the free 20SPT in cellular proteolysis towards native or oxidized proteins remains to be demonstrated.

GENERAL SIGNIFICANCE

The biological significance of proteolysis mediated by the free 20SPT has been elusive since its discovery. The present review critically analyzes the available experimental data supporting the proteolytic role of the free or single capped 20SPT.

Antimicrobial peptides are degraded by the cytosolic proteases of human erythrocytes

Well-studied and promising antimicrobial peptides (AMPs), with potent bactericidal activity, in vitro, have yet to have a significant impact in human medicine beyond topical applications. We previously showed that interactions of AMPs with concentrated human erythrocytes inhibit many of them, and suggested that screens and assays should be done in their presence to mimic host cell inhibition. Here, we use AMPs to characterize the activity of proteases that are associated with human erythrocytes. The representative AMPs, ARVA and indolicidin, are degraded significantly during incubation with dilute, washed erythrocytes and yield a variety of degradation products, suggesting significant exopeptidase activity. Comparison of these fragments with those obtained from incubation with serum shows that the proteolytic activity associated with cells yields unique products that are not explained by residual serum proteases. By separately testing the membrane and cytosolic fractions, we show that erythrocyte proteolytic activity is found only in the cytosol. Finally, we incubated a diverse cross-section of natural and synthetic linear AMPs with human erythrocyte cytosolic extracts and observed degradation of all of them. These results show that, in addition to cell binding, proteolysis can also contribute significantly to host cell inhibition of AMPs in vitro and possibly also in vivo.

The Immunoproteasome in oxidative stress, aging, and disease

The Immunoproteasome has traditionally been viewed primarily for its role in peptide production for antigen presentation by the major histocompatibility complex, which is critical for immunity. However, recent research has shown that the Immunoproteasome is also very important for the clearance of oxidatively damaged proteins in homeostasis, and especially during stress and disease. The importance of the Immunoproteasome in protein degradation has become more evident as diseases characterized by protein aggregates have also been linked to deficiencies of the Immunoproteasome. Additionally, there are now diseases defined by mutations or polymorphisms within Immunoproteasome-specific subunit genes, further suggesting its crucial role in cytokine signaling and protein homeostasis (or "proteostasis"). The purpose of this review is to highlight our growing understanding of the importance of the Immunoproteasome in the management of protein quality control, and the detrimental impact of its dysregulation during disease and aging.

Regulation of ubiquitin-proteasome and autophagy pathways after acute LPS and epoxomicin administration in mice

Background

The ubiquitin-proteasome pathway (UPP) is a major protein degradation pathway that is activated during sepsis and has been proposed as a therapeutic target for preventing skeletal muscle loss due to cachexia. Although several studies have investigated the modulation of proteasome activity in response to LPS administration, none have characterized the overall UPP response to LPS administration in the fate of proteasome inhibition.

Methods

Here, we determined the modulation pattern of the main key components of the UPP in the gastrocnemius (GAS) of mice during the acute phase of lipopolysaccharide (LPS)-mediated endotoxemia (7.5 mg/kg – 8 h) by measuring all three β1, β2 and β5 activites of the 20S and 26S proteasomes, the levels of steady state polyubiquitinated proteins, mRNA levels of muscle ligases, as well as signaling pathways regulating the UPP. Another goal was to assess the effects of administration of a specific proteasome inhibitor (epoxomicin, 0.5 mg/kg) on UPP response to sepsis.

Results

The acute phase of LPS-induced endotoxemia lowered GAS/body weight ratio and increased MuRF1 and MAFbx mRNA concomitantly to an activation of the pathways known to regulate their expression. Unexpectedly, we observed a decrease in all 20S and 26S proteasome activities measured in GAS, which might be related to oxidative stress, as oxidized proteins (carbonyl levels) increase with LPS. While significantly inhibiting 20S and 26S proteasome β5 activities in heart and liver, epoxomicin did not lower proteasome activity in GAS. However, the increase in mRNA expression of the muscle ligases MuRF1 and MAFbx were partially rescued without affecting the other investigated signaling pathways. LPS also strongly activated autophagy, which could explain the observed GAS atrophy with LPS-induced reduction of proteasome activity.

Conclusions

Our results highlight an opposite regulation of UPP in the early hours of LPS-induced muscle atrophy by showing reduced proteasome activities and increased mRNA expression of muscle specific ligases. Furthermore, our data do not support any preventive effect of epoxomicin in muscle atrophy due to acute cachexia since proteasome activities are not further repressed.

Erythrocyte-based analgesic peptides

Human erythrocyte discards the major organelles in a bid to maximize cellular hemoglobin. Hemoglobin, approximately 98% of the intraerythrocytic protein, serves as the principle transport medium of gaseous conveyance. The accumulated data speaks in favor of erythrocyte not merely engaging in gas exchange, but building molecular signaling as a side job during its 4-month sojourn in blood circulation. The production mechanism of erythrocyte-based bioactive peptides is not clear. Recent studies indicate that proteasome and its subunits persist in mature erythrocyte. The intraerythrocytic proteasome is involved in the formation of hemoglobin-derived analgesic peptides and enables erythrocyte to exert the erythrocrine function. Erythrocrine describes erythrocyte for generation and excretion of signaling molecules and has the potential of shedding light on our understanding of novel actions of erythrocyte. Different types of erythrocrine analgesic peptides are originated from the intraerythrocytic degradation of hemoglobin and manifest the systemic influence in physiology and pathophysiology along its travel through the body via the bloodstream. Translational research from bench to bedside will expand our knowledge of erythrocrine concept and facilitate the development of therapeutic strategies for clinical pain.

Maternal and fetal indicators of oxidative stress in various obstetric complications

The present study demonstrates the incidence of increased lipid peroxidation and protein oxidation in both maternal and fetal erythrocytes as markers of oxygen radical activity in different complications of pregnancy. In fetuses born after premature rupture of membranes, lipid peroxidation was significantly elevated as indicated by increased malondialdehyde levels (p<0.05) as compared to controls. Proteolytic activity in the erythrocytes of mothers in this group was also significantly high (p<0.01). In patients delivered by lower segment cesarian section, lipid peroxidation and proteolytic activity in maternal erythrocytes were significantly high (p<0.05 and p<0.001 respectively). In patients with prolonged second stage of labour, lipid peroxidation and proteolytic activity in maternal erythrocytes was significantly higher than in controls (p<0.001 and p<0.05 respectively). In this group, endogenous protein damage due to oxidative stress was significantly high both in the mother and the fetus (p<0.001 and p<0.05 respectively).

Maternal and fetal indicators of oxidative stress during intrauterine growth retardation (IUGR)

The present study demonstrates the possibility of increased lipid peroxidation and protein oxidation in both maternal and fetal erythrocytes as markers of oxygen radical activity during intrauterine growth retardation. The erythrocyte MDA levels were significantly elevated in mothers of IUGR babies when compared to controls (p<0.01). The endogenous protein damage due to oxidative stress was significantly higher in IUGR mothers when compared to controls (p<0.05). Similarly the proteolytic activity in erythrocyte lysates against oxidatively damaged hemoglobin was significantly increased in mothers of IUGR babies compared to controls (p<0.001).In fetuses born with IUGR, both lipid peroxidation and proteolytic activity were significantly increased when compared to normal newborns (p<0.01).The result of this study indicates that oxidative stress was induced both in IUGR babies and their mothers which is manifested as increased lipid peroxidation and protein oxidant damage.

Non-canonical ubiquitin-based signals for proteasomal degradation

Regulated cellular proteolysis is mediated largely by the ubiquitin-proteasome system (UPS). It is a highly specific process that is time- (e.g. cell cycle), compartment- (e.g. nucleus or endoplasmic reticulum) and substrate quality- (e.g. denatured or misfolded proteins) dependent, and allows fast adaptation to changing conditions. Degradation by the UPS is carried out through two successive steps: the substrate is covalently tagged with ubiquitin and subsequently degraded by the 26S proteasome. The accepted 'canonical' signal for proteasomal recognition is a polyubiquitin chain that is anchored to a lysine residue in the target substrate, and is assembled through isopeptide bonds involving lysine 48 of ubiquitin. However, several 'non-canonical' ubiquitin-based signals for proteasomal targeting have also been identified. These include chains anchored to residues other than internal lysine in the substrates, chains assembled through linking residues other than lysine 48 in ubiquitin, and mixed chains made of both ubiquitin and a ubiquitin-like protein. Furthermore, some proteins can be degraded following modification by a single ubiquitin (monoubiquitylation) or multiple single ubiquitins (multiple monoubiquitylation). Finally, some proteins can be proteasomally degraded without prior ubiquitylation (the process is also often referred to as ubiquitination). In this Commentary, we describe these recent findings and discuss the possible physiological roles of these diverse signals. Furthermore, we discuss the possible impact of this signal diversity on drug development.

Subcellular fractionation of stored red blood cells reveals a compartment-based protein carbonylation evolution

During blood banking, erythrocytes undergo storage lesions, altering or degrading their metabolism, rheological properties, and protein content. Carbonylation is a hallmark of protein oxidative lesions, thus of red blood cell oxidative stress. In order to improve global erythrocyte protein carbonylation assessment, subcellular fractionation has been established, allowing us to work on four different protein populations, namely soluble hemoglobin, hemoglobin-depleted soluble fraction, integral membrane and cytoskeleton membrane protein fractions. Carbonylation in erythrocyte-derived microparticles has also been investigated. Carbonylated proteins were derivatized with 2,4-dinitrophenylhydrazine (2,4-DNPH) and quantified by western blot analyses. In particular, carbonylation in the cytoskeletal membrane fraction increased remarkably between day 29 and day 43 (P<0.01). Moreover, protein carbonylation within microparticles released during storage showed a two-fold increase along the storage period (P<0.01). As a result, carbonylation of cytoplasmic and membrane protein fractions differs along storage, and the present study allows explaining two distinct steps in global erythrocyte protein carbonylation evolution during blood banking. This article is part of a Special Issue entitled: Integrated omics.

Inhibition of intraerythrocytic proteasome retards the generation of hemorphins

Hemorphins are a set of hemoglobin-derived opioid peptides. The production mechanism of these structural overlap peptides remains unclear. Based on the sequences of hemorphins, it could be inferred that hemorphins are probably generated by cleavage of hemoglobin β chain at sites favored by the chymotrypsin-like protease. 20S proteasome possesses the chymotrypsin-like activity and still persists in mature erythrocytes. This study attempts to clarify whether the intraerythrocytic proteasome involves in the formation of hemorphins. Hemorphins containing hemorphin-7 and V-hemorphin-7 are isolated by immunoprecipitation from culture supernatant of human erythrocytes. Bortezomib inhibits the chymotrypsin-like activity of intraerythrocytic proteasome and prevents the yield of hemorphins in a dose-dependent manner. The present study suggests that intraerythrocytic proteasome contributes to the generation of hemorphins.

Redox control of the ubiquitin-proteasome system: from molecular mechanisms to functional significance

In their natural environments, cells are regularly exposed to oxidizing conditions that may lead to protein misfolding. If such misfolded proteins are allowed to linger, they may form insoluble aggregates and pose a serious threat to the cell. Accumulation of misfolded, oxidatively damaged proteins is characteristic of many diseases and during aging. To counter the adverse effects of oxidative stress, cells can initiate an antioxidative response in an attempt to repair the damage, or rapidly channel the damaged proteins for degradation by the ubiquitin-proteasome system (UPS). Recent studies have shown that elements of the oxidative stress response and the UPS are linked on many levels. To manage the extra burden of misfolded proteins, the UPS is induced by oxidative stress, and special proteasome subtypes protect cells against oxidative damage. In addition, the proteasome is directly associated with a thioredoxin and other cofactors that may adjust the particle's response during an oxidative challenge. Here, we give an overview of the UPS and a detailed description of the degradation of oxidized proteins and of the crosstalk between oxidative stress and protein degradation in health and disease.

Protein quality control during erythropoiesis and hemoglobin synthesis

Erythrocytes must regulate hemoglobin synthesis to limit the toxicities of unstable free globin chain subunits. This regulation is particularly relevant in β-thalassemia, in which β-globin deficiency causes accumulation of free α-globin, which forms intracellular precipitates that destroy erythroid precursors. Experimental evidence accumulated over more than 40 years indicates that erythroid cells can neutralize moderate amounts of free α-globin through generalized protein quality control mechanisms, including molecular chaperones, the ubiquitin-proteasome system, and autophagy. In many ways, β-thalassemia resembles protein aggregation disorders of the nervous system, liver, and other tissues, which occur when levels of unstable proteins overwhelm cellular compensatory mechanisms. Information gained from studies of nonerythroid protein aggregation disorders may be exploited to further understand and perhaps treat β-thalassemia.

The immunoproteasome, the 20S proteasome and the PA28αβ proteasome regulator are oxidative-stress-adaptive proteolytic complexes

Oxidized cytoplasmic and nuclear proteins are normally degraded by the proteasome, but accumulate with age and disease. We demonstrate the importance of various forms of the proteasome during transient (reversible) adaptation (hormesis), to oxidative stress in murine embryonic fibroblasts. Adaptation was achieved by 'pre-treatment' with very low concentrations of H2O2, and tested by measuring inducible resistance to a subsequent much higher 'challenge' dose of H2O2. Following an initial direct physical activation of pre-existing proteasomes, the 20S proteasome, immunoproteasome and PA28αβ regulator all exhibited substantially increased de novo synthesis during adaptation over 24 h. Cellular capacity to degrade oxidatively damaged proteins increased with 20S proteasome, immunoproteasome and PA28αβ synthesis, and was mostly blocked by the 20S proteasome, immunoproteasome and PA28 siRNA (short interfering RNA) knockdown treatments. Additionally, PA28αβ-knockout mutants achieved only half of the H2O2-induced adaptive increase in proteolytic capacity of wild-type controls. Direct comparison of purified 20S proteasome and immunoproteasome demonstrated that the immunoproteasome can selectively degrade oxidized proteins. Cell proliferation and DNA replication both decreased, and oxidized proteins accumulated, during high H2O2 challenge, but prior H2O2 adaptation was protective. Importantly, siRNA knockdown of the 20S proteasome, immunoproteasome or PA28αβ regulator blocked 50-100% of these adaptive increases in cell division and DNA replication, and immunoproteasome knockdown largely abolished protection against protein oxidation.

Oxidized proteins: mechanisms of removal and consequences of accumulation

Elevated levels of oxidized proteins are reported in diseased tissue from age-related pathologies such as atherosclerosis, neurodegenerative disorders, and cataract. Unlike the precise mechanisms that exist for the repair of nucleic acids, lipids, and carbohydrates, the primary pathway for the repair of oxidized proteins is complete catabolism to their constitutive amino acids. This process can be inefficient as is evidenced by their accumulation. It is generally considered that damaged proteins are degraded by the proteasome; however, this is only true for mildly oxidized proteins, because substrates must be unfolded to enter the narrow catalytic core. Rather, evidence suggests that moderately or heavily oxidized proteins are endocytosed and enter the endosomal/lysosomal system, indicating co-operation between the proteasomes and the lysosomes. Heavily modified substrates are incompletely degraded and accumulate within the lysosomal compartments resulting in the formation of lipofuscin-like, autofluorescent aggregates. Accumulation eventually results in impaired turnover of large organelles such as proteasomes and mitochondria, lysosomal destablization, leakage of proteases into the cytosol and apoptosis. In this review, we summarize reports published since our last assessments of the field of oxidized protein degradation including a role for modified proteins in the induction of apoptosis.

Heat shock and oxygen radicals stimulate ubiquitin-dependent degradation mainly of newly synthesized proteins

Accumulation of misfolded oxidant-damaged proteins is characteristic of many diseases and aging. To understand how cells handle postsynthetically damaged proteins, we studied in Saccharomyces cerevisiae the effects on overall protein degradation of shifting from 30 to 38°C, exposure to reactive oxygen species generators (paraquat or cadmium), or lack of superoxide dismutases. Degradation rates of long-lived proteins (i.e., most cell proteins) were not affected by these insults, even when there was widespread oxidative damage to proteins. However, exposure to 38°C, paraquat, cadmium, or deletion of SOD1 enhanced two- to threefold the degradation of newly synthesized proteins. By 1 h after synthesis, their degradation was not affected by these treatments. Degradation of these damaged cytosolic proteins requires the ubiquitin–proteasome pathway, including the E2s UBC4/UBC5, proteasomal subunit RPN10, and the CDC48–UfD1–NPL4 complex. In yeast lacking these components, the nondegraded polypeptides accumulate as aggregates. Thus, many cytosolic proteins proceed through a prolonged “fragile period” during which they are sensitive to degradation induced by superoxide radicals or increased temperatures.

Effects of chronic alcohol drinking on receptor-binding, internalization, and degradation of human immunodeficiency virus 1 envelope protein gp120 in hepatocytes

Although alcohol drinking increases susceptibility to human immunodeficiency virus (HIV) infection, possible mechanisms underlying the effects of alcohol are not yet known. Since the HIV envelope protein gp120 plays a key role in progression of HIV infection, the aim of the present study was to evaluate the toxicity and degradation of gp120 in hepatocytes isolated from liver of alcohol-preferring rats drinking either 15% ethanol in water or pure water for 70 days. The hypothesis was that alcohol drinking augmented the toxicity, but suppressed degradation of gp120. Hepatocytes from water-drinking rats (C-cells) or ethanol-drinking rats (Et-cells) were treated with laptacystin, anti-CD4 antibodies, CCR5 antagonist, or mannose, followed by [(125)I]gp120 or native gp120. At predetermined intervals, control (C) and ethanol exposed (Et) cells were analyzed for toxicity and degradation of gp120. In C-cells, [(125)I]gp120 binding and internalization peaked within 5-45 min and remained elevated for up to 10h and then decreased gradually. In Et-cells, [(125)I]gp120 binding peaked comparably to C-cells, but the binding remained to the peak level throughout the experimental period. C-cells exhibited the lysosomal/ubiquitin-mediated degradation of intracellular gp120, resulting in released gp120 fragments into the incubation medium that suppressed gp120-CD4 binding, improved cell viability, and inhibited gp120-induced apoptosis. Ethanol drinking suppressed gp120 degradation in and release of gp120 fragments from hepatocytes. The incubation medium of Et-cells did not suppress gp120-CD4 binding or the gp120-mediated apoptosis in hepatocytes. Thus, chronic alcohol drinking augmented the adverse effects of gp120 possibly by suppressing its degradation in hepatocytes. The present observation also suggests that a number of CCR5 or ubiquitin-based therapeutic drugs may not be effective in suppressing HIV infection in alcohol-drinking subjects.

Regulation of Murine Cardiac 20S Proteasomes

Our recent studies have provided a proteomic blueprint of the 26S proteasome complexes in the heart, among which 20S proteasomes were found to contain cylinder-shaped structures consisting of both α and β subunits. These proteasomes exhibit a number of features unique to the myocardium, including striking differences in post-translational modifications (PTMs) of individual subunits and novel PTMs that have not been previously reported. To date, mechanisms contributing to the regulation of this myocardial proteolytic core system remain largely undefined; in particular, little is known regarding PTM-dependent regulation of cardiac proteasomes. In this investigation, we seek to elucidate the function and regulation of 20S proteasome complexes in the heart. Functionally viable murine cardiac 20S proteasomes were purified. Tandem mass spectrometry analyses, combined with native gel electrophoresis, immunoprecipitation, and immunoblotting, revealed the identification of 2 previously unrecognized functional partners in the endogenous intact cardiac 20S complexes: protein phosphatase 2A (PP2A), and protein kinase A (PKA). Furthermore, our results demonstrated that PP2A and PKA profoundly impact the proteolytic function of 20S proteasomes: phosphorylation of 20S complexes enhances the peptidase activity of individual subunits in a substrate-specific fashion. Moreover, inhibition of PP2A or the addition of PKA significantly modified both the serine- and threonine-phosphorylation profile of proteasomes; multiple individual subunits of 20S (eg, α1 and β2) were targets of PP2A and PKA. Taken together, these studies provide the first demonstration that the function of cardiac 20S proteasomes is modulated by associating partners and that phosphorylation may serve as a key mechanism for regulation.

Purification and Characterization of a Hemoglobin Degrading Aspartic Protease from the Malarial Parasite Plasmodium vivax

Aspartic proteases of human malarial parasites are thought to play key roles in essential pathways of merozoite release, invasion and host cell hemoglobin degradation during the intraerythrocytic stages of their life cycle. Therefore, we have purified and characterized Plasmodium vivax aspartic protease, to determine if this enzyme can be used as potential drug target/immunogen, and its inhibitors as potential antimalarial drug. The P. vivax aspartic protease has been purified by a combination of ion exchange and size exclusion chromatographies and HPLC. Its properties were examined in order to define a role in the hemoglobin degradation process. The purified enzyme migrated as a single band on native PAGE and SDS/PAGE with a molecular mass of 40 kDa. Gelatin zymogram analyses revealed a clear zone of proteolytic activity corresponding to the band obtained on native PAGE and SDS/PAGE. The enzyme has an optimal pH of 4.0 and exhibits its highest activity at 37 degrees C. The enzyme is inhibited by pepstatin, but not by other inhibitors including o-phenanthroline, EDTA, PMSF or E-64, supporting its designation as an aspartic protease; its IC50 value was found to be 3.0 microM. A Lineweaver Burk double reciprocal plot with pepstatin shows that the inhibition is competitive with respect to the substrate. Ca2+ and Mg2+ ions enhance the protease activity, whereas Cu2+ and Hg2+ ions were found to be inhibitory. The pivotal role of aspartic protease in initiating hemoglobin degradation in P. vivax malaria parasite is also demonstrated.

Correlation between the physicochemical property of some nonsteroidal anti-inflammatory drugs and changes in adenosine triphosphate, glutathione and hemoglobin in rat erythrocytes

This study was conducted to explore the relationship between physicochemical property and toxic effectiveness using rat red blood cells (RBCs). The toxic effectiveness of acid nonsteroidal anti-inflammatory drugs (NSAIDs) was systemically examined by the depletion of intracorpuscular adenosine triphosphate (ATP), glutathione (GSH), and hemoglobin (Hb) at various doses, increased every 5 fmol/RBC. When the RBCs were incubated with NSAIDs, the drugs attained maximum levels within RBC, and the levels were then reduced. The ATP depletion seemed to be observed on the excretion of the drugs prior to the depletions of GSH and Hb. The physicochemical properties of NSAIDs were obtained from QMPRPlus, SMILES code, and CS ChemRaw Ultra. Correlation between their physicochemical properties and their doses for the depletions of ATP, GSH and Hb was performed in comparison with those of the membrane bound enzyme (MBE) inhibiting- and methemoglobin (MHb)-generating drugs. The ATP depletion by NSAIDs was correlated with the GSH depletion and intracorpuscular levels of the drugs, but not with the Hb depletion. The GSH depletion was correlated with the Hb depletion and participated in the lipophilicity of the drugs.

Selective degradation of oxidatively modified protein substrates by the proteasome

Oxidative stress in mammalian cells is an inevitable consequence of their aerobic metabolism. Oxidants produce modifications to proteins leading to loss of function (or gain of undesirable function) and very often to an enhanced degradation of the oxidized proteins. For several years it has been known that the proteasome is involved in the degradation of oxidized proteins. This review summarizes our knowledge about the recognition of oxidized protein substrates by the proteasome in in vitro systems and its applicability to living cells. The majority of studies in the field agree that the degradation of mildly oxidized proteins is an important function of the proteasomal system. The major recognition motif of the substrates seems to be hydrophobic surface patches that are recognized by the 20S 'core' proteasome. Such hydrophobic surface patches are formed by partial unfolding and exposure of hydrophobic amino acid residues during oxidation. Oxidized proteins appear to be relatively poor substrates for ubiquitination, and the ubiquitination system does not seem to be involved in the recognition or targeting of oxidized proteins. Heavily oxidized proteins appear to first aggregate (new hydrophobic and ionic bonds) and then to form covalent cross-links that make them highly resistant to proteolysis. The inability to degrade extensively oxidized proteins may contribute to the accumulation of protein aggregates during diseases and the aging process.

Overexpression of oxidized protein hydrolase protects COS-7 cells from oxidative stress-induced inhibition of cell growth and survival

Oxidized protein hydrolase (OPH) preferentially degrades oxidatively damaged proteins in vitro and is widely distributed in various cells and tissues. The role of OPH in intact cells exposed to oxidative stress was examined. For this purpose, using COS-7, a cell line derived from African green monkey kidney, COS-7-OPH cells that stably overexpressed OPH were established. When COS-7-OPH cells were exposed to oxidative stress induced by H(2)O(2) and paraquat, accumulation of protein carbonyls in the cells was apparently lower than that of parental COS-7 cells, and COS-7-OPH cells were significantly resistant to the oxidative stress compared with parental COS-7 cells. The majority of overexpressed OPH in the cells was found to be located uniformly in cytosol, and its location was not altered by H(2)O(2)-induced oxidative stress. Above results indicate that OPH in intact cells plays a preventive role against oxidative stress and suggest that OPH relieves cells from accumulation of oxidatively damaged proteins.

Catecholamines inhibit Ca(2+)-dependent proteolysis in rat skeletal muscle through beta(2)-adrenoceptors and cAMP

Overall proteolysis and the activity of skeletal muscle proteolytic systems were investigated in rats 1, 2, or 4 days after adrenodemedullation. Adrenodemedullation reduced plasma epinephrine by 95% and norepinephrine by 35% but did not affect muscle norepinephrine content. In soleus and extensor digitorum longus (EDL) muscles, rates of overall proteolysis increased by 15-20% by 2 days after surgery but returned to normal levels after 4 days. The rise in rates of protein degradation was accompanied by an increased activity of Ca(2+)-dependent proteolysis in both muscles, with no significant change in the activity of lysosomal and ATP-dependent proteolytic systems. In vitro rates of Ca(2+)-dependent proteolysis in soleus and EDL from normal rats decreased by ~35% in the presence of either 10(-5) M clenbuterol, a beta(2)-adrenergic agonist, or epinephrine or norepinephrine. In the presence of dibutyryl cAMP, proteolysis was reduced by 62% in soleus and 34% in EDL. The data suggest that catecholamines secreted by the adrenal medulla exert an inhibitory control of Ca(2+)-dependent proteolysis in rat skeletal muscle, mediated by beta(2)-adrenoceptors, with the participation of a cAMP-dependent pathway.

Protein degradation by peroxide catalyzed by chromium (III): role of coordinated ligand

In order to understand the role of coordinated ligands in controlling the biotoxicity of chromium (III), interactions of three types of chromium (III) complexes viz. trans-diaquo [1,2 bis (salicyledeneamino) ethane chromium (III) perchlorate, [(Cr(salen)(H(2)O)(2)](ClO(4)); tris (ethylenediamine) chromium (III) chloride, [Cr(en)(3)]Cl(3), and monosodium ethylene diamine tetraacetato monoaquo chromiate (III), [Cr(EDTA)(H(2)O)]Na with BSA has been investigated. Spectroscopic and equilibrium dialysis studies show that the two cationic complexes Cr(salen)(H(2)O)(+)(2) and Cr(en)(3+)(3) bind to the protein with a protein-metal ratio of 1:8 and 1:4. The anionic complex Cr(EDTA)(H(2)O)(-) binds to the protein with a protein-metal ratio of 1:2. The binding constant K(b) as estimated from the fluorescence quenching studies has been found to be 7.6 +/- 0.4 x 10(3) M(-1), 3.1 +/- 0.2 x 10(2) M(-1), and 1.8 +/- 0.2 x 10(2) M(-1) for Cr(salen)(H(2)O)(+)(2), Cr(en)(3+)(3), and Cr(EDTA)(H(2)O)(-) respectively indicating that the thermodynamic stability of protein-chromium complex is Cr(salen)(H(2)O)(+)(2) > Cr(en)(3+)(3) approximately Cr(EDTA)(H(2)O)(-). The complexes Cr(salen)(H(2)O)(+)(2) and Cr(EDTA)(H(2)O)(-) in the presence of hydrogen peroxide have been found to bring about protein degradation, whereas Cr(en)(3+)(3) does not bring about any protein damage. This clearly shows that the nature of the chromium (III) complex plays a major role in the biotoxicity of chromium (III).

Effect of guanethidine-induced adrenergic blockade on the different proteolytic systems in rat skeletal muscle

Overall proteolysis and the activity of skeletal muscle proteolytic systems were investigated in rats submitted to guanethidine-induced adrenergic blockade for 4 days. In soleus, overall proteolysis increased by 15-20% during the first 2 days of guanethidine treatment but decreased to levels below control values after 4 days. Extensor digitorum longus (EDL) did not show the initial increase in total proteolysis, which was already reduced after 2 days of guanethidine treatment. The initial rise in the rate of protein degradation in soleus was accompanied by an increased activity of the Ca(2+)-dependent proteolytic pathway. In both soleus and EDL, the reduction in overall proteolysis was paralleled by decreased activities of the Ca(2+)-dependent and ATP-dependent proteolytic processes. No change was observed in the activity of the lysosomal proteolytic system. Overall proteolysis in soleus and EDL from nontreated rats was partially inhibited by isoproterenol, in vitro. The data suggest an acute inhibitory control of skeletal muscle proteolysis by the adrenergic system, well evident in the oxidative muscle, with an important participation of the Ca(2+)-dependent pathway.

Posttranslational modifications of cardiac and skeletal muscle proteins by reactive oxygen species after burn injury in the rat

OBJECTIVE

To determine the involvement of oxidative damage in muscle wasting after burn injury.

SUMMARY BACKGROUND DATA

Burn injury damages tissue at the site of the burn and also affects peripheral tissue. There is evidence to suggest that reactive oxygen species may be generated in increased amounts after burn, and these may contribute to wound healing and to posttranslational modifications of tissue constituents distant from the wound site.

METHODS

The oxidation of muscle proteins was assessed, using the dinitrophenylhydrazine assay for carbonyl content, in muscles of rats after a full-thickness skin scald burn covering 20% of the total body surface area, over a 6-week period. In this model, rats failed to incur normal body weight or muscle weight gain.

RESULTS

Soleus, extensor digitorum longus, diaphragm, and heart ventricle proteins were oxidatively damaged after injury. The extent of tissue protein oxidation, however, differed depending on the time points studied. In general, higher levels of protein carbonyl group formation, an indicator of oxidative damage, were found to occur within 1 to 5 days after injury, and the oxidized protein content of the various tissues decreased during the later stages. Both sarcoplasmic and myofibrillar carbonyl-containing proteins accumulated in diaphragm 3 days after burn injury and were rapidly removed from the tissue during a 2-hour in vitro incubation. This coincided with increased proteolytic activity in diaphragm.

CONCLUSIONS

These observations suggest that the loss of proteins modified by reactive oxygen species may contribute to the burn-induced protein wasting in respiratory and other muscles by a proteolytically driven mechanism.

Identification of a 28 kDa secretory protein from rat olfactory epithelium as a thiol-specific antioxidant

The 28 kDa secretory protein is one of the abundant water-soluble proteins in olfactory epithelium of mammals. Analysis of partial amino acid sequence of the 28 kDa protein strongly suggested that it belongs to a new family of highly conserved antioxidant proteins requiring thiol for their antioxidant activity (TSA/AhpC family). In the present study, we found the 28 kDa protein to have thiol-dependent antioxidant activity, thereby protecting radical-sensitive proteins such as glutamine synthetase and hemoglobin from oxidative modification caused by thiol-dependent metal ion-catalyzed oxidation system. The purified 28 kDa protein did not possess catalase or glutathione peroxidase activities, and required thiols to exhibit its antioxidant activity. The 28 kDa protein is the first member of the family of thiol-specific antioxidants identified in olfactory epithelium and the first secretory protein shown to be thiol-specific antioxidant.

Characterization of membrane-bound serine protease related to degradation of oxidatively damaged erythrocyte membrane proteins

It has been shown that erythrocyte membrane proteins become susceptible to degradation by membrane-bound serine protease activity after oxidative modification of the membranes (M. Beppu, M. Inoue, T. Ishikawa, K. Kikugawa, Biochim. Biophys. Acta 1196 (1994) 81-87). The aim of the present study was to clarify the presence of the serine protease in oxidized erythrocyte membranes and to characterize the selectivity of the enzyme to oxidized proteins. Human erythrocytes were oxidized in vitro with xanthine/xanthine oxidase/Fe(III) and oxidized membranes isolated. Proteolytic activity of the membranes toward spectrin obtained from oxidized membranes and bovine serum albumin oxidized with H2O2/horseradish peroxidase was increased by membrane oxidation, and the degradability of the substrates was increased by substrate oxidation. The proteolytic activity was inhibited by the serine protease inhibitor diisopropyl fluorophosphate (DFP). The 72 kDa and 80 kDa proteins in the membranes were labeled by [3H]DFP when detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions and subsequent fluorography. The 72 kDa protein was found to be a serine enzyme, acetylcholine esterase. The 80 kDa protein appeared to be responsible for the degradation of oxidatively damaged proteins. The 80 kDa protein was loosely bound to membranes and readily solubilized into a 0.1% NP-40 detergent solution. The presence of the same 80 kDa protease in intact erythrocyte cytosol was suggested. The increased serine protease activity in oxidized membranes can result from the increased adherence of the cytosolic 80 kDa serine protease to the membranes due to oxidation.

SP-22 is a thioredoxin-dependent peroxide reductase in mitochondria

SP-22 is a mitochondrial antioxidant protein in bovine adrenal cortex. The protein is homologous to thioredoxin peroxidase and other antioxidant proteins. It protects radical-sensitive enzymes from oxidative damage by a radical-generating system (Fe2+/dithiothreitol) in the presence of a small amount of serum. In this study we purified a second mitochondrial protein with Mr 11,777, which cooperates with SP-22 to protect glutamine synthetase and other proteins from Fe2+/dithiothreitol-mediated damage. Without SP-22, the protein had no protecting activity. We determined amino acid and nucleotide sequences of the protein and its cDNA, respectively, and found that it was a protein of the thioredoxin family. The protein, designated as mt-Trx (mitochondrial thioredoxin), had a presequence composed of 59 amino acids that seemed to be a mitochondrial targeting signal. Mitochondrial extract prepared from adrenal cortex contained NADPH-dependent 5,5'dithiobis(2-nitrobenzoic acid) (Nbs2) reductase activity. The enzyme was thought to have thioredoxin reductase activity, since the Nbs2-reducing activity was stimulated by mt-Trx. We partially purified the Nbs2 reductase from bovine adrenocortical mitochondria. In the presence of the partially purified reductase, mt-Trx, and NADPH, SP-22 showed the activity to protect oxyhemoglobin against ascorbate-induced damage. Furthermore, with the three protein components (Nbs2 reductase, mt-Trx, and SP-22) NADPH was oxidized in the presence of hydrogen peroxide or tert-butyl hydroperoxide. The oxidation of NADPH was concomitant with the disappearance of an equimolar amount of hydrogen peroxide. Without any one of the protein components no hemoglobin-protecting and peroxide-dependent NADPH-oxidizing activities were observed. From these results we concluded that SP-22 is thioredoxin-dependent peroxide reductase or so-called thioredoxin peroxidase in mitochondria from the adrenal cortex.

Degradation of glycated hemoglobin. Role of erythrocytic proteolytic enzymes and oxidant damage

Glycated hemoglobin can be degraded by proteolytic enzyme(s) in the erythrocyte. The enzyme(s) co-elutes with glycated hemoglobin when the latter is separated from erythrocyte lysates using the cation-exchanger Bio Rex-70. A further purification of the Bio Rex eluant on DEAE Sephadex A-50 separated the enzyme(s) from glycated hemoglobin. Studies with the Bio Rex eluant showed that degradation of glycated hemoglobin is maximum at 37 degrees C at pH 8.6. Proteolytic degradation is inhibited by 5 mM N-ethylmaleimide (NEM), 5 mM ethylenediamine tetraacetic acid (EDTA) and 0.6 mM n-p-tosyl-L-lysine choromethyl ketone (TLCK) (100-87 and 76% inhibition respectively). This study also examines the possibility that oxidative-damage to glycated hemoglobin increases its susceptibility to proteolytic degradation. When incubated with various anti-oxidants like DTPA, uric acid, mannitol and butylated hydroxy toluene (BHT), proteolytic degradation of glycated hemoglobin decreased by 66.1, 50.7 and 38% respectively.

Ubiquitin C-terminal hydrolase is an immediate-early gene essential for long-term facilitation in Aplysia

The switch from short-term to long-term facilitation of the synapses between sensory and motor neurons mediating gill and tail withdrawal reflexes in Aplysia requires CREB-mediated transcription and new protein synthesis. We isolated several downstream genes, one of which encodes a neuron-specific ubiquitin C-terminal hydrolase. This rapidly induced gene encodes an enzyme that associates with the proteasome and increases its proteolytic activity. This regulated proteolysis is essential for long-term facilitation. Inhibiting the expression or function of the hydrolase blocks induction of long-term but not short-term facilitation. We suggest that the enhanced proteasome activity increases degradation of substrates that normally inhibit long-term facilitation. Thus, through induction of the hydrolase and the resulting up-regulation of the ubiquitin pathway, learning recruits a regulated form of proteolysis that removes inhibitory constraints on long-term memory storage.

Hydrogen peroxide-mediated alteration of the heme prosthetic group of metmyoglobin to an iron chlorin product: evidence for a novel oxidative pathway

Treatment of metmyoglobin with H2O2 is known to lead to the crosslinking of an active site tyrosine residue to the heme [Catalano, C. E., Y. S. Choe, and P. R. Ortiz de Montellano (1989) J. Biol. Chem. 264, 10534-10541]. We have found in this study that this reaction also leads to an altered heme product not covalently bound to the protein. This product was characterized by visible absorption, infrared absorption, and mass and NMR spectrometry as an iron chlorin product formed from the saturation of the double bond between carbon atoms at positions 17 and 18 of pyrrole ring D with concomitant addition of a hydroxyl group on the carbon atom at position 18 and lactonization of the propionic acid to the carbon atom at position 17. Studies with the use of (18)O-labeled H2O2, O2, and H2O clearly indicate that the source of the added oxygen on the heme is water. Evidently, water adds regiospecifically to a cationic site formed on a carbon atom at position 18 after oxidation of the ferric heme prosthetic group with peroxide. Prolonged incubation of the reaction mixture containing the iron hydroxychlorin product led to the formation of an iron dihydroxychlorin product, presumably from a slow addition of water to the initial iron hydroxychlorin. The iron chlorin products characterized in this study are distinct from the meso-oxyheme species, which is thought to be formed during peroxide-mediated degradation of metmyoglobin, cytochrome P450, ferric heme, and model ferric hemes, and give further insight into the mechanism of H2O2-induced heme alterations.

Importance of the ATP-ubiquitin-proteasome pathway in the degradation of soluble and myofibrillar proteins in rabbit muscle extracts

Recent studies have suggested that activation of the ubiquitin-proteasome pathway is primarily responsible for the rapid loss of muscle proteins in various types of atrophy. The present studies were undertaken to test if different classes of muscle proteins are degraded by this pathway. In extracts of rabbit psoas muscle, the complete degradation of soluble proteins to amino acids was stimulated up to 6-fold by ATP. Peptide aldehyde inhibitors of the proteasome or the removal of proteasomes markedly inhibited only the ATP-dependent process. Addition of purified myosin, actin, troponin, or tropomyosin to these extracts showed that these proteins served as substrates for the ubiquitin-proteasome pathway. By contrast, degradation of myoglobin did not require ATP, proteasomes, or any known proteases in muscles. When myosin, actin, and troponin were added as actomyosin complexes or as intact myofibrils to these extracts, they were not hydrolyzed at a significant rate, probably because in these multicomponent complexes, these proteins are protected from degradation. Accordingly, actin (but not albumin or troponin) inhibited the degradation of 125I-myosin, and actin was found to selectively inhibit ubiquitin conjugation to 125I-myosin. Also, the presence of tropomyosin inhibited the degradation of 125I-troponin. However, neither actin nor tropomyosin inhibited the degradation of 125I-lysozyme or soluble muscle proteins. Thus, specific interactions between the myofibrillar proteins appear to protect them from ubiquitin-dependent degradation, and the rate-limiting step in their degradation is probably their dissociation from the myofibril.

Role of different proteolytic pathways in degradation of muscle protein from streptozotocin-diabetic rats

In vitro rats of overall proteolysis and the activities of four different proteolytic pathways (lysosomal, Ca2+ dependent, ATP dependent, and ATP independent), as well as rates of protein synthesis, were measured in soleus and extensor digitorum longus (EDL) muscles from streptozotocin-diabetic rats. In the acute phase (1-3 days) of diabetes, there was an increase in overall proteolysis that coincided with an increased activity of the Ca(2+)-dependent pathway in both soleus and EDL and of the ATP-dependent pathway in EDL. After longer periods (5-10 days) of diabetes, the overall rate of protein degradation decreased and reached values similar to or even lower than those of controls as a result of a reduction in the activities of Ca(2+)-dependent and ATP-dependent pathways. No change was detected at any time interval in the activity of the intralysosomal proteolytic system in muscles from diabetic animals. Rates of protein synthesis were already reduced 24 h after diabetes induction and decreased further thereafter. Insulin treatment restored to normal the activities of the proteolytic pathways and rates of protein synthesis.

Degradation of oxidized proteins in K562 human hematopoietic cells by proteasome

Exposure to various forms of oxidative stress (H2O2 and O2.-) significantly increased the intracellular degradation of both "short-lived" and "long-lived" cellular proteins in the human hematopoietic cell line K562. Oxidatively modified hemoglobin and superoxide dismutase used as purified proteolytic substrates were also selectively degraded by K562 cell lysates, but exposure of these protein substrates to very high hydrogen peroxide concentrations actually decreased their proteolytic susceptibility. Our studies found little or no change in the overall capacity of cells and cell lysates to degrade "foreign" oxidized proteins after treatment of K562 cells with hydrogen peroxide or paraquat, a finding supported by proteasome Western blots and unchanged capacity of cell lysates to degrade the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-4-methylcoumarin-7-amide. Six days of daily treatment of K562 cells with an antisense oligodeoxynucleotide directed against the initiation codon region of the human proteasome C2 subunit gene dramatically depressed hydrogen peroxide-induced degradation of metabolically radiolabeled intracellular proteins. The actual amount of proteasome in antisense-treated K562 cells was also severely depressed, as revealed by Western blots and by measurements of the degradation of the fluorogenic peptide succinyl-leucine-leucine-valine-tyrosine-4-methylcoumarin-7-amide. The degradation of oxidatively modified foreign protein substrates was also markedly depressed in lysates prepared from K562 cells treated with the proteasome C2 antisense dideoxynucleotide. The inhibitor profile for the degradation of H2O2-modified hemoglobin by K562 cell lysates was consistent with a major role for the ATP-independent 20 S "core" proteasome complex. We conclude that proteasome, probably the 20 S core proteasome complex, is primarily responsible for the selective degradation of oxidatively damaged proteins in human hematopoietic cells. Since "oxidative marking" of cellular proteins by lipoxygenase has been proposed as an important step in red blood cell maturation, it is important to determine which protease or proteases could recognize and degrade such modified substrates. Our results provide evidence that proteasome can, indeed, conduct such selective degradation and appears to be the major cellular protease capable of fulfilling such a role in maturation.

Regulatory features of multicatalytic and 26S proteases

It should be clear from the foregoing accounts that our understanding of MCP and 26S regulation is still rudimentary. Moreover, we have only recently identified about a dozen natural substrates of these two proteases. Those outside the field may view the situation with some dismay. Those who study the MCP and 26S enzymes are provided with rich opportunities to address fundamental questions of protein catabolism and metabolic regulation.

Covalent crosslinking of the heme prosthetic group to myoglobin by H2O2: toxicological implications

It is known that treatment of myoglobin with H2O2 leads to covalent alteration of the heme prosthetic group with concomitant formation of a protein bound heme adduct and transforms myoglobin from an oxygen storage protein to an oxidase. In the current study it was shown, with the use of 14C-labeled heme reconstituted into apomyoglobin, that up to 88% of the oxidatively altered heme can be accounted for by the protein bound product. Furthermore, a partially purified preparation of the protein bound heme adduct was introduced into human fibroblasts using the method of osmotic lysis of pinosomes and found to cause cell death (40%) within 1 h, as evidenced by trypan blue exclusion. Native myoglobin introduced into cells in the same manner or extracellular treatment by the protein bound heme adduct had no effect on cell viability. The extent of cell death could be decreased (50%) by N-acetyl-L-cysteine, indicating a potential role for reactive oxygen intermediates in this process. These results show that the covalently altered myoglobin can elicit cellular damage and suggests that similar processes may occur in vivo in pathologic conditions such as that involving cardiac ischemia and reperfusion injury, where covalently altered myoglobin may form.

Ubiquitin and the enigma of intracellular protein degradation

Contrary to widespread belief, the regulation and mechanism of degradation for the mass of intracellular proteins (i.e. differential, selective protein turnover) in vertebrate tissues is still a major biological enigma. There is no evidence for the conclusion that ubiquitin plays any role in these processes. The primary function of the ubiquitin-dependent protein degradation pathway appears to lie in the removal of abnormal, misfolded, denatured or foreign proteins in some eukaryotic cells. ATP/ubiquitin-dependent proteolysis probably also plays a role in the degradation of some so-called 'short-lived' proteins. Evidence obtained from the covalent modification of such natural substrates as calmodulin, histones (H2A, H2B) and some cell membrane receptors with ubiquitin indicates that the reversible interconversion of proteins with ubiquitin followed by concomitant functional changes may be of prime importance.

Degradation of differentially oxidized alpha-crystallins in bovine lens epithelial cells

There is a growing consensus that altered proteins are more susceptible to degradation than native proteins. The enhancement of degradation of damaged proteins may be of significance since it prevents the accumulation of damaged proteins in cells. Several proteolytic pathways have been discovered in the lens. These include ATP-independent, ATP-dependent and ATP/ubiquitin-dependent proteolytic pathways. However, the extent of involvement of these proteolytic pathways in degradation of damaged proteins is not well described. alpha-Crystallin was oxidized by exposure to 0.03-3.2 mol.OH (mol protein)-1. Modifications to the oxidized alpha-crystallin and proteolytic susceptibility of the oxidized alpha-crystallin were studied. Exposure to > 0.32 mol.OH per mole of subunit produced aggregates and fragments of alpha-crystallin. Changes in isoelectric points of the proteins were observed after exposure to 0.64 mol.OH (mol protein)-1. The extent of loss of tryptophan and sulfhydryl groups was related to the level of .OH-exposure. Carbonyl content increased progressively with increasing oxidation. When incubated with a supernatant of bovine lens epithelial cells, the .OH-modified proteins were proteolytically degraded up to three times faster than untreated alpha-crystallin. ATP stimulated the degradation of native alpha-crystallin and alpha-crystallin which was exposed to 1.6 mol.OH (mol subunit protein)-1 (alpha 1.6). Sixty-seven per cent and 100% of the ATP-dependent degradation of native alpha-crystallin and alpha 1.6 was ubiquitin-dependent, respectively. The data indicate that alpha-crystallins oxidized by .OH are recognized and degraded rapidly by cytoplasmic proteolytic systems in bovine lens epithelial cells. Both ATP-independent and ATP/ubiquitin-dependent proteolytic pathways are involved in the degradation of native and oxidized alpha-crystallin.

Presence of membrane-bound proteinases that preferentially degrade oxidatively damaged erythrocyte membrane proteins as secondary antioxidant defense

Human erythrocytes were oxidized with xanthine/xanthine oxidase/ferric ion or ADP/ferric ion at 37 degrees C for several hours. Band 3 protein and spectrin of the oxidized cells were found to be significantly modified as analyzed by radiolabeling with tritiated borohydride. Sodium dodecylsulfate-polyacrylamide gel electrophoresis of the xanthine/xanthine oxidase/ferric iron-oxidized cells and subsequent immunoblotting with anti band 3 protein showed that band 3 protein was fragmented into smaller molecular-weight fragments. When the cell membrane obtained from the oxidized cells were incubated at pH 7.4 and 37 degrees C for several hours in the presence of alpha-tocopherol, extensive degradation of band 3 protein and spectrin was observed. Band 3 protein was found to be most susceptible to the degradation. Degradation of band 3 protein was also observed after similar incubation of the membrane from the ADP/ferric ion-oxidized cells. Membrane-bound serine- and metalloproteinases were responsible for the degradation of band 3 protein, because the degradation was remarkably inhibited by diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride, and partially by ethylenediaminetetraacetic acid. Hence, the membrane proteins became susceptible to membrane-bound proteinases by oxidative stress. This observation suggests that these membrane-bound proteinases exist to remove oxidatively damaged proteins from the cell membrane.