Dietary restriction attenuates age-associated muscle atrophy by lowering oxidative stress in mice even in complete absence of CuZnSOD

Age-related loss of muscle mass and function, sarcopenia, has a major impact on the quality of life in the elderly. Among the proposed causes of sarcopenia are mitochondrial dysfunction and accumulated oxidative damage during aging. Dietary restriction (DR), a robust dietary intervention that extends lifespan and modulates age-related pathology in a variety of species, has been shown to protect from sarcopenia in rodents. Although the mechanism(s) by which DR modulates aging are still not defined, one potential mechanism is through modulation of oxidative stress and mitochondrial dysfunction. To directly test the protective effect of DR against oxidative stress-induced muscle atrophy in vivo, we subjected mice lacking a key antioxidant enzyme, CuZnSOD (Sod1) to DR (60% of ad libitum fed diet). We have previously shown that the Sod1(-/-) mice exhibit an acceleration of sarcopenia associated with high oxidative stress, mitochondrial dysfunction, and severe neuromuscular innervation defects. Despite the dramatic atrophy phenotype in the Sod1(-/-) mice, DR led to a reversal or attenuation of reduced muscle function, loss of innervation, and muscle atrophy in these mice. DR improves mitochondrial function as evidenced by enhanced Ca(2+) regulation and reduction of mitochondrial reactive oxygen species (ROS). Furthermore, we show upregulation of SIRT3 and MnSOD in DR animals, consistent with reduced mitochondrial oxidative stress and reduced oxidative damage in muscle tissue measured as F(2) -isoprostanes. Collectively, our results demonstrate that DR is a powerful mediator of mitochondrial function, mitochondrial ROS production, and oxidative damage, providing a solid protection against oxidative stress-induced neuromuscular defects and muscle atrophy in vivo even under conditions of high oxidative stress.     

Absence of CuZn superoxide dismutase leads to elevated oxidative stress and acceleration of age-dependent skeletal muscle atrophy

We describe a novel phenotype in mice lacking the major antioxidant enzyme, CuZn-superoxide dismutase (Sod1(-/-) mice), namely a dramatic acceleration of age-related loss of skeletal muscle mass. Sod1(-/-) mice are 17 to 20% smaller and have a significantly lower muscle mass than wild-type mice as early as 3 to 4 months of age. Muscle mass in the Sod1(-/-) mice is further reduced with age and by 20 months, the hind-limb muscle mass in Sod1(-/-) mice is nearly 50% lower than in age-matched wild-type mice. Skeletal muscle tissue from young Sod1(-/-) mice has elevated oxidative damage to proteins, lipids, and DNA compared to muscle from young wild-type mice. The reduction in muscle mass and elevated oxidative damage are accompanied by a 40% decrease in voluntary wheel running by 6 months of age and decreased performance on the Rota-rod test at 13 months of age, but are not associated with a decline in overall spontaneous activity. In some of the old Sod1(-/-) mice, the loss in muscle mass is also associated with the presence of tremors and gait disturbances. Thus, the absence of CuZnSOD imposes elevated oxidative stress, loss of muscle mass, and physiological consequences that resemble an acceleration of normal age-related sarcopenia.     

Physical exercise regulates p53 activity targeting SCO2 and increases mitochondrial COX biogenesis in cardiac muscle with age

The purpose of this study was to outline the timelines of mitochondrial function, oxidative stress and cytochrome c oxidase complex (COX) biogenesis in cardiac muscle with age, and to evaluate whether and how these age-related changes were attenuated by exercise. ICR/CD-1 mice were treated with pifithrin-μ (PFTμ), sacrificed and studied at different ages; ICR/CD-1 mice at younger or older ages were randomized to endurance treadmill running and sedentary conditions. The results showed that mRNA expression of p53 and its protein levels in mitochondria increased with age in cardiac muscle, accompanied by increased mitochondrial oxidative stress, reduced expression of COX subunits and assembly proteins, and decreased expression of most markers in mitochondrial biogenesis. Most of these age-related changes including p53 activity targeting cytochrome oxidase deficient homolog 2 (SCO2), p53 translocation to mitochondria and COX biogenesis were attenuated by exercise in older mice. PFTμ, an inhibitor blocking p53 translocation to mitochondria, increased COX biogenesis in older mice, but not in young mice. Our data suggest that physical exercise attenuates age-related changes in mitochondrial COX biogenesis and p53 activity targeting SCO2 and mitochondria, and thereby induces antisenescent and protective effects in cardiac muscle.     

The histone deacetylase inhibitor butyrate improves metabolism and reduces muscle atrophy during aging

Sarcopenia, the loss of skeletal muscle mass and function during aging, is a major contributor to disability and frailty in the elderly. Previous studies found a protective effect of reduced histone deacetylase activity in models of neurogenic muscle atrophy. Because loss of muscle mass during aging is associated with loss of motor neuron innervation, we investigated the potential for the histone deacetylase (HDAC) inhibitor butyrate to modulate age‐related muscle loss. Consistent with previous studies, we found significant loss of hindlimb muscle mass in 26‐month‐old C57Bl/6 female mice fed a control diet. Butyrate treatment starting at 16 months of age wholly or partially protected against muscle atrophy in hindlimb muscles. Butyrate increased muscle fiber cross‐sectional area and prevented intramuscular fat accumulation in the old mice. In addition to the protective effect on muscle mass, butyrate reduced fat mass and improved glucose metabolism in 26‐month‐old mice as determined by a glucose tolerance test. Furthermore, butyrate increased markers of mitochondrial biogenesis in skeletal muscle and whole‐body oxygen consumption without affecting activity. The increase in mass in butyrate‐treated mice was not due to reduced ubiquitin‐mediated proteasomal degradation. However, butyrate reduced markers of oxidative stress and apoptosis and altered antioxidant enzyme activity. Our data is the first to show a beneficial effect of butyrate on muscle mass during aging and suggests HDACs contribute to age‐related muscle atrophy and may be effective targets for intervention in sarcopenia and age‐related metabolic disease.     

PARK2/Parkin-mediated mitochondrial clearance contributes to proteasome activation during slow-twitch muscle atrophy via NFE2L1 nuclear translocation

Skeletal muscle atrophy is thought to result from hyperactivation of intracellular protein degradation pathways, including autophagy and the ubiquitin–proteasome system. However, the precise contributions of these pathways to muscle atrophy are unclear. Here, we show that an autophagy deficiency in denervated slow-twitch soleus muscles delayed skeletal muscle atrophy, reduced mitochondrial activity, and induced oxidative stress and accumulation of PARK2/Parkin, which participates in mitochondrial quality control (PARK2-mediated mitophagy), in mitochondria. Soleus muscles from denervated Park2 knockout mice also showed resistance to denervation, reduced mitochondrial activities, and increased oxidative stress. In both autophagy-deficient and Park2-deficient soleus muscles, denervation caused the accumulation of polyubiquitinated proteins. Denervation induced proteasomal activation via NFE2L1 nuclear translocation in control mice, whereas it had little effect in autophagy-deficient and Park2-deficient mice. These results suggest that PARK2-mediated mitophagy plays an essential role in the activation of proteasomes during denervation atrophy in slow-twitch muscles.     

Two subpopulations of mitochondria in the aging rat heart display heterogenous levels of oxidative stress

Cardiac mitochondria are composed of two distinct subpopulations: one beneath the sarcolemma (subsarcolemmal mitochondria: SSM), and another along the myofilaments (interfibrillary mitochondria: IFM). Previous studies suggest a preferential loss of IFM function with age; however, the age-related changes in oxidative stress in these mitochondrial subpopulations have not been examined. To this end, the changes in mitochondrial antioxidant capacity, oxidant output, and oxidative damage to Complex IV in IFM and SSM from young and old rats were studied. Results show no apparent differences in any parameters examined between IFM and SSM from young rats. However, relative to young, only IFM from old rats had a significantly higher rate of oxidant production and a decline in mitochondrial ascorbate levels and GSH redox status. The age-related decline in mitochondrial antioxidant capacity in IFM was accompanied by a marked loss in glutaredoxin and GSSG reductase activities, suggesting a diminished reductive capacity in IFM with age. Moreover, the loss in Complex IV activity was limited to the IFM of old rats, which was accompanied by a 4-fold increase in 4-hydroxynonenal-modified Complex IV. Thus, mitochondrial decay is not uniform and further indicates that myofibrils may be uniquely under oxidative stress in the aging heart.     

Curcumin ameliorates CKD-induced mitochondrial dysfunction and oxidative stress through inhibiting GSK-3β activity

Curcumin has been reported to attenuate muscle atrophy. However, the underling mechanism remains unclear. The aim of this study was to investigate whether curcumin could improve chronic kidney disease (CKD)-induced muscle atrophy and mitochondrial dysfunction by inhibiting glycogen synthase kinase-3β (GSK-3β) activity. The sham and CKD mice were fed either a control diet or an identical diet containing 0.04% curcumin for 12 weeks. The C2C12 myotubes were treated with H₂O₂ in the presence or absence of curcumin. In addition, wild-type and muscle-specific GSK-3β knockout (KO) CKD model mice were made by 5/6 nephrectomy, and the sham was regarded as control. Curcumin could exert beneficial effects, including weight maintenance and improved muscle function, increased mitochondrial biogenesis, alleviated mitochondrial dysfunction by increasing adenosine triphosphate levels, activities of mitochondrial electron transport chain complexes and basal mitochondrial respiration and suppressing mitochondrial membrane potential. In addition, curcumin modulated redox homeostasis by increasing antioxidant activity and suppressed mitochondrial oxidative stress. Moreover, the protective effects of curcumin had been found to be mediated via inhibiting GSK-3β activity in vitro and in vivo. Importantly, GSK-3β KO contributed to improved mitochondrial function, attenuated mitochondrial oxidative damage and augmented mitochondrial biogenesis in muscle of CKD. Overall, this study suggested that curcumin alleviated CKD-induced mitochondrial oxidative damage and mitochondrial dysfunction via inhibiting GSK-3β activity in skeletal muscle.     

A Link between Mitochondrial Dysregulation and Idiopathic Autism Spectrum Disorder (ASD): Alterations in Mitochondrial Respiratory Capacity and Membrane Potential

Autism spectrum disorder (ASD) is a neurological disorder triggered by various factors through complex mechanisms. Research has been done to elucidate the potential etiologic mechanisms in ASD, but no single cause has been confirmed. The involvement of oxidative stress is correlated with ASD and possibly affects mitochondrial function. This study aimed to elucidate the link between mitochondrial dysregulation and idiopathic ASD by focusing on mitochondrial respiratory capacity and membrane potential. Our findings showed that mitochondrial function in the energy metabolism pathway was significantly dysregulated in a lymphoblastoid cell line (LCL) derived from an autistic child (ALCL). Respiratory capacities of oxidative phosphorylation (OXPHOS), electron transfer of the Complex I and Complex II linked pathways, membrane potential, and Complex IV activity of the ALCL were analyzed and compared with control cell lines derived from a developmentally normal non-autistic sibling (NALCL). All experiments were performed using high-resolution respirometry. Respiratory capacities of OXPHOS, electron transfer of the Complex I- and Complex II-linked pathways, and Complex IV activity of the ALCL were significantly higher compared to healthy controls. Mitochondrial membrane potential was also significantly higher, measured in the Complex II-linked pathway during LEAK respiration and OXPHOS. These results indicate the abnormalities in mitochondrial respiratory control linking mitochondrial function with autism. Correlating mitochondrial dysfunction and autism is important for a better understanding of ASD pathogenesis in order to produce effective interventions.     

4-Hydroxy-2(E)-nonenal inhibits CNS mitochondrial respiration at multiple sites

A destructive cycle of oxidative stress and mitochondrial dysfunction is proposed in neurodegenerative disease. Lipid peroxidation, one outcome of oxidative challenge, can lead to the formation of 4-hydroxy-2(E)-nonenal (HNE), a lipophilic alkenal that forms stable adducts on mitochondrial proteins. In this study, we characterized the effects of HNE on brain mitochondrial respiration. We used whole rat brain mitochondria and concentrations of HNE comparable to those measured in patients with Alzheimer's disease. Our results showed that HNE inhibited respiration at multiple sites. Complex I-linked and complex II-linked state 3 respirations were inhibited by HNE with IC50 values of approximately 200 microM HNE. Respiration was apparently diminished owing to the inhibition of complex III activity. In addition, complex II activity was reduced slightly. The lipophilicity and adduction characteristics of HNE were responsible for the effects of HNE on respiration. The inhibition of respiration was not prevented by N-acetylcysteine or aminoguanidine. Studies using mitochondria isolated from porcine cerebral cortex also demonstrated an inhibition of complex I- and complex II-linked respiration. Thus, in neurodegenerative disease, oxidative stress may impair mitochondrial respiration through the production of HNE.     

PARK2/Parkin-mediated mitochondrial clearance contributes to proteasome activation during slow-twitch muscle atrophy via NFE2L1 nuclear translocation

Skeletal muscle atrophy is thought to result from hyperactivation of intracellular protein degradation pathways, including autophagy and the ubiquitin–proteasome system. However, the precise contributions of these pathways to muscle atrophy are unclear. Here, we show that an autophagy deficiency in denervated slow-twitch soleus muscles delayed skeletal muscle atrophy, reduced mitochondrial activity, and induced oxidative stress and accumulation of PARK2/Parkin, which participates in mitochondrial quality control (PARK2-mediated mitophagy), in mitochondria. Soleus muscles from denervated Park2 knockout mice also showed resistance to denervation, reduced mitochondrial activities, and increased oxidative stress. In both autophagy-deficient and Park2-deficient soleus muscles, denervation caused the accumulation of polyubiquitinated proteins. Denervation induced proteasomal activation via NFE2L1 nuclear translocation in control mice, whereas it had little effect in autophagy-deficient and Park2-deficient mice. These results suggest that PARK2-mediated mitophagy plays an essential role in the activation of proteasomes during denervation atrophy in slow-twitch muscles.     

Acute and severe hypobaric hypoxia increases oxidative stress and impairs mitochondrial function in mouse skeletal muscle.

Severe high-altitude hypoxia exposure is considered a triggering stimulus for redox disturbances at distinct levels of cellular organization. The effect of an in vivo acute and severe hypobaric hypoxic insult (48 h at a pressure equivalent to 8,500 m) on oxidative damage and respiratory function was analyzed in skeletal muscle mitochondria isolated from vitamin E-supplemented (60 mg/kg ip, 3 times/wk for 3 wk) and nonsupplemented mice. Forty male mice were randomly divided into four groups: control + placebo, hypoxia + placebo (H + P), control + vitamin E, and hypoxia + vitamin E. Significant increases in mitochondrial heat shock protein 60 expression and protein carbonyls group levels and decreases in aconitase activity and sulfhydryl group content were found in the H + P group when compared with the control + placebo group. Mitochondrial respiration was significantly impaired in animals from the H + P group, as demonstrated by decreased state 3 respiratory control ratio and ADP-to-oxygen ratio and by increased state 4 with both complex I- and II-linked substrates. Using malate + pyruvate as substrates, hypoxia decreased the respiratory rate in the presence of carbonyl cyanide m-chlorophenylhydrazone and also stimulated oligomycin-inhibited respiration. However, vitamin E treatment attenuated the effect of hypoxia on the mitochondrial levels of heat shock protein 60 and markers of oxidative stress. Vitamin E was also able to prevent most mitochondrial alterations induced by hypobaric hypoxia. In conclusion, hypobaric hypoxia increases mitochondrial oxidative stress while decreasing mitochondrial capacity for oxidative phosphorylation. Vitamin E was an effective preventive agent, which further supports the oxidative character of mitochondrial dysfunction induced by hypoxia.     

Alterations in intrinsic mitochondrial function with aging are fiber type-specific and do not explain differential atrophy between muscles

To determine whether mitochondrial dysfunction is causally related to muscle atrophy with aging, we examined respiratory capacity, H(2) O(2) emission, and function of the mitochondrial permeability transition pore (mPTP) in permeabilized myofibers prepared from four rat muscles that span a range of fiber type and degree of age-related atrophy. Muscle atrophy with aging was greatest in fast-twitch gastrocnemius (Gas) muscle (-38%), intermediate in both the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (Sol) muscles (-21%), and non-existent in adductor longus (AL) muscle (+47%). In contrast, indices of mitochondrial dysfunction did not correspond to this differential degree of atrophy. Specifically, despite higher protein expression for oxidative phosphorylation (oxphos) system in fast Gas and EDL, state III respiratory capacity per myofiber wet weight was unchanged with aging, whereas the slow Sol showed proportional decreases in oxphos protein, citrate synthase activity, and state III respiration. Free radical leak (H(2) O(2) emission per O(2) flux) under state III respiration was higher with aging in the fast Gas, whereas state II free radical leak was higher in the slow AL. Only the fast muscles had impaired mPTP function with aging, with lower mitochondrial calcium retention capacity in EDL and shorter time to mPTP opening in Gas and EDL. Collectively, our results underscore that the age-related changes in muscle mitochondrial function depend largely upon fiber type and are unrelated to the severity of muscle atrophy, suggesting that intrinsic changes in mitochondrial function are unlikely to be causally involved in aging muscle atrophy.     

Reduced coupling of oxidative phosphorylation in vivo precedes electron transport chain defects due to mild oxidative stress in mice

Oxidative stress and mitochondrial function are at the core of many degenerative conditions. However, the interaction between oxidative stress and in vivo mitochondrial function is unclear. We used both pharmacological (2 week paraquat (PQ) treatment of wild type mice) and transgenic (mice lacking Cu, Zn-superoxide dismutase (SOD1(-/-))) models to test the effect of oxidative stress on in vivo mitochondrial function in skeletal muscle. Magnetic resonance and optical spectroscopy were used to measure mitochondrial ATP and oxygen fluxes and cell energetic state. In both models of oxidative stress, coupling of oxidative phosphorylation was significantly lower (lower P/O) at rest in vivo in skeletal muscle and was dose-dependent in the PQ model. Despite this reduction in efficiency, in vivo mitochondrial phosphorylation capacity (ATPmax) was maintained in both models, and ex vivo mitochondrial respiration in permeabilized muscle fibers was unchanged following PQ treatment. In association with the reduced P/O, PQ treatment led to a dose-dependent reduction in PCr/ATP ratio and increased phosphorylation of AMPK. These results indicate that oxidative stress uncouples oxidative phosphorylation in vivo and results in energetic stress in the absence of defects in the mitochondrial electron transport chain.     

Involvement of oxidative stress and caspase 2-mediated intrinsic pathway signaling in age-related increase in muscle cell apoptosis in mice

Apoptosis has been implicated as a mechanism of loss of muscle cells in normal aging and plays an important role in age-related sarcopenia. To test the hypothesis that caspase 2 and c-Jun NH₂-terminal kinase (JNK)-mediated intrinsic pathway signaling contribute to skeletal muscle cell apoptosis in aging, we compared activation of caspase 2 and JNK and the in vivo expression of 4-hydroxynonenal protein adducts (4-HNE), inducible nitric oxide synthase (iNOS), glucose-6-phosphate dehydrogenase (G6PDH), B-cell lymphoma-2 (BCL-2), BAX, and phospho-BCL-2 in gastrocnemius muscles of young (5 months old) and old (25 months old) mice. A distinct age-related increase in 4-HNE and iNOS expression was readily detected in mice. Increased oxidative stress and iNOS induction were further accompanied by a decrease in G6PDH expression, activation of caspase 2 and JNK, and inactivation of BCL-2 through phosphorylation at serine 70, and caspase 9 activation. Regression analysis further revealed that increased muscle cell death in aging was significantly correlated with changes in the levels of these molecules. Taken together, our data indicate that caspase 2 and JNK-mediated intrinsic pathway signaling is one of the mechanisms involved in age-related increase in muscle cell apoptosis.     

Mitochondrial dysfunction and reduced prostaglandin synthesis in skeletal muscle of Group VIB Ca2+-independent phospholipase A2γ-deficient mice

Group VIB Ca²⁺-independent phospholipase A₂γ (iPLA₂γ) is a membrane-bound iPLA₂ enzyme with unique features, such as the utilization of distinct translation initiation sites and the presence of mitochondrial and peroxisomal localization signals. Here we investigated the physiological functions of iPLA₂γ by disrupting its gene in mice. iPLA₂γ-knockout (KO) mice were born with an expected Mendelian ratio and appeared normal and healthy at the age of one month but began to show growth retardation from the age of two months as well as kyphosis and significant muscle weakness at the age of four months. Electron microscopy revealed swelling and reduced numbers of mitochondria and atrophy of myofilaments in iPLA₂γ-KO skeletal muscles. Increased lipid peroxidation and the induction of several oxidative stress-related genes were also found in the iPLA₂γ-KO muscles. These results provide evidence that impairment of iPLA₂γ causes mitochondrial dysfunction and increased oxidative stress, leading to the loss of skeletal muscle structure and function. We further found that the compositions of cardiolipin and other phospholipid subclasses were altered and that the levels of myoprotective prostanoids were reduced in iPLA₂γ-KO skeletal muscle. Thus, in addition to maintenance of homeostasis of the mitochondrial membrane, iPLA₂γ may contribute to modulation of lipid mediator production in vivo.     

Early induction of oxidative stress in mouse model of Alzheimer disease with reduced mitochondrial superoxide dismutase activity

While oxidative stress has been linked to Alzheimer's disease, the underlying pathophysiological relationship is unclear. To examine this relationship, we induced oxidative stress through the genetic ablation of one copy of mitochondrial antioxidant superoxide dismutase 2 (Sod2) allele in mutant human amyloid precursor protein (hAPP) transgenic mice. The brains of young (5-7 months of age) and old (25-30 months of age) mice with the four genotypes, wild-type (Sod2(+/+)), hemizygous Sod2 (Sod2(+/-)), hAPP/wild-type (Sod2(+/+)), and hAPP/hemizygous (Sod2(+/-)) were examined to assess levels of oxidative stress markers 4-hydroxy-2-nonenal and heme oxygenase-1. Sod2 reduction in young hAPP mice resulted in significantly increased oxidative stress in the pyramidal neurons of the hippocampus. Interestingly, while differences resulting from hAPP expression or Sod2 reduction were not apparent in the neurons in old mice, oxidative stress was increased in astrocytes in old, but not young hAPP mice with either Sod2(+/+) or Sod2(+/-). Our study shows the specific changes in oxidative stress and the causal relationship with the pathological progression of these mice. These results suggest that the early neuronal susceptibility to oxidative stress in the hAPP/Sod2(+/-) mice may contribute to the pathological and behavioral changes seen in this animal model.     

Effect of oxidative stress on telomere maintenance in aortic smooth muscle cells

Reactive oxygen species (ROS) and telomere dysfunction are both associated with aging and the development of age-related diseases. Although there is evidence for a direct relationship between ROS and telomere dysfunction as well as an independent association of oxidative stress and telomere attrition with age-related disorders, there has not been sufficient exploration of how the interaction between oxidative stress and telomere function may contribute to the pathophysiology of cardiovascular diseases (CVD). To better understand the complex relationships between oxidative stress, telomerase biology and pathophysiology, we examined the telomere biology of aortic smooth muscle cells (ASMCs) isolated from mutant mouse models of oxidative stress. We discovered that telomere lengths were significantly shorter in ASMCs isolated from superoxide dismutase 2 heterozygous (Sod2^+/?) mice, which exhibit increased arterial stiffness with aging, and the observed telomere attrition occurred over time. Furthermore, the telomere erosion occurred even though telomerase activity increased. In contrast, telomeres remained stable in wild-type and superoxide dismutase 1 heterozygous (Sod1^+/?) mice, which do not exhibit CVD phenotypes. The data indicate that mitochondrial oxidative stress, in particular elevated superoxide levels and decreased hydrogen peroxide levels, induces telomere erosion in the ASMCs of the Sod2^+/? mice. This reduction in telomere length occurs despite an increase in telomerase activity and correlates with the onset of disease phenotype. Our results suggest that the oxidative stress caused by imbalance in mitochondrial ROS, from deficient SOD2 activity as a model for mitochondrial dysfunction results in telomere dysfunction, which may contribute to pathogenesis of CVD.     

Elevated hepatic mitochondrial and peroxisomal oxidative capacities in fed and starved adult obese (ob/ob) mice.

Hepatic mitochondrial and peroxisomal oxidative capacities were studied in young (4-5 weeks old) and adult (6-9 months old) lean and obese ob/ob mice that were fed or starved for 24 or 48 h. The adult obese mice showed elevated capacity for mitochondrial oxidation (ng-atoms of O consumed/min per mg of protein) of lipid and non-lipid substrates, with the exception of pyruvate + malate, and elevated activities of citrate synthase and total carnitine palmitoyltransferase. Oxidative rates and enzyme activities were not affected by starvation of lean or obese mice, and both males and females responded similarly. Peroxisomal palmitoyl-CoA oxidation (nmol/min per mg of peroxisomal protein) was also increased in livers of adult obese mice and did not change with starvation. In young mice, hepatic mitochondrial and peroxisomal oxidative capacities in lean and obese mice were comparable. The increased mitochondrial and peroxisomal oxidative capacities appear to develop with maturation in obese ob/ob mice.     

Phosphomannosyl receptors of lysosomal enzymes in cardiac and skeletal muscles of young and old mice

The endogenous activity and the binding of high-uptake beta-N-acetylglucosaminidase were assayed in the membranes of heart and skeletal muscles of young (2 months) and old (15 months) NMRI-mice (Mus musculus) to evaluate the age-related changes in the phosphomannosyl receptors of lysosomal enzymes in muscular membranes. The total activities of beta-N-acetylglucosaminidase were significantly higher in cardiac and skeletal muscles of old than young mice. The total and the specific (inhibited by mannose-6-phosphate) binding of beta-N-acetylglucosaminidase to the membranes of cardiac muscle, but not to those of skeletal muscle, were higher in old mice than in young ones. The endogenous activity of beta-N-acetylglucosaminidase was significantly higher in the membranes of skeletal muscles of old mice than in those of young mice. The membranes of heart muscles did not show any difference in the endogenous activities. The saturation properties of the binding of beta-N-acetylglucosaminidase to the phosphomannosyl receptors were very similar in the membranes of heart and skeletal muscles of both age groups. We conclude that during aging the number of phosphomannosyl receptors of lysosomal enzymes increases in the membranes of heart muscle while the occupancy of phosphomannosyl receptors with endogenous ligands increases in the membranes of skeletal muscle.