Mitochondrial creatine kinase is a prime target of peroxynitrite-induced modification and inactivation

Multiple interference of anthracyclines with mitochondrial creatine kinases: preferential damage of the cardiac isoenzyme and its implications for drug cardiotoxicity

Anthracyclines are among the most efficient drugs of cancer chemotherapy, but their use is limited by a significant risk of cardiotoxicity, which is still far from being understood. This study investigates whether impairment of mitochondrial creatine kinase (MtCK), a key enzyme in cellular energy metabolism, could be involved in anthracycline cardiotoxicity. We have analyzed the effects of three anthracyclines, doxorubicin, daunorubicin, and idarubicin, on two MtCK isoenzymes, sarcomeric/cardiac sMtCK and ubiquitous uMtCK, from human and chicken. Using surface plasmon resonance, gel filtration, and enzyme assays, we have quantified properties that are of basic importance for MtCK functioning in vivo: membrane binding, octameric state, and enzymatic activity. Anthracyclines significantly impaired all three properties with differences in dose-, time-, and drug-dependence. Membrane binding and enzymatic activity were already affected at low anthracycline concentrations (5-100 microM), indicating high clinical relevance. Effects on membrane binding were immediate, probably because of competitive binding of the drug to cardiolipin. In contrast, dissociation of MtCK octamers into dimers, enzymatic inactivation and cross-linking occurred only after hours to days. Different protection assays suggest that the deleterious effects were caused by oxidative damage, mainly affecting the highly susceptible MtCK cysteines, followed by generation of free oxygen radicals at higher drug concentrations. Enzymatic inactivation occurred mainly at the active site and involved Cys278, as indicated by experiments with protective agents and sMtCK mutant C278G. All anthracycline effects were significantly more pronounced for sMtCK than for uMtCK. These in vitro results suggest that sMtCK damage may play a role in anthracycline cardiotoxicity.

Nitric oxide and peroxynitrite interactions with mitochondria

Nitric oxide (*NO) and peroxynitrite (ONOO-) avidly interact with mitochondrial components, leading to a range of biological responses spanning from the modulation of mitochondrial respiration, mitochondrial dysfunction to the signaling of apoptotic cell death. Physiological levels of *NO primarily interact with cytochrome c oxidase, leading to a competitive and reversible inhibition of mitochondrial oxygen uptake. In turn, this leads to alterations in electrochemical gradients, which affect calcium uptake and may regulate processes such as mitochondrial transition pore (MTP) opening and the release of pro-apoptotic proteins. Large or persistent levels of *NO in mitochondria promote mitochondrial oxidant formation. Peroxynitrite formed either extra- or intra-mitochondrially leads to oxidative damage, most notably at complexes I and II of the electron transport chain, ATPase, aconitase and Mn-superoxide dismutase. Mitochondrial scavenging systems for peroxynitrite and peroxynitrite-derived radicals such as carbonate (CO3*-) and nitrogen dioxide radicals (*NO2) include cytochrome c oxidase, glutathione and ubiquinol and serve to partially attenuate the reactions of these oxidants with critical mitochondrial targets. Detection of nitrated mitochondrial proteins in vivo supports the concept that mitochondria constitute central loci of the toxic effects of excess reactive nitrogen species. In this review we will provide an overview of the biochemical mechanisms by which *NO and ONOO- regulate or alter mitochondrial functions.

Cigarette smoke exposure and hypercholesterolemia increase mitochondrial damage in cardiovascular tissues

BACKGROUND

A shared feature among cardiovascular disease risk factors is increased oxidative stress. Because mitochondria are susceptible to damage mediated by oxidative stress, we hypothesized that risk factors (secondhand smoke and hypercholesterolemia) are associated with increased mitochondrial damage in cardiovascular tissues.

METHODS AND RESULTS

Atherosclerotic lesion formation, mitochondrial DNA damage, protein nitration, and specific activities of mitochondrial proteins in cardiovascular tissues from age-matched C57 and apoE(-/-) mice exposed to filtered air or secondhand smoke were quantified. Both secondhand smoke and hypercholesterolemia were associated with significantly increased mitochondrial DNA damage and protein nitration. Tobacco smoke exposure also resulted in significantly decreased specific activities of mitochondrial enzymes. The combination of secondhand smoke and hypercholesterolemia resulted in increased atherosclerotic lesion formation and even greater levels of mitochondrial damage.

CONCLUSIONS

These data are consistent with the hypothesis that cardiovascular disease risk factors cause mitochondrial damage and dysfunction.

Peroxynitrite oxidation of tubulin sulfhydryls inhibits microtubule polymerization

Considerable evidence both in vitro and in vivo implicates protein damage by peroxynitrite as a probable mechanism of cell death. Herein, we report that treatment of bovine brain microtubule protein, composed of tubulin and microtubule-associated proteins, with peroxynitrite led to a dose-dependent inhibition of microtubule polymerization. The extent of cysteine oxidation induced by peroxynitrite correlated well with inhibition of microtubule polymerization. Disulfide bonds between the subunits of the tubulin heterodimer were detected by Western blot as a result of peroxynitrite-induced cysteine oxidation. Addition of disulfide reducing agents including dithiothreitol and beta-mercaptoethanol restored a significant portion of the polymerization activity that was lost following peroxynitrite addition. Thus, peroxynitrite-induced disulfide bonds are at least partially responsible for the observed inhibition of polymerization. Sodium bicarbonate protected microtubule protein from the peroxynitrite-induced inhibition of polymerization. Tyrosine nitration of microtubule protein by 1 mM peroxynitrite increased approximately twofold when sodium bicarbonate was present whereas the extent of cysteine oxidation decreased from 7.5 to 6.3 mol cysteine/mol tubulin. These results indicate that cysteine oxidation of tubulin by peroxynitrite, rather than tyrosine nitration, is the primary mechanism of inhibition of microtubule polymerization.

Mouse models for mitochondrial disease

Mutations in mitochondrial genes encoded by both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) genes have been implicated in a wide range of neuromuscular diseases. MtDNA base substitution and rearrangement mutations generally inactivate one or more tRNA or rRNA genes and can cause myopathy, cardiomyopathy, cataracts, growth retardation, diabetes, etc. nDNA mutations can cause Leigh syndrome, cardiomyopathy, and nephropathy, due to defects in oxidative phosphorylation (OXPHOS) enzyme complexes; cartilage-hair hypoplasia (CHH) and mtDNA depletion syndrome, through defects in mitochondrial nucleic acid metabolism; and ophthalmoplegia with multiple mtDNA deletions, caused by adenine nucleotide translocator-1 (ANT1) mutations. Mouse models have been prepared that recapitulate a number of these diseases. The mtDNA 16S rRNA chloramphenicol (CAP) resistance mutation was introduced into the mouse female germline and caused cataracts and rod and cone abnormalities in chimeras and neonatal lethal myopathy and cardiomyopathy in mutant animals. A mtDNA deletion was introduced into the mouse germline and caused myopathy, cardiomyopathy, and nephropathy. Conditional inactivation of the nDNA mitochondrial transcription factor (Tfam) gene in the heart resulted in neonatal lethal cardiomyopathy, while its inactivation in the pancreatic beta-cells caused diabetes. The ATP/ADP ratio was implicated in mitochondrial diabetes through transgenic modification of the beta-cell ATP-sensitive K(+) channel (K(ATP)). Mutational inactivation of the mouse Ant1 gene resulted in myopathy, cardiomyopathy, and multiple mtDNA deletions in association with elevated reactive oxygen species (ROS) production. Inactivation of uncoupler proteins (Ucp) 1-3 revealed that mitochondrial Delta Psi regulated ROS production. The role of mitochondrial ROS toxicity in disease and aging was confirmed by inactivating glutathione peroxidase (GPx1), resulting in growth retardation, and by total and partial inactivation of Mn superoxide dismutase (MnSOD; Sod2), resulting in neonatal lethal dilated cardiomyopathy and accelerated apoptosis in aging, respectively. The importance of mitochondrial ROS in degenerative diseases and aging was confirmed by treating Sod2 -/- mice and C. elegans with catalytic antioxidant drugs.

Gene expression profiling of aging using DNA microarrays

We have previously employed high density oligonucleotide arrays representing thousands of genes to determine the gene expression profile of the aging process in skeletal muscle (gastrocnemius) and brain (cerebellum and neocortex) of male C57BL/6 mice. Specific gene expression profiles are associated with the aging process of individual organs, and caloric restriction can prevent or retard the establishment of these gene expression alterations. The use of DNA microarrays may provide a new tool to measure biological age on a tissue-specific basis and to evaluate at the molecular level the efficacy of interventions designed to retard the aging process.

Mitochondrial dysfunction in cardiac disease: ischemia--reperfusion, aging, and heart failure

Mitochondria contribute to cardiac dysfunction and myocyte injury via a loss of metabolic capacity and by the production and release of toxic products. This article discusses aspects of mitochondrial structure and metabolism that are pertinent to the role of mitochondria in cardiac disease. Generalized mechanisms of mitochondrial-derived myocyte injury are also discussed, as are the strengths and weaknesses of experimental models used to study the contribution of mitochondria to cardiac injury. Finally, the involvement of mitochondria in the pathogenesis of specific cardiac disease states (ischemia, reperfusion, aging, ischemic preconditioning, and cardiomyopathy) is addressed.

Creatine transporter and mitochondrial creatine kinase protein content in myopathies

Total creatine or phosphocreatine, or both, are reduced in the skeletal muscle of patients with inflammatory myopathy, mitochondrial myopathy, and muscular dystrophy/congenital myopathy. We used Western blotting techniques to measure skeletal muscle creatine transporter protein and sarcomeric mitochondrial creatine kinase (mtCK) protein content in patients with inflammatory myopathy (N = 8), mitochondrial myopathy (N = 5), muscular dystrophy (N = 7), and congenital myopathy (N = 3), as compared to a control group without a neuromuscular diagnosis (N = 8). Creatine transporter protein content was lower for all groups compared to control subjects (P < 0.05; P < 0.01 for congenital myopathy). Mitochondrial CK (mtCK) was lower for inflammatory myopathy (P < 0.05), higher for mitochondrial myopathy (P < 0.05), not different for muscular dystrophy, and markedly lower for the congenital myopathy group (P < 0.01), compared to control subjects. Together, these data suggest that the reduction in total creatine or phosphocreatine in patients with certain myopathies is correlated with creatine transporter and not mtCK protein content. This further supports the belief that creatine monohydrate supplementation may benefit patients with low muscle creatine stores, although the reduction in creatine transporter protein may have implications for dosing.

Microarray profiling of gene expression in aging and its alteration by caloric restriction in mice

An active research area in biological gerontology concerns the mechanisms by which caloric restriction (CR) retards the aging process in laboratory rodents. We used high density oligonucleotide arrays representing 6347 genes to determine the gene expression profile of the aging process in gastrocnemius muscle of male C57BL/6 mice. Aging resulted in a differential gene expression pattern indicative of a marked stress response and lower expression of metabolic and biosynthetic genes. Most alterations were completely or partially prevented by CR. Transcriptional patterns of muscle from calorie-restricted animals suggest that CR retards the aging process by causing a metabolic shift toward increased protein turnover and decreased macromolecular damage. The use of high density oligonucleotide microarrays provides a new tool to measure biological age on a tissue-specific basis and to evaluate at the molecular level the efficacy of nutritional interventions designed to retard the aging process.

Phosphorylation-dependent alteration in myofilament ca2+ sensitivity but normal mitochondrial function in septic heart

The subcellular mechanisms responsible for myocardial depression during sepsis remain unclear. Recent data suggest a role for impaired energy generation and utilization, resulting in altered contractile function. Here, we studied the energetic and mechanical properties of skinned fibers isolated from rabbit ventricle in a nonlethal but hypotensive model of endotoxemia. Thirty-six hours after lipopolysaccharide (LPS) injection (in the presence of altered myocardial contractility), mitochondrial respiration, coupling between oxidation and phosphorylation, and creatine kinase function were similar in preparations from endotoxemic (LPS) and control animals. The maximal Ca2+-activated force was similar in LPS and control preparations. However, the Ca2+ concentration corresponding to half-maximal force (pCa50, where pCa = -log10[Ca2+]) was 5.55 +/- 0.01 (n = 11) in LPS fibers versus 5.61 +/- 0.01 (n = 10) in control fibers (p < 0.01). Both protein kinase A (PKA) and alkaline phosphatase treatment led to the disappearance in the difference between control and LPS pCa50 values. Incubation of control fibers with the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) did not change the Ca2+ sensitivity after subsequent skinning, whereas isoproterenol decreased pCa50 from 5.62 +/- 0.01 to 5.55 +/- 0.01 (p < 0.01). These data suggest that during sepsis, cardiac mitochondrial and creatine kinase systems remain unaltered, whereas protein phosphorylation decreases myofibrillar Ca2+ sensitivity and may contribute to the depression of cardiac contractility.

On the mechanisms of neuroprotection by creatine and phosphocreatine

Creatine and phosphocreatine were evaluated for their ability to prevent death of cultured striatal and hippocampal neurons exposed to either glutamate or 3-nitropropionic acid (3NP) and to inhibit the mitochondrial permeability transition in CNS mitochondria. Phosphocreatine (PCr), and to a lesser extent creatine (Cr), but not (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK801), dose-dependently ameliorated 3NP toxicity when applied simultaneously with the 3NP in Mg2+-free media. Pre-treatment of PCr for 2 or 5 days and Cr for 5 days protected against glutamate excitotoxicity equivalent to that achieved by MK801 post-treatment. The combination of PCr or Cr pre-treatment and MK801 post-treatment did not provide additional protection, indicating that both prevented the toxicity attributable to secondary glutamate release. To determine if Cr or PCr directly inhibited the permeability transition, mitochondrial swelling and depolarization were assayed in isolated, purified brain mitochondria. PCr reduced the amount of swelling induced by calcium by 20%. Cr decreased mitochondrial swelling when inhibitors of creatine kinase octamer-dimer transition were present. However, in brain mitochondria prepared from rats fed a diet supplemented with 2% creatine for 2 weeks, the extent of calcium-induced mitochondrial swelling was not altered. Thus, the neuroprotective properties of PCr and Cr may reflect enhancement of cytoplasmic high-energy phosphates but not permeability transition inhibition.

Neuronal plasticity and stressor toxicity during aging

Brain aging, Alzheimer disease and stroke share common elements of deficits in calcium regulation, declines in mitochondrial function, increases in generation of reactive oxygen species (ROS), accumulated damage from ROS and immune system dysfunction. The problem is to distinguish less significant side reactions, such as gray hair, from aspects of aging that contribute to disease. Toward establishing cause and effect relationships, a neuron cell culture system is described that allows comparisons with age under uniform environmental conditions. This neuron culture model indicates that susceptibility to death by apoptosis and consequences of the inflammatory response from beta-amyloid are age-related and an inherent characteristic of the neurons. Further mechanistic investigations are possible. New therapeutic approaches are suggested that combine inhibition of calcium overloads (calcium channel blockers), reduced ROS damage (melatonin, N-acetyl-cysteine), and bolstered mitochondrial function and energy generation (creatine). Together with newly demonstrated capabilities for adult and aged neuron regeneration and multiplication, i.e. plasticity, these approaches offer new hope toward reversing age-related decrements and damage from neurodegenerative disease.

Potential benefits of creatine monohydrate supplementation in the elderly

Creatine plays a role in cellular energy metabolism and potentially has a role in protein metabolism. Creatine monohydrate supplementation has been shown to result in an increase in skeletal muscle total and phosphocreatine concentration, increase fat-free mass, and enhance high-intensity exercise performance in young healthy men and women. Recent evidence has also demonstrated a neuroprotective effect of creatine monohydrate supplementation in animal models of Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, and after ischemia. A low total and phosphocreatine concentration has been reported in human skeletal muscle from aged individuals and those with neuromuscular disorders. A few studies of creatine monohydrate supplementation in the elderly have not shown convincing evidence of a beneficial effect with respect to muscle mass and/or function. Future studies will be required to address the potential for creatine monohydrate supplementation to attenuate age-related muscle atrophy and strength loss, as well as to protect against age-dependent neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease.

Protein nitration

Various proteins/enzymes obtained commercially were tested for the presence of endogenously nitrated tyrosine by Western blot analysis omitting reducing agent in the step of SDS-PAGE. Histones II-S and VIII-S, IgG, cAMP-dependent protein kinase (PKA), phosphorylase b, and phosphorylase kinase exhibited strong immunoreactive bands. Histone VI-S, glycogen synthase, lactate dehydrogenase, actin, thyroglobulin, and macroglobulin exhibited moderate immunoreactivity. Histone III-S, casein, acetyl cholinesterase, DNase I, and lipase had only traceable immunoreactivity. Whereas histone VII-S, pyruvate kinase, trypsin, pepsin, chymotrypsin, protease IV, and protease XIII, and glutathione S-transferase lacked immunoreactivity. A variation of immunoreactivity between hypertensive and normaltensive rat hearts was found in the histone-agarose fractions of crude extracts. Additionally, nitrotyrosine immunoreactivity was observed in non-mammalian organisms including Eschericia coli, Saccharomyces cerevisiae and Triticum vulgaris. Upon the treatment of 15 microM peroxynitrite (PN), strong oxidant derived from nitric oxide (NO), the apparent Km of PKA for cAMP increased from approximately 10(-8) to 10(-6) M. The results imply that the varied nitration of tyrosine residues in proteins/enzymes may occur as a post-translational modification in vivo, and such discriminative nitration may be vital in PN/NO-regulated signal transduction cascade.

Is greater tissue activity of creatine kinase the genetic factor increasing hypertension risk in black people of sub-Saharan African descent?

We postulate that the genetic factor increasing the propensity of black people of sub-Saharan African descent to develop high blood pressure is the relatively high activity of creatine kinase, predominantly in vascular and cardiac muscle tissue. Such greater activity of creatine kinase has been reported in skeletal muscle of black untrained subjects has been reported to be almost twice the activity found in white subjects. Creatine kinase, a key enzyme of cellular energy metabolism, increases the capacity of the cell to function under high demands. The enzyme regulates, buffers and transports, via phosphocreatine and creatine, energy produced by glycolysis and oxidative phosphorylation to sites of energy consumption such as myofibrils and membrane ion pumps. At these cellular locations, it is involved in the contraction process and active trans- membranous transport by readily providing the ATP needed for these processes. In addition, creatine kinase is increasingly reported to be involved in trophic responses. Furthermore, by using H+ and ADP to synthesize ATP, creatine kinase prevents acidification of the cell, providing relative protection against the effects of ischaemia. Greater creatine kinase activity in cardiovascular muscle and other tissues with high energy demands could increase cardiovascular contractile reserve, enhance trophic responses and increase renal tubular ability to retain salt. This could facilitate the development of arterial hypertension under chronic provocative circumstances, with higher mean blood pressures, more left ventricular hypertrophy and relatively fewer ischaemic events. Therefore, greater cellular activity of creatine kinase might explain the greater hypertension risk and the clinical characteristics of hypertensive disease observed in the black population.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Neuroprotective effects of creatine in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative illness for which there is no effective therapy. We examined whether creatine, which may exert neuroprotective effects by increasing phosphocreatine levels or by stabilizing the mitochondrial permeability transition, has beneficial effects in a transgenic mouse model of HD (line 6/2). Dietary creatine supplementation significantly improved survival, slowed the development of brain atrophy, and delayed atrophy of striatal neurons and the formation of huntingtin-positive aggregates in R6/2 mice. Body weight and motor performance on the rotarod test were significantly improved in creatine-supplemented R6/2 mice, whereas the onset of diabetes was markedly delayed. Nuclear magnetic resonance spectroscopy showed that creatine supplementation significantly increased brain creatine concentrations and delayed decreases in N-acetylaspartate concentrations. These results support a role of metabolic dysfunction in a transgenic mouse model of HD and suggest a novel therapeutic strategy to slow the pathological process.

Octamers of mitochondrial creatine kinase isoenzymes differ in stability and membrane binding

Octamer stability and membrane binding of mitochondrial creatine kinase (MtCK) are important for proper functioning of the enzyme and were suggested as targets for regulatory mechanisms. A quantitative analysis of these properties, using fluorescence spectroscopy, gel filtration, and surface plasmon resonance, revealed substantial differences between the two types of MtCK isoenzymes, sarcomeric (sMtCK) and ubiquitous (uMtCK). As compared with human and chicken sMtCK, human uMtCK showed a 23-34 times slower octamer dissociation rate, a reduced reoctamerization rate and a superior octamer stability as deduced from the octamer/dimer ratios at thermodynamic equilibrium. Octamer stability of sMtCK increased with temperature up to 30 degrees C, indicating a substantial contribution of hydrophobic interactions, while it decreased in the case of uMtCK, indicating the presence of additional polar dimer/dimer interactions. These conclusions are consistent with the recently solved x-ray structure of the human uMtCK (Eder, M., Fritz-Wolf, K., Kabsch, W., Wallimann, T., and Schlattner, U. (2000) Proteins 39, 216-225). When binding to 16% cardiolipin membranes, sMtCK showed slightly faster on-rates and higher affinities than uMtCK. However, human uMtCK was able to recruit the highest number of binding sites on the vesicle surface. The observed divergence of ubiquitous and sarcomeric MtCK is discussed with respect to their molecular structures and the possible physiological implications.

Mitogen-activated protein kinase phosphorylation in kidneys of beta(s) sickle cell mice

Previous studies in beta(s) sickle cell mice demonstrated renal immunostaining for nitrotyrosine, which is putative evidence of peroxynitrite (ONOO(-)) formation. ONOO(-) is known to nitrate tyrosine residues of various enzymes, thereby interfering with phosphorylation and inactivating them. The present study examined the state of phosphorylation of mitogen-activated protein (MAP) kinase signal transduction enzymes, i.e., p38, c-Jun NH(2)-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK). Western blot performed with antibodies directed against specific phosphorylated threonine/tyrosine residues of these enzymes demonstrated reduced phosphorylation of renal p38 and a trend toward reduced phosphorylation of ERK. In contrast, phosphorylation of renal JNK was markedly increased compared with normal mice. The abundance of MAP kinase phosphatase-1 (MKP-1), a key upstream enzyme that modulates phosphorylation of MAP kinases, was not different in beta(s) versus normal mice. To determine whether nitration of tyrosine by ONOO(-) was responsible for reduced phosphorylation of p38 and ERK, mercaptoethylguanidine (MEG), a compound known to reduce inducible isoform of nitric oxide synthase activity and to scavenge ONOO(-), was administered to beta(s) mice for 5 d. MEG was found to restore phosphorylation of p38 and ERK toward normal levels. These observations provide evidence that ONOO(-) (or closely related reaction products of NO) contributes to dephosphorylation of p38 and ERK, and presumably reduces activity of these enzymes. The increased phosphorylation of JNK, which suggests activation of this signaling pathway by extracellular stress signals, may play a role in apoptosis in the kidneys of these mice. The changes in phosphorylation of MAP kinase pathways found in this study could have important consequences for regulation of nuclear transcription factors, and thus renal function and pathology in sickle cell kidneys.

Crystal structure of human ubiquitous mitochondrial creatine kinase

Creatine kinase (CK), catalyzing the reversible trans-phosphorylation between ATP and creatine, plays a key role in the energy metabolism of cells with high and fluctuating energy requirements. We have solved the X-ray structure of octameric human ubiquitous mitochondrial CK (uMtCK) at 2.7 A resolution, representing the first human CK structure. The structure is very similar to the previously determined structure of sarcomeric mitochondrial CK (sMtCK). The cuboidal octamer has 422 point group symmetry with four dimers arranged along the fourfold axis and a central channel of approximately 20 A diameter, which extends through the whole octamer. Structural differences with respect to sMtCK are found in isoform-specific regions important for octamer formation and membrane binding. Octameric uMtCK is stabilized by numerous additional polar interactions between the N-termini of neighboring dimers, which extend into the central channel and form clamp-like structures, and by a pair of salt bridges in the hydrophobic interaction patch. The five C-terminal residues of uMtCK, carrying positive charges likely to be involved in phospholipid-binding, are poorly defined by electron density, indicating a more flexible region than the corresponding one in sMtCK. The structural differences between uMtCK and sMtCK are consistent with biochemical studies on octamer stability and membrane binding of the two isoforms.

Protective effect of the energy precursor creatine against toxicity of glutamate and beta-amyloid in rat hippocampal neurons

The loss of ATP, which is needed for ionic homeostasis, is an early event in the neurotoxicity of glutamate and beta-amyloid (A(beta)). We hypothesize that cells supplemented with the precursor creatine make more phosphocreatine (PCr) and create larger energy reserves with consequent neuroprotection against stressors. In serum-free cultures, glutamate at 0.5-1 mM was toxic to embryonic hippocampal neurons. Creatine at >0.1 mM greatly reduced glutamate toxicity. Creatine (1 mM) could be added as late as 2 h after glutamate to achieve protection at 24 h. In association with neurotoxic protection by creatine during the first 4 h, PCr levels remained constant, and PCr/ATP ratios increased. Morphologically, creatine protected against glutamate-induced dendritic pruning. Toxicity in embryonic neurons exposed to A(beta) (25-35) for 48 h was partially prevented by creatine as well. During the first 6 h of treatment with A(beta) plus creatine, the molar ratio of PCr/ATP in neurons increased from 15 to 60. Neurons from adult rats were also partially protected from a 24-h exposure to A(beta) (25-35) by creatine, but protection was reduced in neurons from old animals. These results suggest that fortified energy reserves are able to protect neurons against important cytotoxic agents. The oral availability of creatine may benefit patients with neurodegenerative diseases.

Free radical induced inactivation of creatine kinase: sites of interaction, protection, and recovery

The study aims at a clarification of the oxidative damage of creatine kinase isoenzymes by X-ray-induced water radiolysis. The radical species generated by this method (under appropriate conditions) are similar to those discussed in the context of mitochondrial energy metabolism. The decay of the enzyme activity is accompanied by a strong decrease of the number of accessible SH groups and by a reduction of the endogenous tryptophan fluorescence. Free radical effects are diminished if irradiation is carried out in the presence of 2-mercaptoethanol. Partial recovery of the activity (repair) is observed if 2-mercaptoethanol is added after irradiation. The experiments suggest a twofold importance of thiol reagents (RSH): to reduce the concentration of free radicals by scavenger reactions and to modify the inactivation mechanism in such a way that efficient repair of enzyme damage may be achieved. Cysteine 282 of MM-CK (Cys-278 in the case of Mi-CK) seems to play a crucial role in this respect. Blockage of the SH group of cysteine 282 by oxidized glutathione effectively protects the enzyme against inactivation by NO(*)(2) radicals. In the absence of nitrogen dioxide and of thiol reagents, however, inactivation seems to proceed via a less specific mechanism involving additional targets of the enzyme.

Neuroprotective effects of creatine administration against NMDA and malonate toxicity

We examined whether creatine administration could exert neuroprotective effects against excitotoxicity mediated by N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid. Oral administration of 1% creatine significantly attenuated striatal excitotoxic lesions produced by NMDA, but had no effect on lesions produced by AMPA or kainic acid. Both creatine and nicotinamide can exert significant protective effects against malonate-induced striatal lesions. We, therefore, examined whether nicotinamide could exert additive neuroprotective effects with creatine against malonate-induced lesions. Nicotinamide with creatine produced significantly better neuroprotection than creatine alone against malonate-induced lesions. Creatine can, therefore, produce significant neuroprotective effects against NMDA mediated excitotoxic lesions in vivo and the combination of nicotinamide with creatine exerts additive neuroprotective effects.

Gene expression profile of aging and its retardation by caloric restriction

The gene expression profile of the aging process was analyzed in skeletal muscle of mice. Use of high-density oligonucleotide arrays representing 6347 genes revealed that aging resulted in a differential gene expression pattern indicative of a marked stress response and lower expression of metabolic and biosynthetic genes. Most alterations were either completely or partially prevented by caloric restriction, the only intervention known to retard aging in mammals. Transcriptional patterns of calorie-restricted animals suggest that caloric restriction retards the aging process by causing a metabolic shift toward increased protein turnover and decreased macromolecular damage.

Peroxynitrite: reactive, invasive and enigmatic

The reactive oxygen and nitrogen species superoxide ion, nitric oxide, nitrogen dioxide and peroxynitrite ion all react with biological target molecules. Some of these interactions are carefully orchestrated segments of signal transduction cascades or part of the armamentarium of the immune system, others are pathological events and may lie at the root of many diseases. As a result of these small reactive molecules, proteins, particularly metalloproteins, can be altered with loss of function, DNA can be cleaved and lipid components can be oxidized to disrupt membranes. The interactions of these species with each other and their aftermath can be sensed by the cell, resulting in a variety of responses including gene regulation and transcription. Indeed, there is recent, tantalizing evidence that the currency of reactive oxygen and nitrogen species is central to the life and death cellular decisions in homeostasis or the initiation of apoptosis. New families of metallopharmaceuticals may serve both to probe the nature and mechanisms of these events and to effect the outcome.

Nitric oxide inhibits cardiac energy production via inhibition of mitochondrial creatine kinase

Nitric oxide biosynthesis in cardiac muscle leads to a decreased oxygen consumption and lower ATP synthesis. It is suggested that this effect of nitric oxide is mainly due to the inhibition of the mitochondrial respiratory chain enzyme, cytochrome c oxidase. However, this work demonstrates that nitric oxide is able to inhibit soluble mitochondrial creatine kinase (CK), mitochondrial CK bound in purified mitochondria, CK in situ in skinned fibres as well as the functional activity of mitochondrial CK in situ in skinned fibres. Since mitochondrial isoenzyme is functionally coupled to oxidative phosphorylation, its inhibition also leads to decreased sensitivity of mitochondrial respiration to ADP and thus decreases ATP synthesis and oxygen consumption under physiological ADP concentrations.

Some new aspects of creatine kinase (CK): compartmentation, structure, function and regulation for cellular and mitochondrial bioenergetics and physiology

Creatine kinase (CK) isoenzymes, specifically located at places of energy demand and energy production, are linked by a phosphocreatine/creatine (PCr/Cr) circuit, found in cells with intermittently high energy demands. Cytosolic CKs, in close conjunction with Ca(2+)-pumps, play a crucial role for the energetics of Ca(2+)-homeostasis. Mitochondrial Mi-CK, a cuboidal-shaped octamer with a central channel, binds and crosslinks mitochondrial membranes and forms a functionally coupled microcompartment with porin and adenine nucleotide translocase for vectorial export of PCr into the cytosol. The CK system is regulated by AMP-activated protein kinase via PCr/Cr and ATP/AMP ratios. Mi-CK stabilizes and cross-links cristae- or inner/outer membranes to form parallel membrane stacks and, if overexpressed due to creatine depletion or cellular energy stress, forms those crystalline intramitochondrial inclusions seen in some mitochondrial cytopathy patients. Mi-CK is a prime target for free radical damage by peroxynitrite. Mi-CK octamers, together with CK substrates have a marked stabilizing and protective effect against mitochondrial permeability transition pore opening, thus providing a rationale for creatine supplementation of patients with neuromuscular and neurodegenerative diseases.