Just another sebaceous cyst?

Two cases are presented where an incorrect diagnosis of a sebaceous cyst delayed the treatment of a more serious underlying problem. The history and examination findings pointed to the diagnosis in both cases. Although not rare entities in themselves, these cases illustrate the importance of formulating a differential diagnosis even when confronted with an apparently straightforward condition.

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.

Effects of Synovex-S and recombinant bovine growth hormone (Somavubove) on growth responses of steers: III. Muscle growth and protein responses.

We conducted this study to determine whether the growth responses of specific skeletal muscles in crossbred beef steers were differentially affected by treatment with recombinant bovine growth hormone (Somavubove, SbV, .1 mg/kg BW, i.m., daily), Synovex-S (200 mg progesterone + 20 mg 17-beta estradiol benzoate, SYN, ear implant), or a combination of the two. Starting body weights of steers averaged 182+/-1.8 kg. Five steers were used at this average BW to obtain data on weight and composition of individual muscles at d 0, and 20 other steers were assigned in equal numbers to control (C, no implant and placebo daily injection), SYN, SbV, and SYN + SbV treatment groups. After 56 d of treatment with placebo or growth promoters, complete rectus femoris (RF), triceps brachii (TB), supraspinatus (SS), psoas major (PM), and semitendinosus (ST) muscles were dissected, weighed, and then ground for determination of moisture, total protein, and fat. To calculate the average daily muscle wet weight, protein, and fat gains, the initial weight, protein content, and fat content of a muscle were subtracted from those obtained at slaughter and the difference divided by 56. Muscle weight was increased over C in TB and SS by SYN (P < .1); in TB by SbV (P < .09); and in RF (P < .05), TB (P < .03), and SS (P < .03) by SYN + SbV. Overall average daily wet tissue gain was increased over C by SbV + SYN (P < .05) in RF, TB, and SS. Average daily protein gain in RF and TB was increased by SYN (P < .1), SbV (P < .06), and SYN + SbV (P < .01) over that calculated for C. For RF, TB, and SS, average daily protein gain was greater (P < . 1) in SbV + SYN than that obtained with SbV or SYN alone. These data suggest that administration of growth promoters, such as somatotropin and Synovex, to cattle differentially affects growth characteristics in certain muscles and can have additive effects on protein gain when used together.

Sepsis increases oxidatively damaged proteins in skeletal muscle.

BACKGROUND: Muscle wasting and negative nitrogen balance are common findings in septic patients. It is not clear what signals this loss of body protein. Proteins modified by reactive oxygen species have been shown to be rapidly degraded. OBJECTIVE: To test the hypothesis that sepsis results in an increased amount of oxidatively damaged proteins in skeletal muscle. METHODS: Exposure of proteins to reactive oxygen species results in the incorporation of carbonyl groups into amino acids with metal binding sites. The formation of carbonyl group derivatives in sarcoplasmic and myofibrillar proteins was measured in the extensor digitorum longus and soleus muscles of septic rats 4 to 48 hours after cecal ligation and puncture and in control rats that underwent sham operation. RESULTS: Protein carbonyl content was increased 8 and 16 hours after cecal ligation and puncture in the extensor digitorum longus and soleus muscles, respectively. When muscles were incubated in vitro, the carbonyl content in protein decreased in muscles from septic rats but not in muscles from rats that had the sham operation. The loss of carbonyl groups in incubated septic muscles occurred also in energy-depleted muscles. CONCLUSIONS: Muscle proteins are oxidatively damaged during sepsis and an energy-independent proteolytic pathway participates in the degradation of these proteins. Damage to muscle proteins by reactive oxygen species may signal the selective removal of postsynthetically modified proteins, contributing to accelerated muscle protein degradation in sepsis.

Activation of the multicatalytic endopeptidase by oxidants. Effects on enzyme structure.

It is well established that the functional properties of proteins can be compromised by oxidative damage and, in vivo, proteins modified by oxidants are rapidly degraded. It was hypothesized that oxidants may also affect the ability of proteases to hydrolyze peptides and proteins. We therefore examined the effect of oxidants on the endopeptidase activities of the 650 kDa 20S proteasome or multicatalytic endopeptidase (MCP), which is thought to play a central role in nonlysosomal protein breakdown. Treatment of the MCP with the oxidant system, FeSO4-EDTA-ascorbate, stimulated the peptidase activities of the MCP while H2O2 treatment showed little or no stimulation. However, treatment of the MCP with FeSO4-EDTA-ascorbate or H2O2 stimulated proteinase activity by 480% and 730%, respectively. An endogenous activator of the MCP, PA28, stimulated the acidic, basic, and hydrophobic peptidase activities of the MCP, but had no effect on proteolytic activity. Treatment of PA28 with oxidants in the presence of MCP or alone did not greatly affect PA28's ability to activate the peptidase activities of the MCP. Using nondenaturing polyacrylamide gel electrophoresis, structural alterations in the enzyme which may be responsible for the activation of peptidase and protease activities following exposure to oxidants were investigated. Treatment of the MCP with reagents that activate proteolysis, including H2O2, as well as the serine protease inhibitor 3,4-dichloroisocoumarin and the cysteine protease inhibitor p-(chloromercuri) benzenesulfonic acid, all caused dissociation of the 650 kDa MCP. However, exposure to FeSO4-EDTA-ascorbate resulted in little or no dissociation of the complex. The MCP complex dissociated by p-(chloromercuri) benzenesulfonic acid could be reassociated upon treatment with the reducing agent dithiothreitol, but dithiothreitol failed to completely reassociate 3,4-dichloroisocoumarin- or H2O2 treated MCP. Therefore, chemical modification of the MCP can cause activation with varying degrees of complex dissociation. These results suggest that metabolites, such as reactive oxygen species, in addition to endogenous proteins, such as PA28, are capable of modulating MCP activity.

ATP-stimulated degradation of oxidatively modified superoxide dismutase by cathepsin D in cardiac tissue extracts.

Proteins modified by oxidants are rapidly degraded by intracellular proteases. Oxidatively modified superoxide dismutase (Ox-SOD) was degraded 2-8 times faster at both acidic and alkaline pH than the native protein in bovine cardiac tissue extracts. At acidic pH, Ox-SOD hydrolysis was stimulated by ATP and by non-hydrolyzable ATP analogs by up to 50%, but degradation was not stimulated by ATP at alkaline pH. The aspartic protease inhibitor pepstatin completely inhibited the acid Ox-SOD hydrolyzing activity and its stimulation by ATP. This activity eluted from gel filtration with a molecular size of 34-48 kDa and contained the single chain and two mature forms of cathepsin D. Purified cathepsin D degraded Ox-SOD and ATP enhanced the affinity of cathepsin D for oxidatively modified proteins. Thus cardiac tissue proteins modified by oxidants may be substrates for the lysosomal protease cathepsin D.

Protective effect of ascorbic acid on the breakdown of proteins exposed to hydrogen peroxide in chicken skeletal muscle.

Ascorbic acid is believed to protect cells from oxidative damage by reacting with oxygen-derived free radicals. We investigated whether ascorbic acid would affect the rate of breakdown of skeletal muscle proteins in extracts exposed to hydrogen peroxide. Ascorbic acid (20 mmol/L) alone had little or no effect on the rate of ATP-independent or ATP-dependent breakdown of proteins in chicken skeletal muscle. Pretreatment of chicken skeletal muscle extracts with 10 mmol/L H2O2 resulted in a complete loss of ATP-dependent proteolysis and a significant increase (14- to 15-fold) in the rate of ATP-independent protein breakdown. Ascorbic acid (20 mmol/L) did not prevent H2O2 (10 mmol/L) from inactivating the ATP-dependent proteolytic pathway in skeletal muscle. However, ascorbic acid (20 mmol/L) prevented the H2O2-induced increase in the ATP-independent proteolysis of endogenous muscle proteins. Ascorbic acid also slowed the rate of hydrolysis of exogenously added [3H]superoxide dismutase exposed to H2O2 and inhibited the enhanced degradation of [3H]lysozyme and H2O2-treated [3H]superoxide dismutase by the proteolytic systems exposed to H2O2. Thus ascorbic acid seems to inhibit the H2O2-induced increase in ATP-independent proteolysis 1) by preventing damage to proteins by H2O2 resulting in a decreased supply of substrates for the ATP-independent degradative system and 2) by preventing activation of the proteolytic enzymes that participate in the energy-independent degradation of H2O2-treated proteins.

The ATP-independent pathway in red blood cells that degrades oxidant-damaged hemoglobin.

Studies were carried out to characterize further the cytoplasmic ATP- and ubiquitin-independent proteolytic system in red blood cells that degrades hemoglobin damaged by exposure to oxidants (Fagan, J. M., Waxman, L., and Goldberg, A. L. (1986) J. Biol. Chem. 261, 5705-5713). Several proteases were ruled out as having a major role in the degradation of oxidant-treated hemoglobin (Ox-Hb). Acid hydrolases are not active in this process since the degradation of Ox-Hb has a pH optimum between 6 and 8. The calpains are also not involved since inhibitors of cysteine proteases (leupeptin and trans-epoxysuccinyl-L-leucylamido-(3-methyl)butane) did not diminish the increased proteolysis in intact erythrocytes treated with oxidants or in lysates to which Ox-Hb was added. The degradation of Ox-Hb was unaffected by inhibitors of serine and aspartic proteases. Removal of the high M(r) multicatalytic proteinase by immunoprecipitation also did not significantly affect the degradation of Ox-Hb in erythrocyte lysates. The degradation of Ox-Hb was sensitive to metal chelators and sulfhydryl-modifying reagents but not to specific inhibitors of known metalloproteases. Insulin, which is rapidly degraded in lysates, completely blocked the degradation of Ox-Hb. Insulin- and Ox-Hb-hydrolyzing activity was also inhibited following immunoprecipitation of the 100-kDa metalloinsulinase. The metalloinsulinase, which is inhibited by sulfhydryl-modifying reagents and which requires divalent metals, may therefore participate in the degradation of hemoglobin damaged by oxidants in erythrocytes.

Comparison of the multicatalytic proteinases isolated from the nucleus and cytoplasm of chicken red blood cells.

1. Two chromatographically distinct multicatalytic proteinases (MCP's) were isolated from the cytoplasm of chicken red blood cells and one MCP was purified from the nuclei. 2. The nuclear and the majority (97-99%) of the cytoplasmic multicatalytic proteolytic activity were chromatographically similar and differed from the minor cytoplasmic activity in their elution from hydroxylapatite, number of subunits on 2D-SDS-PAGE, and in their sensitivity to proteinase inhibitors. 3. Dichloroisocoumarin, a serine proteinase inhibitor, inhibited the hydrolysis of fluorogenic peptides but stimulated the degradation of casein by the multicatalytic proteinases suggesting that this enzyme has distinct active sites for protein and peptide hydrolysis.

ATP depletion stimulates calcium-dependent protein breakdown in chick skeletal muscle.

The contribution of metabolic energy to the degradation of intracellular proteins in skeletal muscle was investigated. Isolated chick skeletal muscles deprived of oxygen and muscles incubated in buffer under nonphysiological conditions containing inhibitors of glycolysis and mitochondrial respiration had lower concentrations or undetectable levels of ATP and faster rates of proteolysis. Both total protein breakdown and the breakdown of myofibrillar proteins were stimulated 35-124% in ATP-depleted tissues. However, ATP-depleted muscles incubated in buffer to which no Ca2+ was added showed slower rates of total protein breakdown and no significant change in myofibrillar proteolysis compared with control muscles. Trans-epoxysuccinyl-L-leucylamido(4-guanidino)butane (E-64), a compound that inhibits the calpains and the lysosomal cysteine proteases, completely blocked the Ca(2+)-stimulated breakdown of nonmyofibrillar and myofibrillar proteins in ATP-depleted muscles. However, Ca(2+)-stimulated proteolysis was not inhibited in ATP-depleted muscles incubated with weak bases to prevent lysosome function. These data suggest that intracellular proteins can be degraded in skeletal muscle in the absence of metabolic energy and that the calpains play a major role in the enhanced proteolysis in skeletal muscles depleted of ATP.

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 soluble enzyme from C3H/10T1/2 cells which is inhibited by the Bowman-Birk proteinase inhibitor.

The anticarcinogenic Bowman-Birk proteinase inhibitor (BBI) inhibits a 70-kDa serine proteinase in C3H/10T1/2 transformed fibroblasts. Two serine proteinases, the proline endopeptidase and a novel neutral proteolytic activity, both having a mass of approximately 70-kDa, were isolated from the cytoplasm of C3H/10T1/2 cells. BBI did not inhibit diisopropylfluorophosphate binding to the proline endopeptidase or its ability to hydrolyze peptides. However, BBI blocked the binding of diisopropylfluorophosphate and inhibited the cleavage of peptides by the novel cytoplasmic enzyme. Thus BBI does not inhibit the proline endopeptidase but another soluble 70-kDa serine proteinase from C3H/10T1/2 cells.

Purification of the multicatalytic proteinase from the nucleus and cytoplasm of chicken red blood cells.

We have purified the multicatalytic proteinase from the nucleus and cytoplasm of chicken red blood cells. Both enzymes have an Mr of 700 kDa and can be separated into 8-10 bands (22-32 kDa) on 1D-SDS-PAGE.

A novel ATP-requiring protease from skeletal muscle that hydrolyzes non-ubiquitinated proteins.

Previously, we isolated an ATP-dependent proteolytic pathway in muscle, liver, and reticulocytes that requires ubiquitin and the enzymes which conjugate ubiquitin to proteins. We report here that skeletal muscle contains another soluble alkaline energy-dependent (but ubiquitin-independent) proteolytic activity. The cleavage of non-ubiquitinated protein substrates by the partially purified protease requires ATP hydrolysis since ATP in the absence of Mg2+, nonhydrolyzable ATP analogs, and pyrophosphate all fail to stimulate proteolysis. Proteolytic activity is also stimulated by UTP, CTP, and GTP, although not as effectively as by ATP (Km(ATP) = 0.027 mM). The enzyme is inactivated by the serine protease inhibitors diisopropyl fluorophosphate and 3,4-dichloroisocoumarin, but not by specific inhibitors of aspartic, thiol, or metalloproteases. It is maximally active at pH 8 and has a molecular weight of approximately 600,000. This new activity differs from the 720-kDa multicatalytic proteinase, but resembles the soluble ATP-dependent proteolytic system that we previously isolated from murine erythroleukemia cells.

Polyamines inhibit the ATP-dependent proteolytic pathway in rabbit reticulocyte lysates.

Reticulocytes contain a soluble nonlysosomal proteolytic pathway that requires ATP and ubiquitin. Polyamines at physiological concentrations were found to inhibit rapidly the ATP-dependent proteolytic system in reticulocyte lysates; spermidine and putrescine inhibited this process by 26-72% and spermine by 71-96%. Spermine had little effect on the ATP-independent breakdown of oxidant-treated hemoglobin. By fractionating the ATP-dependent system, we show that polyamines inhibit the ATP-dependent degradation of ubiquitin-protein conjugates.

Effects of oxygen deprivation on incubated rat soleus muscle.

Isolated soleus muscle deprived of oxygen produces more lactate and alanine than oxygen-supplied muscle. Oxygenated muscle synthesized glutamine, while anoxic muscle used this amino acid. Oxygen deprivation decreased adenine nucleotides leading to the efflux of nucleosides. Protein synthesis and degradation responded differently to anoxia. Synthesis almost completely ceased, while proteolysis increased. Therefore, protein degradation in soleus muscle is enhanced when energy supplies and oxygen tension are low.

Activation of protein breakdown and prostaglandin E2 production in rat skeletal muscle in fever is signaled by a macrophage product distinct from interleukin 1 or other known monokines.

During sepsis or after injection of endotoxin into rats, there is a large increase in muscle protein breakdown and prostaglandin E2 (PEG2) production. Prior studies showed that partially purified interleukin 1 (IL-1) from human monocytes can stimulate these processes when added to isolated rat muscles. The availability of pure recombinant IL-1 and other monokines has allowed us to investigate the identity of the active agent in this process. Incubation of muscles with recombinant human or murine IL-1 alpha or IL-1 beta or with IL-1 plus a phorbol ester did not stimulate muscle proteolysis or PGE2 production. Homogeneous natural porcine IL-1 ("catabolin") and mouse or human IL-1 beta were also not effective in vitro. In addition, a variety of other human cytokines, including tumor necrosis factor ("cachectin"), epidermal thymocyte-activating factor, eosinophil cytotoxicity-enhancing factor, interferon-alpha, beta, and gamma, platelet-derived growth factor, and transforming growth factor (TGF) beta, which are all released by activated macrophages, TGF-alpha, or mixtures of these polypeptides, also failed to activate proteolysis or PGE2 production. By contrast, a large increase in net protein breakdown could be induced in the rat soleus by polypeptides released from porcine monocytes or by the serum from febrile cattle which had been injected with Pasteurella haemolytica or bovine rhinotracheitis virus. Therefore, a still-unidentified product of activated monocytes appears to be responsible for the negative nitrogen balance that accompanies infectious illness.

Skeletal muscle and liver contain a soluble ATP + ubiquitin-dependent proteolytic system.

Although protein breakdown in most cells seems to require metabolic energy, it has only been possible to establish a soluble ATP-dependent proteolytic system in extracts of reticulocytes and erythroleukemia cells. We have now succeeded in demonstrating in soluble extracts and more purified preparations from rabbit skeletal muscle a 12-fold stimulation by ATP of breakdown of endogenous proteins and a 6-fold stimulation of 125I-lysozyme degradation. However, it has still not been possible to demonstrate such large effects of ATP in similar preparations from liver. Nevertheless, after fractionation by DEAE-chromatography and gel filtration, we found that extracts from liver as well as muscle contain both the enzymes which conjugate ubiquitin to 125I-lysozyme and an enzyme which specifically degrades the ubiquitin-protein conjugates. When this proteolytic activity was recombined with the conjugating enzymes, ATP + ubiquitin-dependent degradation of many proteins was observed. This proteinase is unusually large, approx. 1500 kDa, requires ATP hydrolysis for activity and resembles the ubiquitin-protein-conjugate degrading activity isolated from reticulocytes. Thus the ATP + ubiquitin-dependent pathway is likely to be present in all mammalian cells, although certain tissues may contain inhibitory factors.

Demonstration of two distinct high molecular weight proteases in rabbit reticulocytes, one of which degrades ubiquitin conjugates.

Reticulocytes contain a nonlysosomal proteolytic pathway that requires ATP and ubiquitin. By DEAE chromatography and gel filtration, we were able to fractionate the ATP-dependent system into a 30-300-kDa fraction that catalyzes the ATP-dependent conjugation of ubiquitin to substrates ("Conjugation Fraction") and a high mass fraction (greater than 450 kDa) necessary for hydrolysis of the conjugated proteins. The latter contains two distinct proteases. One protease is unusually large, approximately 1500 kDa, and degrades proteins only when ATP and the conjugating fractions are added. This activity precipitates at 0-38% (NH4)2SO4 saturation and is essential for ATP-dependent proteolysis. Like crude extracts, it is labile in the absence of nucleotides and is inhibited by heparin, poly(Glu-Ala-Tyr), 3,4-dichloroisocoumarin, hemin, decavanadate, N-ethylmaleimide, and various peptide chloromethyl ketones. It lacks amino-peptidase and insulin-degrading activities and does not require tRNA for activity. The ubiquitin-conjugate degrading enzyme, which we suggest be named UCDEN, is inactive against substrates that cannot undergo ubiquitin conjugation. The smaller protease (670 kDa), which precipitates at 40-80% (NH4)2SO4 saturation, does not require ATP or ubiquitin and is therefore not required for ATP-dependent proteolysis. It is stimulated by N-ethylmaleimide and 3,4-dichloroisocoumarin and is stable at 37 degrees C. It hydrolyzes fluorometric tetrapeptides and proteins, including proteins which cannot be conjugated to ubiquitin. Thus, reticulocytes contain two large cytosolic proteases: one is essential for the degradation of ubiquitin conjugates, while the function of the other is uncertain.

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

It is generally accepted that ATP is required for intracellular protein breakdown. Reticulocytes contain a soluble ATP-dependent pathway for the degradation of highly abnormal proteins and for the elimination of certain proteins during cell maturation. Reticulocytes and erythrocytes also selectively degrade proteins damaged by oxidation. When these cells were exposed to oxidants, such as phenylhydrazine or nitrite, they showed a large increase in protein breakdown. This oxidant-induced proteolysis was not inhibited in cells depleted of ATP. However, ATP depletion did prevent the degradation of pre-existent cell proteins. In reticulocyte extracts, phenylhydrazine-treated hemoglobin is also degraded rapidly by an ATP-independent process, unlike endogenous proteins and many exogenous polypeptides. This lack of an ATP requirement means that the degradation of oxidant-damaged proteins does not require ligation to ubiquitin (even though phenylhydrazine treatment does make hemoglobin a very good substrate for ubiquitin conjugation). In many respects, the pathway for breakdown of oxidant-treated hemoglobin differs from the ATP-dependent process. The latter has a much higher activation energy than the degradation of oxidized proteins. The ATP-dependent process is inhibited by hemin, 3,4-dichloroisocoumarin, diisopropylfluorophosphate and N-ethylmaleimide. The ATP-independent pathway is less sensitive to N-ethylmaleimide, hemin, and 3,4-dichloroisocoumarin and is not affected by diisopropylfluorophosphate. In addition, only the ATP-dependent proteolytic process is inactivated by dilution or incubation at 37 degrees C in the absence of nucleotides. Reticulocytes thus contain multiple soluble systems for degrading proteins and can rapidly hydrolyze certain types of abnormal proteins by either an ATP-independent or ATP-dependent process. Erythrocytes lack the ATP-dependent process present in reticulocytes; however, erythrocytes retain the capacity to degrade oxidant-damaged hemoglobin. These two processes probably are active in the elimination of different types of abnormal proteins.