Hydrogen peroxide-induced oxidative stress to the mammalian heart-muscle cell (cardiomyocyte): nonperoxidative purine and pyrimidine nucleotide depletion

Aluminum as a Possible Cause Toward Dyslipidemia

Aluminum, the third most abundant metal present in the earth's crust, is present almost in all daily commodities we use, and exposure to it is unavoidable. The interference of aluminum with various biochemical reactions in the body leads to detrimental health effects, out of which aluminum-induced neurodegeneration is widely studied. However, the effect of aluminum in causing dyslipidemia cannot be neglected. Dyslipidemia is a global health problem, which commences to the cosmic of non-communicable diseases. The interference of aluminum with various iron-dependent enzymatic activities in the tri-carboxylic acid cycle and electron transport chain results in decreased production of mitochondrial adenosine tri-phosphate. This ultimately contributes to oxidative stress and iron-mediated lipid peroxidation. This mitochondrial dysfunction along with modulation of α-ketoglutarate and L-carnitine perturbs lipid metabolism, leading to the atypical accumulation of lipids and dyslipidemia. Respiratory chain disruption because of the accumulation of reduced nicotinamide adenine di-nucleotide as a consequence of oxidative stress and the stimulatory effect of aluminum exposure on glycolysis causes many health issues including fat accumulation, obesity, and other hepatic disorders. One major factor contributing to dyslipidemia and enhanced pro-inflammatory responses is estrogen. Aluminum, being a metalloestrogen, modulates estrogen receptors, and in this world of industrialization and urbanization, we could corner down to metals, particularly aluminum, in the development of dyslipidemia. As per PRISMA guidelines, we did a literature search in four medical databases to give a holistic view of the possible link between aluminum exposure and various biochemical events leading to dyslipidemia.

Cardiovascular Complications in Community-Acquired Pneumonia

Community-acquired pneumonia (CAP) is accountable for high mortality in both pediatric and adult populations worldwide, about one-third of hospitalized patients pass away within a year of being discharged from the facility. The high mortality and morbidity rates are closely related to cardiovascular complications that are consequent or concomitant to the acute episode of pneumonia. An updated perspective on the major pathophysiological mechanisms, prevalence, risk factors, outcomes, and relevant treatments of cardiovascular events in CAP patients is provided in the current study. It is possible to evaluate the pathophysiology of cardiac disease in this population based on plaque-related events, such as acute myocardial infarction, or events unrelated to plaque, such as arrhythmias and heart failure. With an absolute rate of cardiovascular problems ranging broadly from 10% to 30%, CAP raises the risk of both plaque-related and plaque-unrelated events. Both in- and out-patients may experience these issues at admission, throughout hospitalization, or even up to a year following discharge. At long-term follow-up, cardiac events account for more than 30% of deaths in CAP patients, making them a significant cause of mortality. If patients at risk for cardiac events are stratified, diagnostic tools, monitoring, and preventive measures may be applied to these patients. A prospective evaluation of cardioprotective treatments is urgently required from a research point of view.

Phosphorylated cofilin-2 is more prone to oxidative modifications on Cys39 and favors amyloid fibril formation

Cofilins are small protein of the actin depolymerizing family. Actin polymerization/depolymerization is central to a number of critical cellular physiological tasks making cofilin a key protein for several physiological functions of the cell. Cofilin activity is mainly regulated by phosphorylation on serine residue 3 making this post-translational modification key to the regulation of myofilament integrity. In fact, in this form, the protein segregates in myocardial aggregates in human idiopathic dilated cardiomyopathy. Since myofilament network is an early target of oxidative stress we investigated the molecular changes induced by oxidation on cofilin isoforms and their interplay with the protein phosphorylation state to get insight on whether/how those changes may predispose to early protein aggregation. Using different and complementary approaches we characterized the aggregation properties of cofilin-2 and its phosphomimetic variant (S3D) in response to oxidative stress in silico, in vitro and on isolated cardiomyocytes. We found that the phosphorylated (inactive) form of cofilin-2 is mechanistically linked to the formation of an extended network of fibrillar structures induced by oxidative stress via the formation of a disulfide bond between Cys39 and Cys80. Such phosphorylation-dependent effect is likely controlled by changes in the hydrogen bonding network involving Cys39. We found that the sulfide ion inhibits the formation of such structures. This might represent the mechanism for the protective effect of the therapeutic agent Na2S on ischemic injury.

Intralipid Increases Nitric Oxide Release from Human Endothelial Cells During Oxidative Stress

BACKGROUND

Intralipid (ILP), a lipid emulsion, protects organs against ischemia/reperfusion (IR) injury. We hypothesized that ILP activates endothelial nitric oxide synthase (eNOS) and increases NO release from endothelial cells (ECs) through a fatty-acid translocase cluster of differentiation (CD36) mediated endocytotic mechanism, acting as a potentially protective paracrine signal during oxidative stress.

METHODS

Human umbilical-vein ECs were exposed to 1% ILP for 2 hours followed by oxidative stress with 0.2-mM hydrogen peroxide for 2 hours. Western blots were conducted with anti-CD36, dynamin-2, src-kinase-1, eNOS, and phospho-eNOS; equal protein loading was confirmed with β-actin. CD36 immunoprecipitation was probed for caveolin-1 to determine if CD36 and caveolin-1 were complexed on the cell membrane. NO was measured by fluorescence of ECs.

RESULTS

ILP caused a 227% increase in CD36 expression vs controls. Immunoprecipitation indicated a CD36/caveolin-1 complex on ECs' membrane with exposure to ILP. Dynamin-2 increased 52% and src-kinase-1 340% after ILP treatment vs control cells. eNOS phosphorylation was confirmed by a 63% increase in the phospho-eNOS/eNOS ratio in ILP-treated cells, and NO fluorescence increased 102%.

CONCLUSION

ILP enters ECs via endocytosis by a CD36/caveolin-1 cell membrane receptor complex, which in turn is pulled into the cell by dynamin-2 activity. Upregulation of src-kinase-1 and eNOS phosphorylation suggest downstream mediators. Subsequent NO release from ECs serve as a paracrine signal to neighboring cells for protection against IR injury. Student t-test was utilized for single comparisons and analysis of variance with Bonferroni-Dunn post hoc modification for multiple comparisons; P < .05 was considered statistically significant.

Effects of methylmercury and retinol palmitate co-administration in rats during pregnancy and breastfeeding: Metabolic and redox parameters in dams and their offspring

Ubiquitous low-dose methylmercury (MeHg) exposure through an increased fish consumption represents a global public health problem, especially among pregnant women. A plethora of micronutrients presented in fish affects MeHg uptake/distribution, but limited data is available. Vitamin A (VitA), another fish micronutrient is used in nutritional supplementation, especially during pregnancy. However, there is no information about the health effects arising from their combined exposure. Therefore, the present study aimed to examine the effects of both MeHg and retinyl palmitate administered on pregnant and lactating rats in metabolic and redox parameters from dams and their offspring. Thirty Wistar female rats were orally supplemented with MeHg (0,5 mg/kg/day) and retinyl palmitate (7500 µg RAE/kg/day) via gavage, either individually or in combination from the gestational day 0 to weaning. For dams (150 days old) and their offspring (31 days old), glycogen accumulation (hepatic and cardiac) and retinoid contents (plasma and liver) were analyzed. Hg deposition in liver tissue was quantified. Redox parameters (liver, kidney, and heart) were evaluated for both animals. Cytogenetic damage was analyzed with micronucleus test. Our results showed no general toxic or metabolic alterations in dams and their offspring by MeHg-VitA co-administration during pregnancy and lactation. However, increased lipoperoxidation in maternal liver and a disrupted pro-oxidant response in the heart of male pups was encountered, with apparently no particular effects in the antioxidant response in female offspring. GST activity in dam kidney was altered leading to possible redox disruption of this tissue with no alterations in offspring. Finally, the genomic damage was exacerbated in both male and female pups. In conclusion, low-dose MeHg exposure and retinyl palmitate supplementation during gestation and lactation produced a potentiated pro-oxidant effect, which was tissue-specific. Although this is a pre-clinical approach, we recommend precaution for pregnant women regarding food consumption, and we encourage more epidemiological studies to assess possible modulations effects of MeHg-VitA co-administration at safe or inadvertently used doses in humans, which may be related to specific pathologies in mothers and their children.

Targeting extracellular vesicles to injured tissue using membrane cloaking and surface display

BACKGROUND

Extracellular vesicles (EVs) and exosomes are nano-sized, membrane-bound vesicles shed by most eukaryotic cells studied to date. EVs play key signaling roles in cellular development, cancer metastasis, immune modulation and tissue regeneration. Attempts to modify exosomes to increase their targeting efficiency to specific tissue types are still in their infancy. Here we describe an EV membrane anchoring platform termed "cloaking" to directly embed tissue-specific antibodies or homing peptides on EV membrane surfaces ex vivo for enhanced vesicle uptake in cells of interest. The cloaking system consists of three components: DMPE phospholipid membrane anchor, polyethylene glycol spacer and a conjugated streptavidin platform molecule, to which any biotinylated molecule can be coupled for EV decoration.

RESULTS

We demonstrate the utility of membrane surface engineering and biodistribution tracking with this technology along with targeting EVs for enhanced uptake in cardiac fibroblasts, myoblasts and ischemic myocardium using combinations of fluorescent tags, tissue-targeting antibodies and homing peptide surface cloaks. We compare cloaking to a complementary approach, surface display, in which parental cells are engineered to secrete EVs with fusion surface targeting proteins.

CONCLUSIONS

EV targeting can be enhanced both by cloaking and by surface display; the former entails chemical modification of preformed EVs, while the latter requires genetic modification of the parent cells. Reduction to practice of the cloaking approach, using several different EV surface modifications to target distinct cells and tissues, supports the notion of cloaking as a platform technology.

A framework for large-scale metabolome drug profiling links coenzyme A metabolism to the toxicity of anti-cancer drug dichloroacetate

Metabolic profiling of cell line collections has become an invaluable tool to study disease etiology, drug modes of action and to select personalized treatments. However, large-scale in vitro dynamic metabolic profiling is limited by time-consuming sampling and complex measurement procedures. By adapting a mass spectrometry-based metabolomics workflow for high-throughput profiling of diverse adherent mammalian cells, we establish a framework for the rapid measurement and analysis of drug-induced dynamic changes in intracellular metabolites. This methodology is scalable to large compound libraries and is here applied to study the mechanism underlying the toxic effect of dichloroacetate in ovarian cancer cell lines. System-level analysis of the metabolic responses revealed a key and unexpected role of CoA biosynthesis in dichloroacetate toxicity and the more general importance of CoA homeostasis across diverse human cell lines. The herein-proposed strategy for high-content drug metabolic profiling is complementary to other molecular profiling techniques, opening new scientific and drug-discovery opportunities.

Newt cells secrete extracellular vesicles with therapeutic bioactivity in mammalian cardiomyocytes

Newts can regenerate amputated limbs and cardiac tissue, unlike mammals which lack broad regenerative capacity. Several signaling pathways involved in cell proliferation, differentiation and survival during newt tissue regeneration have been elucidated, however the factors that coordinate signaling between cells, as well as the conservation of these factors in other animals, are not well defined. Here we report that media conditioned by newt limb explant cells (A1 cells) protect mammalian cardiomyocytes from oxidative stress-induced apoptosis. The cytoprotective effect of A1-conditioned media was negated by exposing A1 cells to GW4869, which suppresses the generation of extracellular vesicles (EVs). A1-EVs are similar in diameter (~100–150 nm), structure, and share several membrane surface and cargo proteins with mammalian exosomes. However, isolated A1-EVs contain significantly higher levels of both RNA and protein per particle than mammalian EVs. Additionally, numerous cargo RNAs and proteins are unique to A1-EVs. Of particular note, A1-EVs contain numerous mRNAs encoding nuclear receptors, membrane ligands, as well as transcription factors. Mammalian cardiomyocytes treated with A1-EVs showed increased expression of genes in the PI3K/AKT pathway, a pivotal player in survival signaling. We conclude that newt cells secrete EVs with diverse, distinctive RNA and protein contents. Despite ~300 million years of evolutionary divergence between newts and mammals, newt EVs confer cytoprotective effects on mammalian cardiomyocytes.

Cardiotoxicity during invasive pneumococcal disease

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia and sepsis, with adult hospitalization linked to approximately 19% incidence of an adverse cardiac event (e.g., heart failure, arrhythmia, infarction). Herein, we review the specific host-pathogen interactions that contribute to cardiac dysfunction during invasive pneumococcal disease: (1) cell wall-mediated inhibition of cardiomyocyte contractility; (2) the new observation that S. pneumoniae is capable of translocation into the myocardium and within the heart, forming discrete, nonpurulent, microscopic lesions that are filled with pneumococci; and (3) the bacterial virulence determinants, pneumolysin and hydrogen peroxide, that are most likely responsible for cardiomyocyte cell death. Pneumococcal invasion of heart tissue is dependent on the bacterial adhesin choline-binding protein A that binds to laminin receptor on vascular endothelial cells and binding of phosphorylcholine residues on pneumococcal cell wall to platelet-activating factor receptor. These are the same interactions responsible for pneumococcal translocation across the blood-brain barrier during the development of meningitis. We discuss these interactions and how their neutralization, either with antibody or therapeutic agents that modulate platelet-activating factor receptor expression, may confer protection against cardiac damage and meningitis. Considerable collagen deposition was observed in hearts of mice that had recovered from invasive pneumococcal disease. We discuss the possibility that cardiac scar formation after severe pneumococcal infection may explain why individuals who are hospitalized for pneumonia are at greater risk for sudden death up to 1 year after infection.

Protecting solutions of parenteral nutrition from peroxidation

BACKGROUND

Light exposure induces the generation of peroxides in solutions of total parenteral nutrition (TPN). Peroxide toxicity has been documented in cell, in tissue, and in isolated organs. To decrease the infused peroxide load and to protect the quality of the parenteral nutrients, we tested the photoprotective properties of different infusion sets.

METHODS

Solutions of fat-free TPN and all-in-one total nutrient admixture (TNA) were run through sets of bags (clear and covered) and tubings (clear and colored: black, orange, and yellow) offering different levels of protection against light. Peroxide levels were determined by ferrous oxidation of xylenol orange, thiol functions by the 5,5,-dithiobis(2-nitrobenzoic acid) technique, and absorbance of tubings by spectroscopy.

RESULTS

Protection of only the bag had little effect on peroxide generation. In fat-free TPN solutions kept in covered bags, peroxide concentrations were 1.5 to 2 times higher when run through clear compared with colored tubings. When exposed to phototherapy or in the presence of lipids, peroxides were two to three times higher with the clear compared with the black tubing; meanwhile, orange and yellow tubings offered varying levels of protection related to their light-absorbing properties. Colored tubings offered a greater protection against the disappearance of thiol functions.

CONCLUSIONS

Covering bags and using orange and yellow tubings may be a practical solution to reduce infused peroxide loads from about 400 to 100 microM. This is especially relevant in patients with an immature or a compromised antioxidant capacity or when phototherapy or preparations of TNA are used.

Leukotoxin (9, 10-epoxy-12-octadecenoate) impairs energy and redox state of isolated perfused rat lung

We investigated the perturbation of energy balance and redox state in leukotoxin (9, 10-epoxy-12octadecenoate) (Lx)- and endothelin-1 (ET-1)-induced lung injury, using isolated perfused rat lungs. To examine any relationship between these parameters, intracellular levels of adenine nucleotides, pyridine coenzymes and glutathione were determined by reversed-phase high-performance liquid chromatography (HPLC) in the freeze-dried tissues of isolated rat lungs. The tissue samples were perfused with a physiological salt solution containing either Lx only, Lx plus NG-monomethyl-L-arginine (L-NMMA), Lx plus NG-monomethyl-D-arginine (D-NMMA), Lx plus superoxide dismutase (SOD) or ET-1 only. In isolated perfused lung tissue, 10 mol of Lx caused permeability-increased lung injury, and 10 nM of ET-1, which caused a comparable increase in wet lung weight, evoked pulmonary capillary hypertensive lung injury. Lx-injured lungs showed decreases in the contents of ATP, NADPH, NADH, reduced glutathione (GSH), (2ATP + ADP)/2(ATP + ADP + AMP) ratio (energy charge) and NADH/NAD+ ratio, and increased the contents of ADP and AMP compared with the vehicle control and ET-1-injured lungs. Such effects of Lx were significantly attenuated by pretreatment with 0.4 mM L-NMMA or 500 units/ml of SOD, but not with 0.4 mM D-NMMA. On the other hand, the ET-1-injured lung evidenced decreased tissue GSH. These findings indicate that Lx shifted the lung redox state toward oxidation and that Lx-induced lung injury was involved in the imbalance of the energy and redox state via production of nitric oxide and/or superoxide anion.

Effect of Maharishi AK-4 on H2O2-induced oxidative stress in isolated rat hearts

Oxidative damage to crucial biomolecules due to excess generation of reactive oxygen species has been implicated as a major cause of organ damage and hence compounds capable of negating such damage have potential benefits. Using hydrogen peroxide (H2O2) as a model pro-oxidant to induce oxidative stress, we have examined the ability of natural food supplement Maharishi Amrit Kalash (MAK-4) to decrease oxidative damage in potassium-arrested isolated rat hearts. The protocol was that hearts isolated from male Sprague-Dawley rats were retrograde-perfused with Krebs-Henseleit (K-H) solution for 30 min for equilibration. After this period, the hearts were subjected to cardioplegia with high potassium (26-30 mM), followed by reperfusion with K-H solution in the presence or absence of 200 microM H2O2. As expected, H2O2 treatment following cardioplegia induced a high degree of oxidative stress as assessed by release of lactate dehydrogenase (LDH, a marker of plasma membrane damage) and total glutathione (GSH + GSSG). H2O2 also impaired the ability of heart to regain developed tension during the testing period. However, addition of MAK-4 in the perfusate containing H2O2 decreased oxidative stress in terms of release of LDH and glutathione. In parallel with these biochemical studies, in a few experiments the cardiac function was assessed by measuring developed contractile tension. These preliminary studies also showed that in the presence of MAK-4 the H2O2-treated hearts were able to regain better developed tension. Further in vitro studies to examine the possible mechanisms of MAK-4 action reveal that this formulation contains H2O2 binding activity which resulted in the decreased availability of H2O2 itself. Our studies hence reveal that the ayurvedic food supplement MAK-4 may have potential benefits in reducing oxidative stress.

Adaptation to oxidative stress: quinone-mediated protection of signaling in rat lung epithelial L2 cells

Cells can respond to a sublethal oxidative stress by up-regulating their intracellular glutathione (GSH) pool. Such increased GSH concentration is likely to be protective against further oxidative challenge, and, in fact, pre-exposure to low levels of oxidants confers increased cellular resistance to subsequent greater oxidative stress. Previously, we have shown that pretreatment of rat lung epithelial L2 cells with sublethal concentrations of tert-butylhydroquinone (TBHQ) increases intracellular GSH concentration in a concentration- and time-dependent manner. This increase resulted from up-regulation of both gamma-glutamyltranspeptidase (GGT) and gamma-glutamylcysteine synthetase (GCS). Therefore, we investigated whether such increased GSH concentration protected these cells against a subtle loss in function caused by a subsequent challenge with sublethal concentrations of tert-butyl hydroperoxide (tBOOH) (< or = 200 microM), mimicking a physiological oxidative stress. Activation of L2 cell purinoreceptors with 100 microM ADP caused an elevation of intracellular Ca2+. This response was suppressed by a brief pre-exposure to tBOOH. The inhibition, however, was alleviated dramatically by a 16-hr pretreatment with 50 microM TBHQ. The same TBHQ pretreatment also protected the cells from ATP-depletion induced by tBOOH. L-Buthionine S,R-sulfoximine (BSO), an irreversible inhibitor of GCS, prevented the increase in intracellular GSH and also completely removed the protection by TBHQ in maintaining the ATP level. Thus, pre-exposure to a sublethal level of TBHQ results in protection of cell functions from hydroperoxide toxicity. This protection appears to depend on alteration of the intracellular GSH pool, the modulation of which constitutes an adaptive response to oxidative stress.

Oxidative modulation and inactivation of rabbit cardiac adenylate deaminase

Oxidative stress and adenine nucleotide catabolism occur concomitantly in several disease states, such as cardiac ischaemia-reperfusion, and may act as synergistic determinants of tissue injury. However, the mechanisms underlying this potential interaction remain ill-defined. We examined the influence of oxidative stress on the molecular, kinetic and regulatory properties of a ubiquitous AMP-catabolizing enzyme, adenylate deaminase (AMPD) (EC 3.5.4.6). To this intent, rabbit heart AMPD and an H2O2/ascorbate/iron oxidation system were employed. Enzyme exposure to the complete oxidation system acutely impaired its catalytic activity, lowered the Vmax. by 7-fold within 5 min, and rendered the enzyme unresponsive to nucleotide effectors. Irreversible AMPD inactivation resulted within about 15 min of oxidative insult and was not prevented by free-radical scavengers. Oxidative stress did not affect the molecular mass, tetrameric nature, Km, immunoreactivity or trypsinolytic pattern of the enzyme; nor did it induce carbonyl formation, Zn2+ release from the holoenzyme or net AMPD S-thiolation. This injury pattern is inconsistent with a radical-fragmentation mechanism as the basis for the oxidative AMPD inactivation observed. Rather, the sensitivity of the enzyme to both S-thiolation and thiol alkylation and the significant (3 of 9/mol of denatured enzyme) net loss of DTNB-reactive thiols on exposure to oxidant strongly implicate the conversion of essential thiol moieties into stable higher-oxidation states in the oxidative inactivation of cardiac AMPD. The altered thiol status of the enzyme on oxidative insult may prohibit a catalytically permissible conformation and, in so doing, increase AMP availability to 5'-nucleotidase in vivo.

Ischemic heart disease and antioxidants: mechanistic aspects of oxidative injury and its prevention

The disease state of myocardial ischemia results from a hypoperfusion-induced insufficiency of heart-muscle oxidative metabolism due to inadequate coronary circulation. Myocardial ischemia is an important, lifespan-limiting medical problem and a major economic health-care concern. Reperfusion, although avidly pursued in the clinic as essential to the ultimate survival of acutely ischemic heart muscle, may itself carry an injury component. Cardiac reperfusion injury appears to reflect, at least in part, an oxidant burden established upon reoxygenation of ischemic myocardium. Laboratory evidence demonstrates that oxidative stress to the heart-muscle cell (cardiomyocyte) can elicit the three known types of ischemia-reperfusion injury that directly affect the myocardium: arrhythmia, stunning, and infarction. The limited clinical occurrence of serious reperfusion arrhythmias has restricted the importance of antioxidants as antiarrhythmic agents against this form of myocardial ischemia-reperfusion damage. Despite the utmost clinical significance of lethal cardiomyocyte injury as a negative prognostic indicator for the ischemic heart-disease patient, inconsistent results of antioxidant interventions in reducing infarct size have somewhat tempered interest in antioxidant infarct trials. By contrast, the negative clinical consequences of stunning may indeed be preventable by utilizing antioxidants to help restore postischemic cardiac pump function. Several as yet unanswered questions remain regarding oxidative stress in the reperfused heart, its significance to cardiomyocyte damage, and its ability to elicit specific postischemic myocardial derangements. Targeted mechanistic studies are required to address these questions and to define the pathogenic role of oxidative stress (and, hence, the therapeutic potential of antioxidant intervention) in myocardial ischemia-reperfusion injury. The overall aim of current research in this area is to enable the cardiac surgeon/cardiologist to advance beyond the largely palliative drugs now available for management of the coronary heart-disease patient and attack directly the pathogenic determinants of heart-muscle ischemia-reperfusion injury. Optimal use of antioxidants may help address this important medical need.

Cellular reducing equivalents and oxidative stress

Energy has been proposed to play a role in the ability of cells and tissues to defend against oxidative stress, even though the ultimate antioxidant capacity of a tissue is determined by the supply of reducing equivalents. The pathways involved in supplying reducing equivalents in response to an oxidative stress remain unclear, particularly if competing reactions such as ATP synthesis are active. Glutathione (GSH), a major component of cellular antioxidant systems, is maintained in the reduced form by glutathione reductase. Although this enzyme is specific for NADPH, the ability of intact cells, isolated mitochondria (which are a major source of free radicals and contain antioxidant systems independent of the rest of the cell), and whole tissues to supply reducing equivalents and maintain normal levels of GSH appears to involve NADH. This article reviews available data regarding the source and pathways by which reducing equivalents are made available to reduce exogenous oxidants, and suggests energy is not a factor. An improved understanding of the mechanism by which reducing equivalents are supplied by tissues to respond to an oxidative stress may direct future research toward designing strategies for augmenting the ability of tissues to defend themselves against oxidative stress induced by reperfusion or xenobiotics.

Cardiac adenylate deaminase: molecular, kinetic and regulatory properties under phosphate-free conditions

Adenylate deaminase (EC 3.5.4.6) may help to regulate the adenine nucleotide catabolism characteristic of such disease states as myocardial ischaemia. We report analysis of the molecular, kinetic and allosteric properties of rabbit heart adenylate deaminase when extracted and purified under phosphate-free conditions (i.e., with Hepes/KOH). The enzyme's subunit molecular mass (approximately 81 kDa), pI (6.5), substrate specificity for 5'-AMP, and activation by K+ were identical in the absence or presence of phosphate. At each chromatographic step during isolation without phosphate, cardiac adenylate deaminase showed a lower apparent activity as compared with the enzyme prepared with phosphate present. Kinetic constants for the phosphate-free rabbit heart adenylate deaminase preparation (Km 0.54 mM AMP; Vmax. 1.4 mumol/min per mg of protein) were approximately 10-fold lower than those of the enzyme isolated with phosphate. The same irreversible decrease in kinetic constants could be achieved by dialysing phosphate from the phosphate-containing enzyme preparation. The relationship between enzyme activity and substrate concentration was sigmoidal in the presence of phosphate, but hyperbolic in its absence. Cardiac adenylate deaminase under phosphate-free conditions was no longer allosterically activated by ATP and ADP, yet remained inhibitable by GTP. Enzyme inhibition by the transition-state mimic coformycin was not influenced by phosphate status. The phosphate-free preparation of rabbit heart adenylate deaminase was markedly labile and extremely susceptible to proteolysis by trypsin or chymotrypsin. The inactivation kinetics and fragmentation pattern in response to controlled proteolysis depended on whether the enzyme had been isolated with or without phosphate present, suggesting a conformational difference between the two enzyme preparations. These data constitute direct evidence that the absence of phosphate irreversibly converts cardiac adenylate deaminase into a pseudo-isoenzyme with distinct kinetic, regulatory and stability properties.

Hydroperoxide-induced oxidative stress impairs heart muscle cell carbohydrate metabolism

Hydrogen peroxide (H2O2) may incite cardiac ischemia-reperfusion injury. We evaluate herein the influence of H2O2-induced oxidative stress on heart muscle hexose metabolism in cultured neonatal rat cardiomyocytes, which have a substrate preference for carbohydrate. Cardiomyocyte exposure to 50 microM-1.0 mM bolus H2O2 transiently activated the pentose phosphate cycle and thereafter inhibited cellular glucose oxidation and glycolysis. These metabolic derangements were nonperoxidative in nature (as assessed in alpha-tocopherol-loaded cells) and occurred without acute change in cardiomyocyte hexose transport or glucose/glycogen reserves. Glycolytic inhibition was supported by the rapid, specific inactivation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The degree of GAPDH inhibition correlated directly with the magnitude of the oxidative insult and was independent of both metal-catalyzed H2O2 reduction to free radicals and lipid peroxidation. Severe GAPDH inhibition was required for a rate-limiting effect on glycolytic flux. Cardiomyocyte pyruvate dehydrogenase was also inhibited by H2O2 overload, but to a lesser degree than GAPDH such that entry of hexose-derived acetyl units into the tricarboxylic acid cycle was not as restrictive as GAPDH inactivation to glycolytic ATP production. An increase in phosphofructokinase activity accompanied GAPDH inactivation, leading to the production and accumulation of glycolytic sugar phosphates at the expense of ATP equivalents. Cardiomyocyte treatment with iodoacetate or 2-deoxyglucose indicated that GAPDH inactivation/glycolytic blockade could account for approximately 50% of the maximal ATP loss following H2O2 overload. Partial restoration of GAPDH activity after a brief H2O2 "pulse" afforded some ATP recovery. These data establish that specific aspects of heart muscle hexose catabolism are H2O2-sensitive injury targets. The biochemical pathology of H2O2 overload on cardiomyocyte carbohydrate metabolism has implications for post-ischemic cardiac bioenergetics and function.