Oxidized proteins in erythrocytes are rapidly degraded by the adenosine triphosphate-dependent proteolytic system

Emerging concepts in the molecular cell biology and functions of mammalian erythrocytes

Erythrocytes, or red blood cells, are essential components of vertebrate blood, comprising approximately 45% of human blood volume. Their distinctive features, including small size, biconcave shape, extended lifespan (∼115 days), and lack of a nucleus or other membrane-bound organelles, make them unique among mammalian cell types. Traditionally regarded as passive carriers of oxygen and carbon dioxide, erythrocytes were long thought to function merely as hemoglobin-filled sacs, incapable of gene expression or roles beyond gas transport. However, advancements in molecular biology have revealed a more complex picture. Recent studies have identified various RNA types within erythrocytes, demonstrated globin mRNA translation, and uncovered miRNA-mediated defenses against Plasmodium infection. Beyond gas exchange, erythrocytes play critical roles in regulating regional blood flow via nitric oxide, contribute to innate immunity through toll-like receptors, transport amino acids between tissues, and maintain water homeostasis. Furthermore, emerging technologies have repurposed erythrocytes as drug-delivery vehicles, opening new avenues for therapeutic applications. This review highlights these recent discoveries and explores the expanding functional landscape of erythrocytes, shedding light on their multifaceted roles in physiology and medicine.

The role of proteasomes in tumorigenesis

Protein homeostasis is the basis of normal life activities, and the proteasome family plays an extremely important function in this process. The proteasome 20S is a concentric circle structure with two α rings and two β rings overlapped. The proteasome 20S can perform both ATP-dependent and non-ATP-dependent ubiquitination proteasome degradation by binding to various subunits (such as 19S, 11S, and 200 PA), which is performed by its active subunit β1, β2, and β5. The proteasome can degrade misfolded, excess proteins to maintain homeostasis. At the same time, it can be utilized by tumors to degrade over-proliferate and unwanted proteins to support their growth. Proteasomes can affect the development of tumors from several aspects including tumor signaling pathways such as NF-κB and p53, cell cycle, immune regulation, and drug resistance. Proteasome-encoding genes have been found to be overexpressed in a variety of tumors, providing a potential novel target for cancer therapy. In addition, proteasome inhibitors such as bortezomib, carfilzomib, and ixazomib have been put into clinical application as the first-line treatment of multiple myeloma. More and more studies have shown that it also has different therapeutic effects in other tumors such as hepatocellular carcinoma, non-small cell lung cancer, glioblastoma, and neuroblastoma. However, proteasome inhibitors are not much effective due to their tolerance and singleness in other tumors. Therefore, further studies on their mechanisms of action and drug interactions are needed to investigate their therapeutic potential.

Polyubiquitin Chains Linked by Lysine Residue 48 (K48) Selectively Target Oxidized Proteins In Vivo

Aims: Ubiquitin is a highly conserved protein modifier that heavily accumulates during the oxidative stress response. Here, we investigated the role of the ubiquitination system, particularly at the linkage level, in the degradation of oxidized proteins. The function of ubiquitin in the removal of oxidized proteins remains elusive because of the wide range of potential targets and different roles that polyubiquitin chains play. Therefore, we describe in detail the dynamics of the K48 ubiquitin response as the canonical signal for protein degradation. We identified ubiquitin targets and defined the relationship between protein ubiquitination and oxidation during the stress response. Results: Combining oxidized protein isolation, linkage-specific ubiquitination screens, and quantitative proteomics, we found that K48 ubiquitin accumulated at both the early and late phases of the stress response. We further showed that a fraction of oxidized proteins are conjugated with K48 ubiquitin. We identified ∼750 ubiquitinated proteins and ∼400 oxidized proteins that were modified during oxidative stress, and around half of which contain both modifications. These proteins were highly abundant and function in translation and energy metabolism. Innovation and Conclusion: Our work showed for the first time that K48 ubiquitin modifies a large fraction of oxidized proteins, demonstrating that oxidized proteins can be targeted by the ubiquitin/proteasome system. We suggest that oxidized proteins that rapidly accumulate during stress are subsequently ubiquitinated and degraded during the late phase of the response. This delay between oxidation and ubiquitination may be necessary for reprogramming protein dynamics, restoring proteostasis, and resuming cell growth.

Determinants and Regulation of Protein Turnover in Yeast

Protein turnover maintains the recycling needs of the proteome, and its malfunction has been linked to aging and age-related diseases. However, not all proteins turnover equally, and the factors that contribute to accelerate or slow down turnover are mostly unknown. We measured turnover rates for 3,160 proteins in exponentially growing yeast and analyzed their dependence on physical, functional, and genetic properties. We found that functional characteristics, including protein localization, complex membership, and connectivity, have greater effect on turnover than sequence elements. We also found that protein turnover and mRNA turnover are correlated. Analysis under nutrient perturbation and osmotic stress revealed that protein turnover highly depends on cellular state and is faster when proteins are being actively used. Finally, stress-induced changes in protein and transcript abundance correlated with changes in protein turnover. This study provides a resource of protein turnover rates and principles to understand the recycling needs of the proteome under basal conditions and perturbation.

REGγ Contributes to Regulation of Hemoglobin and Hemoglobin δ Subunit

Hemoglobin (Hb) is a family of proteins in red blood cells responsible for oxygen transport and vulnerable for oxidative damage. Hemoglobin δ subunit (HBD), a member of Hb family, is normally expressed by cells of erythroid lineage. Expression of Hb genes has been previously reported in nonerythroid and hematopoietic stem cells. Here, we report that Hb and HBD can be degraded via REGγ proteasome in hemopoietic tissues and nonerythroid cells. For this purpose, bone marrow, liver, and spleen hemopoietic tissues from REGγ^+/+ and REGγ^−/− mice and stable REGγ knockdown cells were evaluated for the degradation of Hb and HBD via REGγ. Western blot and immunohistochemical analyses exhibited downregulation of Hb in REGγ wild-type mouse tissues. This was validated by dynamic analysis following blockade of de novo synthesis of proteins with CHX. Degradation of HBD only occurred in REGγ WT cells but not in REGγN151Y, a dominant-negative REGγ mutant cell. Notably, downregulation of HBD was found in HeLa shN cells with stimulation of phenylhydrazine, an oxidation inducer, suggesting that the REGγ proteasome may target oxidatively damaged Hbs. In conclusion, our findings provide important implications for the degradation of Hb and HBD in hemopoietic tissues and nonerythroid cells via the REGγ proteasome.

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.

Activation of proteasome by insulin-like growth factor-I may enhance clearance of oxidized proteins in the brain

The insulin-like growth factor type 1 (IGF-I) plays an important role in neuronal physiology. Reduced IGF-I levels are observed during aging and this decrease may be important to age-related changes in the brain. We studied the effects of IGF-I on total protein oxidation in brain tissues and in cell cultures. Our results indicate that in frontal cortex the level of oxidized proteins is significantly reduced in transgenic mice designed to overproduce IGF-I compared with wild-type animals. The frontal cortex of IGF-I-overproducing mice exhibited high chymotrypsin-like activity of the 20S and 26S proteasomes. The proteasome can also be activated in response to IGF-I in cell cultures. Kinetic studies revealed peak activation of the proteasome within 15 min following IGF-I stimulation. The effects of IGF-I on proteasome were not observed in R(-) cells lacking the IGF-I receptor. Experiments using specific kinase inhibitors suggested that activation of proteasome by IGF-I involves phosphatidyl inositol 3-kinase and mammalian target of rapamycin signaling. IGF-I also attenuated the increase in protein carbonyl content induced by proteasome inhibition. Thus, appropriate levels of IGF-I may be important for the elimination of oxidized proteins in the brain in a process mediated by activation of the proteasome.

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 degrees 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 degrees 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.

Short-term mental activation accelerates the age-related decline of high-energy phosphates in rat cerebral cortex

Aging in rats is associated with a significant decline in brain levels of energy-rich phosphates, including ATP and creatine phosphate. To test the effects of transient mental activity mediated by psychometric testing, and of metabolic inhibition of pyruvate dehydrogenase (PDH) (an enzyme complex that generates acetyl coenzyme A (CoA) to feed the mitochondrial tricarboxylic acid cycle), we compared adult (52 to 64-week-old) and aged (104-week-old) rats with and without intracerebral injections of the PDH inhibitor, bromopyruvate (BP), in the presence and in the absence of extensive psychometric testing by standard passive avoidance and hole board test paradigms. As compared with mental rest, short-term mental activation was associated with higher levels of energy-rich phosphates in the cerebral cortex of both adult and aged animals, but did not prevent the age-dependent decline in these phosphates. ATP turnover was markedly increased by mental activity, but was less pronounced in aged animals. In the hippocampus, less marked changes in the energy pool became obvious. The abnormalities in energy metabolism indicate an age-dependent and stress-accentuated reduction of the capacity to meet such energy-dependent demands as mixed function oxidation in the aged brain. BP did not change brain levels of energy-rich phosphates, indicating that the damage caused by decreased PDH activity can be compensated for both in adult and in aged animals.

Endothelial dysfunction and activation as an expression of disease: role of prostacyclin analogs

The endothelium is now considered a real endocrine-paracrine organ, important not only as a structural barrier between the circulation and surrounding tissue, but also because it plays an essential role for local hemodynamics, releasing substances that modulate the vascular calibre and blood cell activation. Here, after a brief but detailed analysis of the importance of the endothelium in vascular homeostasis, in the control of coagulation and in the relations with the different blood cells, we will explain the concept of endothelial dysfunction (altered NO release) and activation (amplified adhesion molecule expression) in inflammatory, connective tissue and post-trasplantation diseases. Furthermore, this review will focus on the activity of prostacyclin and synthetic analogs, especially their ability to interact with the vasodilatation system and their role in modulating cell interaction by surface adhesion molecule expression, cytokines and growth factors release as well as gene transcription factors. Finally, we will consider the therapeutic role of prostacyclin analogs in the prevention and treatment of connective tissue diseases.

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.

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.

Studies on the effects of monosodium glutamate (MSG) on oxidative stress in erythrocytes of adult male mice

Subcutaneous administration of monosodium glutamate (MSG) to normal adult male mice for 6 days at dose levels of 4 and 8 mg/g body weight caused a significant increase in erythrocyte glucose content accompanied by increased lipid peroxidation. The levels of total glutathione and protein-bound glutathione were significantly increased in the erythrocytes, whereas non-protein glutathione was significantly decreased. The administration of 4 and 8 mg/g body weight of MSG significantly increased the activities of glutathione reductase (GR), glutathione-S-transferase (GST) and glutathione peroxidase (GPX). It was observed that MSG, above 4 mg/g body weight, produced oxidative stress which was counteracted by the body by maintaining the level of glutathione, which was done by increasing the activity of its metabolizing enzymes.

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.

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.

Oxidative stress and recovery from oxidative stress are associated with altered ubiquitin conjugating and proteolytic activities in bovine lens epithelial cells

Roles for ubiquitin (an 8.5 kDa polypeptide) involve its conjugation to proteins as a signal to initiate degradation and as a stress protein. We investigated ubiquitin conjugation and ubiquitin-dependent proteolytic activities in cultured bovine lens epithelial cells (BLECs) upon oxidative challenge. A 44% decrease in intracellular glutathione confirmed oxidative stress upon incubation with 1 mM H2O2. After 30 min incubation, endogenous high-molecular-mass ubiquitin conjugates decreased 73%, and intracellular proteolysis decreased about 50%. In the supernatants of the oxidatively treated BLECs, the ability to form high-molecular-mass ubiquitin conjugates with exogenous 125I-labelled ubiquitin decreased 28%, and ATP-dependent degradation of oxidized alpha-crystallin decreased 36%. When the H2O2-treated BLECs were allowed to recover for 60 min, intracellular proteolysis returned to the level of control cells. There was also a subsequent transient enhancement of intracellular proteolysis and a simultaneous recovery of endogenous high-molecular-mass ubiquitin conjugates. In parallel cell-free experiments, conjugating activity with exogenous 125I-labelled ubiquitin and ATP-dependent degradation of oxidized alpha-crystallin increased 35% and 72% respectively compared with non-oxidatively treated BLECs. ATP-independent proteolysis showed little response to exposure or removal of H2O2. These results indicate that (1) the rate of intracellular proteolysis in BLECs is associated with the level of endogenous high-molecular-mass ubiquitin conjugates and (2) oxidative stress may inactivate the ubiquitin conjugation activity with coordinate depression of proteolytic capability. Enhancement in ubiquitin conjugation and proteolytic activities during recovery from oxidative stress may be important in removal of damaged proteins and restoration of normal function of BLECs. The inactivation of ubiquitin-dependent proteolysis by oxidation may be involved in the accumulation of altered proteins and other adverse sequelae in the oxidatively challenged aging lens.

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.

Effect of maternal diabetes and dietary copper on fetal development in rats

To test whether diabetes associated alterations in copper metabolism contribute to diabetes-induced teratogenicity in rats, pregnancy outcome was compared between diabetic and nondiabetic rats fed either a copper adequate (12 micrograms/g diet) or low copper diet (1 microgram/g diet). The dietary regimen was begun two weeks prior to mating and continued throughout pregnancy. To facilitate the reduction of maternal copper stores in the low copper groups, the low copper diet was supplemented with a copper chelator, triethylenetetraamine, at 1% for one week; the chelator was removed from the diet one week prior to mating. Pregnancy was terminated on gestation day 20. Maternal and fetal tissues were assessed for copper concentrations, the activities of the cuproenzymes copper, zinc superoxide dismutase and ceruloplasmin, and the copper binding protein metallothionein. Dams fed the low copper diet had low tissue copper concentrations, and low plasma ceruloplasmin and erythrocyte superoxide dismutase activities compared to copper-adequate dams. Fetuses in the low copper groups were characterized by low liver copper concentrations. Gross structural and skeletal anomalies were only observed in the diabetic groups; maternal copper intake did not influence the frequency of these anomalies. However, fetuses in the low-copper nondiabetic group, and both diabetic groups, were characterized by low liver copper, zinc superoxide dismutase activity suggesting that fetal copper metabolism was influenced by both copper intake and diabetes.

Enhanced proteolysis and changes in membrane-associated calpain following phenylhydrazine insult to human red cells

Phenylhydrazine-mediated protein damage in human red cells has been assessed using HPLC, one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and immunoblot analysis of major membrane proteins. The association of the Ca(2+)-activated neutral protease, calpain, with membrane proteins following hydrazine insult was also examined using immunoblot analysis. HPLC amino acid analysis of red cell suspensions was employed to quantify proteolysis. Phenylhydrazine (4 mM) increased the rate of leucine, lysine, and histidine release by approximately 12-, 7-, and 5-fold, respectively. N-acetylcysteine (20 mM), dithiothreitol (50 mM), and dimethylthiourea (50 mM) decreased the rate of phenylhydrazine-stimulated amino acid release by approximately 30-50%; in contrast, the free radical scavengers and antioxidants dimethylfuran (50 mM) and dimethyl sulfoxide (50 mM) were without significant effect. The calcium chelator, EGTA (10 mM) inhibited phenylhydrazine-stimulated proteolysis by approximately 30%. Phenylhydrazine (4 mM) caused attenuation of the major membrane protein bands present in the SDS-PAGE pattern and extensive smearing of a band in the region of approximately 28 kDa. Free radical scavengers and antioxidants failed to ameliorate significantly membrane protein damage in phenylhydrazine-treated cells as judged by SDS-PAGE. Immunoblot analysis of spectrin confirmed these results. Two-dimensional SDS-PAGE of membrane proteins following phenylhydrazine treatment, however, revealed the appearance of new protein spots and a loss of existing protein spots as compared to control. Western blot analysis of membrane-associated calpain (79 kDa (proenzyme), 77- and 75-kDa forms) was also performed. Phenylhydrazine-treated red blood cells exhibited concentration- and time-dependent changes in the level of membrane-associated procalpain relative to control. The inhibitors N-acetylcysteine, dithiothreitol, dimethylthiourea, and dimethyl sulfoxide in the presence of phenylhydrazine appeared to preserve the level of procalpain in association with the membrane proteins, but only N-acetylcysteine and dithiothreitol protected the 77- and 75-kDa forms. In contrast, dimethylfuran in the presence of phenylhydrazine caused a substantial decrease in all three forms of membrane-associated calpain. In phenylhydrazine-treated hemolysate, the level of the 77- and 75-kDa forms of membrane-associated calpain was decreased relative to control. These forms were absent when EGTA (10 mM) was included in the incubation and the level of proenzyme was decreased. These data suggest that calpain is recruited to the membrane following hydrazine insult, undergoes a Ca(2+)-dependent conversion to the active forms, and may be involved in the degradation of damaged cytosolic and membrane protein(s).

Purification of a protease in red blood cells that degrades oxidatively damaged haemoglobin

Haemoglobin damaged by exposure of red blood cells to oxidants is rapidly degraded by a proteolytic pathway which does not require ATP [Fagan, Waxman & Goldberg (1986) J. Biol. Chem. 261, 5705-5713]. By fractionating erythrocyte lysates, we have purified two proteases which hydrolyse oxidatively damaged haemoglobin (Ox-Hb). One protease hydrolysed small fluorogenic substrates in addition to Ox-Hb. Its molecular mass was approximately 700 kDa and it consisted of several subunits ranging in size from 22 to 30 kDa. This enzyme may be related to the high-molecular-mass multicatalytic proteinase previously isolated from a variety of tissue and cell types. The other Ox-Hb-degrading activity had an apparent molecular mass of 400 kDa on gel filtration, a subunit size of 110 kDa and an isoelectric point between 4.5 and 5.0. This protease also hydrolysed the small polypeptides insulin and glucagon, as well as other large proteins such as lysozyme. Insulin blocked the degradation of Ox-Hb and Ox-Hb blocked the hydrolysis of insulin by the purified protease. Thiol reagents and metal chelators strongly inhibited the hydrolysis of both Ox-Hb and insulin, whereas inhibitors of serine, aspartic and thiol proteases had little effect. These properties suggest that the Ox-Hb-degrading activity purified from rabbit erythrocytes is the cytosolic insulin-degrading enzyme that is believed to play a role in the metabolism of insulin in several tissues. We propose that this enzyme may also function as a key component in a cytoplasmic degradative pathway responsible for removing proteins damaged by oxidants.

Identification of a ubiquitin- and ATP-dependent protein degradation pathway in rat cerebral cortex

To investigate the existence of a ubiquitin-dependent protein degradation system in the brain, the proteolytic activity of the cerebral cortex was examined. The soluble extract of rat cerebral cortex degraded 125I-radiolabeled lysozyme in an ATP-dependent manner. The ATP-dependent proteolysis was suppressed with iodoacetamide, which inhibits ubiquitin conjugation, and was abolished by blocking of the amino residues of lysozyme. These results suggest the participation of ubiquitination in the proteolytic activity. An ATP-dependent 125I-ubiquitin-conjugating activity was detected in fraction II from the cerebral cortex. The presence of ATP-dependent proteolytic activity which acted preferentially on ubiquitinated lysozyme was demonstrated, using ubiquitin-125I-lysozyme conjugates as a substrate. The proteinase had a molecular mass of 1500 kDa and displayed nucleotide dependence and sensitivity to various proteinase inhibitors similar to those of the 26S proteinase complex found in reticulocytes. Dialysis of the soluble fraction caused a decrease in the proteolytic activity of ATP-dependent and preferential for ubiquitin-lysozyme conjugates and a reciprocal increase in the ATP-independent free 125I-lysozyme-degrading activity which was scarcely detected before dialysis. The former ATP-dependent proteolytic activity may play a physiological role in the brain.

Ubiquitin-mediated processes in erythroid cell maturation

Response of the ATP, ubiquitin-dependent system during the enhanced degradation of erythrocyte maturation conforms to the general regulatory features common to several similar but unrelated systems. In erythroid cells enhanced degradation follows three phases: (1) Onset of degradation characterized by an increase in the intracellular concentration of free and conjugated ubiquitin, brought about by reduction in mean cell volume; (2) Active enhanced degradation during cellular remodeling; and (3) Loss of activity as a consequence of spontaneous inactivation of components required for ubiquitin conjugation. The extent of degradative remodeling is probably functionally limited by the loss of these critical ligation enzymes.

Modulation of mannose receptor activity by proteolysis

Macrophages express a receptor on the cell surface that functions to clear glycoproteins from the extracellular milieu. The activity of this receptor is sensitive to treatment with trypsin. In inflammatory situations, macrophages are activated and exposed to increased levels of extracellular proteases. Under these conditions, mannose receptor activity on the macrophages is diminished. We therefore decided to study the effects of trypsin treatment on the structure and activity of cell-associated and purified receptor that might contribute to the activation-associated receptor down-regulation. Trypsin treatment (1 microgram/ml for 3 h) resulted in the production of a 140 kDa, trypsin-resistant fragment from both intact cells and isolated receptor. This fragment was no longer able to bind ligand. The remaining 35 kDa fragment apparently is further degraded into smaller fragments, since no evidence of this domain was found on Coomassie Blue-stained gels. The 140 kDa fragment retained immunoreactivity and contained at least a portion of the iodinated tyrosine residues following surface labelling with Na125I. Neither calcium nor ligand protected the receptor from proteolysis. In addition, prior treatment with oxidants did not increase the susceptibility of the receptor to trypsin digestion. We conclude from these results that the macrophage mannose receptor is clipped by the serine protease trypsin at the cell surface, resulting in the release and further degradation of the binding domain, and the production of a membrane-associated 140 kDa fragment. This trypsin-mediated down-regulation of receptor activity might be important in controlling glycoprotein clearance during inflammation.

Increased susceptibility of ribulose-1,5-bisphosphate carboxylase/oxygenase to proteolytic degradation caused by oxidative treatments

The susceptibility of the chloroplastic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase to proteolysis by trypsin, chymotrypsin, proteinase K, and papain is enhanced by oxidative treatments including spontaneous oxidation of cysteines. Proteinases exhibit a high specificity for the oxidized inactive form of the carboxylase, cleaving its large subunit. Treatment of the inactive enzyme with dithiothreitol results in partial recovery of both carboxylase activity and resistance to proteolysis. This behavior may explain the specific degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase that occurs in vivo during leaf senescence.

Erythrocyte endogenous proteinase activity during blood bank storage

We studied proteolytic alterations of membrane proteins in ghosts derived from human red blood cells, preserved up to 35 days in the liquid state either as whole blood or with additive solution. The study was carried out by performing sodium dodecyl sulfate polyacrylamide gel electrophoresis of stromal proteins from erythrocytes, either previously treated with proteinase inhibitors or previously incubated in conditions promoting proteolysis. To differentiate the effect of erythrocyte from granulocyte proteinases, the investigation was also carried out in leukocyte-free red cell preparations. The results show: (1) the effects of endogenous proteinases on membrane proteins derived from red cells stored under blood bank conditions; (2) a decrease of proteolytic effects in ghosts derived from red cells which have been submitted to a longer storage; (3) a relevant influence of the red cell resuspending medium before lysis on the time-dependent onset and exhaustion of proteolysis in ghosts. The presence of increased proteolysis in ghosts could be regarded as a marker of molecular lesions induced in red cells by storage under blood bank conditions.

Subcellular distribution of lipoxygenase products in rabbit reticulocyte membranes

Mitochondrial membranes and plasma membranes of rabbit reticulocytes contain oxygenated polyenoic fatty acids such as (9Z,11E)-(13S)-13-hydroxy-9,11-octadecadienoic acid, 9S and 9R isomers of (10E,12Z)-9-hydroxy-10,12-octadecadienoic acid and their all-E isomers. Furthermore (5Z,8Z,11Z,13E)-(15S)-15-hydroxy-5,8,11,13-icosa tetraenoic acid, 9- and 13-oxooctadecadienoic acid were detected as minor products. The chemical structure of these products has been identified by co-chromatography with authentic standards, by ultraviolet and infrared spectroscopy, and by gas chromatography/mass spectrometry of the native compounds and their hydrogenated derivatives. The oxygenated fatty acids originate most probably from the intracellular action of the erythroid arachidonate 15-lipoxygenase. In membranes of the mature erythrocyte only small amounts of hydroxy fatty acids were detected. Young peripheral reticulocytes contain more oxygenated polyenoic fatty acids in their membranes than older cells. In mixed cell populations, about 85% of the lipoxygenase products were found esterified to the membrane ester lipids, whereas 15% were associated as free hydroxy fatty acids with the membranes. The hydroxy fatty acid content of the mitochondrial membranes is more than threefold higher than that of the plasma membranes. The pattern of the products isolated from plasma membranes shows a high specificity with (9Z,11E)-(13S)-13-hydroxy-9,11-octadecadienoic acid as the main product. In contrast, the pattern found in the mitochondrial membranes was much more unspecific: a complex mixture of all positional and optical isomers was detected. The data presented indicate that the reticulocyte lipoxygenase in vivo acts on both plasma membranes and mitochondrial membranes. The results are discussed in the light of the involvement of the lipoxygenase in the breakdown of mitochondria and other organelles in reticulocytes during maturation.

Oxidant-induced haemoprotein degradation in rat tissue slices: effect of bromotrichloromethane, antioxidants and chelators

Haemoprotein degradation and lipid peroxidation were evaluated in rat liver, kidney and heart slices incubated for 2 h in the presence and absence of bromotrichloromethane, antioxidants and chelators to obtain information about the relationship between oxidants and damage to haemoproteins. Haemoproteins were modified by bromotrichloromethane, and this modification, measured as loss of ferrohaemoproteins, generally was concurrent with lipid peroxidation measured as thiobarbituric acid-reactive substances. These two processes occurred simultaneously as a function of incubation time and oxidant concentration. Inhibition of the two processes by nordihydroguaiaretic acid, butylated hydroxyanisole and Trolox C, and lack of inhibition by mannitol, catalase and superoxide dismutase also were coincident. However, Methylene blue, EDTA, sodium fluoride, 2,4-dinitrophenol, N-ethylmaleimide and o-phenanthroline affected the two processes differently. The results suggested that haemoproteins may compete with other molecules for oxidant radicals, thus serving as protectors of cells against oxidant radicals. Products of haemoprotein degradation such as protein polymers, free amino acids and bilirubin may be indicators of in vivo oxidative stress.

Oxidant-increased proteolysis in rat liver slices: effect of bromotrichloromethane, antioxidants and effectors of proteolysis

Proteolysis and lipid peroxidation were evaluated in rat liver slices incubated in the presence of the oxidant bromotrichloromethane and effectors of proteolysis. Proteolysis was evaluated by S-amino acids and lipid peroxidation by thiobarbituric acid-reactive substances (TBARS) released into the incubation medium. The increased release of S-amino acids by BrCl3C depended on incubation time and oxidant concentration. S-Amino acid release increased 30% over control value and TBARS increased from 22 to 124 nmol/g liver by incubation for 120 min with 1 mM BrCl3C. Release of S-amino acids and TBARS was decreased when liver slices were treated with nor-dihydroguaiaretic acid (NDG), butylated hydroxyanisole (BHA), Trolox C, or N,N'-diphenyl-1,4-phenylenediamine (DPPD) immediately prior to addition of oxidant, suggesting participation of lipid-soluble free radicals. Oxidant-induced release of S-amino acids but not of TBARS was decreased by mannitol, suggesting participation of hydroxyl radical or a species with similar reactivity; and by superoxide dismutase and catalase, suggesting participation of superoxide and hydrogen peroxide, respectively. The decrease of S-amino acid release by sodium fluoride, sodium arsenate, 2,4-dinitrophenol, chloroquine, leupeptin, phenylmethylsulfonyl fluoride, EDTA and o-phenanthroline was variable, suggesting the presence in liver of several proteases to remove oxidatively-modified proteins.

Hyperthermia, unlike ionizing radiation and chemical oxidative stress, does not stimulate proteolysis in erythrocytes

1. Oxidative stress by phenazine methosulfate stimulated proteolysis in erythrocytes. 2. Gamma-irradiation of erythrocytes in the range of 50-1000 Gy also resulted in the induction of proteolysis. 3. Though it has been suggested that hyperthermia imposes an oxidative stress on a cell, hyperthermic exposure of erythrocytes (30 min, 39-49 degrees C) did not stimulate proteolysis during subsequent incubation of whole cells or hemolysates. 4. Proteolytic degradation of spectrin was accelerated during incubation of membranes isolated from cells heated above 45 degrees C but this effect seems to be due rather to thermal denaturation of spectrin than to oxidative modification of cellular proteins by hyperthermia.

Effect of osmotic stress on protein turnover in Lemna minor fronds

Evidence is presented that although many proteins from the fronds of Lemna minor L. undergo enhanced degradation during osmotic stress, ribulose-1,5-bisphosphate carboxylase (RuBPCase) is not degraded. Instead RuBPCase is converted in a series of steps to a very high-molecular-weight form. The first step involves the induction of an oxidase system which after 24 h of stress converts RuBPCase to an acidic and catalytically inactive form. Subsequently, the oxidised RuBPCase protein is gradually polymerized to a number of very large aggregates (molecular weight of several million).The conversion of RuBPCase to a high-molecular-weight form appears to be correlated with (i) a reduction in the number of-SH residues and (ii) the susceptibility to in-vitro proteolysis. Indeed, the number of-SH groups per RuBPCase molecule decreases from 89 in the native enzyme to 54 and 22 in the oxidised and polymerized forms, respectively. On the other hand, the oxidised enzyme is more susceptible to in-vitro proteolysis than the native form. However, it is the polymerized form of RuBPCase which is particularly susceptible to in-vitro proteolysis.Western-blotting experiments and anti-ubiquitin antibodies were used to detect the presence of ubiquitin conjugates in extracts from osmotically stressed Lemna fronds. The possible involvement of ubiquitin in the formation of the aggregates is discussed.

Proteolysis of N-ethylmaleimide-modified aldolase loaded into erythrocyte ghosts: prevention by inhibitors of calpain

1. When rabbit muscle aldolase labelled with tritium and inactivated by N-ethylmaleimide (NEM) was loaded into erythrocyte ghosts, significant proteolysis of the loaded protein occurred. The major product of this proteolysis, separated by electrophoresis under dissociating conditions, was found to be approx. 2 kDa smaller than the parent protein. 2. Proteolysis was detectable during erythrocyte ghost loading at 0 degrees C, reaching a plateau after approx. 12 min. Subsequent incubation at 37 degrees C to allow resealing of the ghosts resulted in additional proteolysis, and up to 20% of the loaded protein was converted to the smaller 38 kDa derivative. 3. EDTA, EGTA, leupeptin and chymostatin, each inhibitors of calcium-activated neutral proteinases (calpains), were the most effective inhibitors of the proteolysis of NEM-inactivated aldolase in ghosts. Other proteinase inhibitors were ineffective, while phenylmethanesulphonyl fluoride was only partially effective. 4. Inhibition of the proteolysis by EGTA was prevented by CaCl2, supporting the involvement of erythrocyte calpain. 5. Pretreatment of ghosts with EGTA prior to loading of NEM-modified aldolase followed by microinjection of the protein into HeLa cells did not result in a different rate of its overall breakdown to acid-soluble products. EGTA is suggested as a useful agent for the erythrocyte ghost-mediated microinjection of calpain-sensitive proteins.

Involvement of the proteasome in various degradative processes in mammalian cells

Eukaryotic cells contain a 700-kDa proteolytic complex (the "proteasome" or multicatalytic endopeptidase complex), whose role in intracellular protein breakdown is unclear. It has been suggested that the proteasome functions in the rapid degradation of oxidant-damaged proteins and in the ATP-dependent proteolytic pathway. To test these possibilities, oxidant-damaged hemoglobin and albumin were produced by treating hemoglobin and albumin with phenylhydrazine, with hydroxyl radicals, or with both hydroxyl and superoxide radicals. After oxidant damage, these proteins were degraded more rapidly in erythrocyte extracts and also by the purified proteasome. However, complete removal of proteasomes from these extracts by immunoprecipitation (or inhibitors of its proteolytic activity) did not reduce the breakdown of oxidant-damaged hemoglobin and decreased degradation of hydroxyl- and superoxide-treated proteins by only 30-40%. Thus, erythrocytes must contain another proteolytic system for degradation of oxidant-damaged proteins. In contrast, immunoprecipitation of proteasomes with polyclonal or monoclonal antibodies prevented the ATP/ubiquitin-dependent degradation of lysozyme and also blocked the ATP-stimulated degradation of ubiquitin-conjugated lysozyme in reticulocyte and skeletal muscle extracts. These data indicate a critical role of the proteasome in the degradation of ubiquitin-conjugated proteins and suggest that the proteasome is associated with or is a component of the larger ubiquitin-conjugate-degrading enzyme complex.

Microinjection of endogenous and exogenous proteins into primary cultures of rat hepatocytes and the degradation of the injected proteins

1. A method to microinject proteins into cells through packaging proteins to erythrocyte ghosts (erythrocyte-mediated microinjection) was modified partially in order to apply the method to primary cultures of rat hepatocytes. 2. Degradation of the microinjected proteins was examined employing the improved method. The mean half-life of the injected endogenous liver protein was 20 hr. The data suggested that the injected proteins are degraded through both lysosomal and non-lysosomal proteolytic pathways probably depending on their structure. 3. The present method to microinject exogenous proteins into primary cultures of rat hepatocytes can be employed usefully for the investigations of protein metabolism in liver.

Substrate inactivation of enzymes in vitro and in vivo

Some enzymes are inactivated by their natural substrates during catalytic turnover, limiting the ultimate extent of reaction. These enzymes can be separated into three broad classes, depending on the mechanism of the inactivation process. The first type is enzymes which use molecular oxygen as a substrate. The second type is inactivated by hydrogen peroxide, which is present either as a substrate or a product, and are stabilized by high catalase activity. The oxidation of both types of enzymes shares common features with oxidation of other enzymes and proteins. The third type of enzyme is inactivated by non-oxidative processes, mainly reversible loss of cofactors or attached groups. Sub classes are defined within each broad classification based on kinetics and stoichiometry. Reaction-inactivation is in part a regulatory mechanism in vivo, because specific proteolytic systems give rapid turnover of such labelled enzymes. The methods for enhancing the stability of these enzymes under reaction conditions depends on the enzyme type. The kinetics of these inactivation reactions can be used to optimize bioreactor design and operation.

Fragmentation of human hemoglobin by oxidative stress produced by phenylhydrazine

Exposure of purified human hemoglobin to phenylhydrazine induces the oxidation of hemoglobin and the generation of acid soluble peptides. The extent of protein fragmentation depends on the concentration of phenylhydrazine, incubation time and temperature. The fragments, excluded by gel filtration chromatography on Sephadex G-15, are partially degraded by leucine aminopeptidase and are totally converted to amino acids by acid hydrolysis. The addition of inhibitors for serine proteinases (phenylmethylsulfonylfluoride), cysteine proteinases (leupeptin), aspartic proteinases (pepstatin A) and metalloproteinases (EDTA) does not alter the formation of acid soluble peptides, thus excluding the involvement of erythrocyte proteinases in the generation of peptides. It is suggested that oxygen and phenylhydrazine free radicals produced in the course of hemoglobin oxidation might be responsible for protein fragmentation. We also discuss a possible relationship between the fragmentation of oxidized hemoglobin and the ATP-independent proteolysis stimulated by oxidative agents.

Susceptibility of glucose-6-phosphate dehydrogenase deficient red cells to primaquine enantiomers and two putative metabolites--I. Effect on reduced glutathione, methemoglobin content and release of hemoglobin

The effects of the primaquine (PQ) enantiomers, (+)PQ and (-)PQ, and two putative metabolites [5-hydroxyprimaquine (5HPQ) and 6-desmethyl-5-hydroxyprimaquine (6D5HPQ)] on methemoglobin (Met Hb) and glutathione content and release of hemoglobin into plasma from glucose-6-phosphate dehydrogenase (G-6-PD) deficient red cells were studied in vitro. The results show that a 1.5 mM concentration of (-)PQ produced a significantly greater increase in Met Hb content and decrease in reduced glutathione (GSH) level than did (+)PQ. However, the release of plasma hemoglobin was greater with (+)PQ than with (-)PQ. The hydroxy derivatives of primaquine, 5HPQ and 6D5HPQ, were significantly more active than PQ. Their individual effects differed; whereas 5HPQ produced significantly greater reduction in GSH compared to 6D5HPQ, the effect of 6D5HPQ on Met Hb content and release of plasma hemoglobin was greater than that of 5HPQ. The qualitative effects of these compounds on normal, heterozygous and hemizygous G-6-PD deficient red cells were similar, but quantitatively the effects were greatest on hemizygous G-6-PD deficient cells and intermediate on heterozygous cells.

Degradation of native and modified forms of fructose-bisphosphate aldolase microinjected into HeLa cells

The uptake and degradation of radiolabelled rabbit muscle fructose-bisphosphate aldolase (EC 4.1.2.13) was studied in HeLa cells microinjected by the erythrocyte ghost fusion system. Labelled aldolase was progressively modified by treatment with GSSG or N-ethylmaleimide (NEM) before microinjection to determine whether these agents, which inactivate and destabilize the enzyme in vitro, affect the half-life of the enzyme in vivo. Increasing exposure of aldolase to GSSG or NEM before microinjection increased the extent of aldolase transfer into the HeLa cells and decreased the proportion of the protein that could be extracted from the cells after water lysis. Some degradation of the GSSG- and NEM-inactivated aldolases was observed in the ghosts before microinjection; thus a family of radiolabelled proteins was microinjected in these experiments. In spite of the above differences, the 40 kDa subunit of each aldolase form was degraded with a half-life of 30 h in the HeLa cells. In contrast, the progressively modified forms of aldolase were increasingly susceptible to proteolytic action in vitro by chymotrypsin or by cathepsin B and in ghosts. These studies indicate that the rate of aldolase degradation in cells is not determined by attack by cellular proteinases that recognize vulnerable protein substrates; the results are most easily explained by a random autophagic process involving the lysosomal system.

Effects of lead on red blood cell membrane proteins

The effects of lead on red blood cell (RBC) membrane proteins were studied in two groups of workers with different lead exposure levels: Group I (6 subjects employed in a battery plant) with a mean blood lead of 40.1 (SD = 3.7) micrograms/100 ml; Group II (5 workers employed in different industries) with a mean blood lead of 60.6 (SD = 8.0) micrograms/100 ml, compared with a control group with mean blood lead of 15.6 (SD = 9.3) micrograms/100 ml. The analysis of RBC membrane polypeptides was carried out by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and by using a densitometer for percentage measurement of the bands corresponding to protein fractions. The results show a very significant decrease in Band 3 (anion channel) and 4.1 in more exposed workers (Group II) only. The effects of lead on RBC membrane proteins seem to be evident at blood-lead levels higher (greater than 50 micrograms/100 ml) than those previously reported in literature. These results confirm the effects of lead on membrane proteins, even if the exact mechanism, particularly the influence of proteolysis and the meaning of the interference, still needs to be investigated thoroughly.

Characterization of hydrazine-stimulated proteolysis in human erythrocytes

The ability of hydrazine, acetylphenylhydrazine, methylhydrazine, and phenylhydrazine to stimulate proteolysis in red cells has been characterized. All four hydrazines effectively stimulated proteolysis in red cells and in hemolysate as evidenced by a two- to threefold increase in the rate of tyrosine release. The rate of tyrosine release varied linearly with time, increased with increasing concentration of hydrazine, and also increased as a function of hematocrit. The rank order for stimulation of proteolysis in red cells was phenylhydrazine greater than methylhydrazine greater than hydrazine approximately equal to acetylphenylhydrazine. Inhibitors of glycolysis in red cells only minimally (13-27%) decreased the rate of tyrosine release stimulated by the different hydrazines. Agents which diminished electron transport decreased the rate of tyrosine release. NADP inhibited the rate of tyrosine release stimulated by hydrazine, methylhydrazine, and acetylphenylhydrazine by approximately 36 to 41%; 2'-AMP was less effective. The rate of tyrosine release resulting from insult by the hydrazines was increased slightly by methylene blue, moderately inhibited (approximately 10 to 27%) by the chelator o-phenanthroline and inhibited approximately 30 to 40% by N-ethylmaleimide. Use of an oxygen-depleted atmosphere (N2) increased slightly the rate of tyrosine release stimulated by the hydrazines; in contrast, carbon monoxide decreased proteolysis stimulated by hydrazine, methylhydrazine, and acetylphenylhydrazine by approximately 50%. Although the antioxidants dimethylfuran, dimethylthiourea, and methylsulfoxide failed to diminish proteolysis stimulated by the hydrazines, N-acetylcysteine exerted a protective effect, decreasing hydrazine-stimulated tyrosine release in red cells approximately 30 to 50%. Inclusion of 3-amino-1,2,4-triazole in the incubation failed to increase further the rate of hydrazine-stimulated proteolysis. These data suggest that more reactive free radicals generated from the hydrazine are responsible for protein damage, that damaged protein (hemoglobin) is degraded via proteolysis, and that an ATP-independent process primarily participates in the degradation of abnormal proteins in the red cell. Thus, proteolytic enzymes present in the erythrocyte appear to exert a protective effect against cellular damage through the removal of abnormal proteins generated as a consequence of xenobiotic insult. The ability of proteolytic enzymes to recognize and degrade abnormal proteins may be of importance in using protein (hemoglobin)-xenobiotic adducts to assess exposure to toxic agents (risk assessment).