Specific Activities Of Mitochondrial Enzymes Research Articles

Control of muscle bioenergetic gene expression: implications for allometric scaling relationships of glycolytic and oxidative enzymes

Muscle metabolic properties vary with body size, with larger animals relying relatively less on oxidative metabolism as a result of lower specific activities of mitochondrial enzymes and greater specific activities of glycolytic enzymes. While many have argued reasons why such relationships might be grounded in physical relationships, an explanation for the regulatory basis of the differences in enzyme levels remains unexplored. Focusing on skeletal muscle, we review potential cellular and genetic explanations for the relationship between bioenergetic enzymes and body mass. Differences in myonuclear domain (the ratio of fiber volume to nuclei number) in conjunction with constitutive expression may explain part of the variation in mitochondrial content among fiber types and species. Superimposed on such constitutive determinants are (1) extrinsic signalling pathways that control the muscle contractile and metabolic phenotype and (2) intrinsic signalling pathways that translate changes in cellular milieu (ions, metabolites, oxygen, redox) arising through the contractile phenotype into changes in enzyme synthesis. These signalling pathways work through transcriptional regulation, as well as post-transcriptional, translational and post-translational regulation, acting via synthesis and degradation.

β-Amyloid Fragment 25–35 Causes Mitochondrial Dysfunction in Primary Cortical Neurons

β-Amyloid deposition and compromised energy metabolism both occur in vulnerable brain regions in Alzheimer's disease. It is not known whether β-amyloid is the cause of impairment of energy metabolism, nor whether impaired energy metabolism is specific to neurons. Our results, using primary neuronal cultures, show that 24-h incubation with Aβ 25–35 caused a generalized decrease in the specific activity of mitochondrial enzymes per milligram of cellular protein, induced mitochondrial swelling, and decreased total mitochondrial number. Incubation with Aβ 25–35 decreased ATP concentration to 58% of control in neurons and 71% of control in astrocytes. Levels of reduced glutathione were also lowered by Aβ 25–35 in both neurons (from 5.1 to 2.9 nmol/mg protein) and astrocytes (from 25.2 to 14.9 nmol/mg protein). We conclude that 24-h treatment with extracellular Aβ 25–35 causes mitochondrial dysfunction in both astrocytes and neurons, the latter being more seriously affected. In astrocytes mitochondrial impairment was confined to complex I inhibition, whereas in neurons a generalized loss of mitochondria was seen.

Cigarette smoke exposure and hypercholesterolemia increase mitochondrial damage in cardiovascular tissues.

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

Relation between growth rate and metabolic organization of white muscle, liver and digestive tract in cod, Gadus morhua

To determine whether the aerobic capacity of tissues required for growth specifically reflects growth rates, we monitored the activities of key enzymes of oxidative, glycolytic and amino acid metabolism in muscle, liver and intestine of Atlantic cod (Gadus morhua) growing at different rates. Fish were maintained individually in small tanks at 10°C and fed on rations that allowed growth rates ranging from-0.6 to 1.6% per day. The correlation between growth rate and muscle enzyme activity was pronounced for the glycolytic enzymes (LDH, PFK and PK). The activities of glycolytic enzymes were more than four times higher for fish having higher growth rates compared to those that did not grow. Mitochondrial enzyme (cytochrome c oxidase, citrate synthase and β-hydroxyacyl-CoA dehydrogenase) activities remained unchanged in fish with positive growth. The liver seems to respond to requirements of growth by an increase in size. In the liver, the activities of the enzymes of amino acid metabolism expressed as units · μg DNA-1 specifically increases with growth rate. In contrast to the two other tissues, the specific activities of mitochondrial enzymes in the intestine increased with growth rate while the relative mass of the intestine remained constant. Intestinal cytochrome c oxidase activity increased from a minimum of about 2 to more than 8 units · g intestine-1. Cytochrome c oxidase activity increased in parallel with the food conversion efficiency. This suggests that the aerobic capacity of the intestine may initially limit the rates of digestion and growth in this species.

Interactions between sustained contractile activity and beta-adrenergic receptors in regulation of gene expression in skeletal muscles

Continuous electrical stimulation for 10-21 days of the motor nerve innervating the anterior compartment muscles of adult rabbits increased both the density of beta-adrenergic receptors (beta-AR) and tissue concentrations of adenosine 3',5'-cyclic monophosphate (cAMP) by two to threefold. Changes in cAMP and in beta-AR occurred in parallel with stimulation-induced adaptations in the specific activity of mitochondrial enzymes (2- to 6-fold increases) and with changes in steady-state concentrations of mitochondrial RNA, beta-F1ATPase mRNA, and myoglobin mRNA (2- to 11-fold increases). These increases in muscle cAMP, in beta-AR, and in expression of protein and mRNA products of genes encoding proteins of oxidative metabolism occurred even in animals receiving high doses of propranolol during the period of electrical stimulation. In contrast to genes that encode proteins of oxidative metabolism, the direction and the time course of activity-induced changes in expression of the glycolytic enzyme aldolase A appeared to be unrelated to changes in muscle cAMP; suppression of steady-state concentrations of aldolase A mRNA was maximal (20-25% of control) at early time points preceding the maximal rise in cAMP. In addition, administration of propranolol attenuated the suppressive effect of continuous contractile activity on expression of aldolase A, even in the absence of an effect of this drug on cAMP in stimulated muscles. We conclude that activity-induced changes in cAMP, in beta-AR, and in expression of genes that encode proteins important for oxidative metabolism occur as a direct consequence of contractile activity and do not require concomitant stimulation of beta-AR.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic alcohol intake induces the oxidative capacity of brown adipose tissue in the rat

The present study was carried out to elucidate the effect of long-term alcohol intake on the oxidative capacity of brown adipose tissue in the rat. Rats housed at room temperature were given water containing 10% ethanol for six months, while controls received water alone. Fully cold-acclimated rats (exposed to +4°C for 6 weeks) served as the second control group. Alcohol did not alter the food intake of the rats compared with the controls kept at room temperature, but it did cause a mean decrease of 8 ml in fluid consumption. There was no difference in the increase in body weight between the groups housed at room temperature. Body weight of the rats exposed to cold did not change during cold acclimation. No morphological liver changes were observed in alcohol-fed rats, but some changes related to long-term alcohol consumption were found in the myocardium. Chronic alcohol intake increased the quantity of brown adipose tissue and its protein content but changes were not as great as in the cold-acclimated rats nor did alcohol increase protein content per unit of the adipose tissue as did cold. On the other hand, the specific activity of mitochondrial cytochrome oxidase increased by 90% and that of succinate dehydrogenase by 130% in alcohol-fed rats, whereas specific activities of these enzymes displayed little or no change in the cold-acclimated rats. Results suggest that chronic alcohol ingestion induces the oxidative capacity of the interscapular brown adipose tissue in the rat, increasing the mass of BAT and specific activities of mitochondrial enzymes.

SALICYLATE INHIBITS CITRULLINE SYNTHESIS IN MITOCHONDRIA: IMPLICATIONS FOR REY'S SYNDROME

In Reye's Syndrome (RS) there is a general decrease in the specific activities of mitochondrial enzymes, including two of the urea cycle, carbamyl phosphate synthetass and ornithine carbamyl-transferase. These two enzymes catalyze the synthesis of citrul-line (CIT) from ornithine, NH3, and CO2; the functional impairment of this reaction sequence probably contributes to hyperammonemia in RS. Salicylate (SA) has been implicated as a synergistic factor in the pathogenesis of RS. In this study we investigated whether SA inhibits CIT synthesis in isolated rat liver mitochondria. CIT synthesis was assayed at 37°C by measuring 14CO2 fixation in the presence of ornithine and NH3 (ABB 178:19,1977). SA inhibited CIT formation (μmol/hr/mg mito protein) as follows:Gentisic acid, salicyluric acid and acetaminophen had no effect at 5 mM. CIT synthesis is dependent on matrix ATP: a mechanism of inhibition was suggested by further experiments showing that SA uncouples oxidative phosphorylation and decreases the rate of ATP synthesis. Matrix ATP/ADP ratios were decreased by SA as follows: 1.15±.04 (O SA); 0.90±.21 (0.25 mM SA); 0.63±.09 (0.5 mM SA); 0.11±.04 (1 mM SA). Conclusion: SA is a potent inhibitor of CIT synthesis at physiological concentrations, probably as a result of its uncoupling action which lowers matrix ATP. The implication for RS is that SA might inhibit urea cycle function beyond the already compromised level. (NIH 14936)

The regulation of synthesis of mitochondrial enzymes in regreening and division-synchronized Euglena cultures

The effect of light and carbon nutrition on the synthesis of citrate synthase (EC 4.1.3.7) and malate dehydrogenase (EC 1.1.1.37) in dark-grown resting (carbon deficient) and in phototrophic division-synchronized cultures of Euglena gracilis Klebs strain z were investigated. Exposure of dark-grown Euglena to white or red light produced a transient increase in the specific activities of citrate synthase and malate dehydrogenase but blue light (of equal energy) was ineffective. Citrate-synthase activity increased at the end of the light phase and in early dark phase in phototrophic cultures division-synchronized by a regime of 14 h light-10 h dark. The addition of ethanol or malate produced a twofold increase in citrate-synthase activity compared with phototrophic cultures. White and blue light, but not red light, produced a transient repression of the metabolite-induced increase in citrate-synthase activity in division-synchronized cultures. Since only red light could effect a transient increase in the specific activity of mitochondrial enzymes, and the blue-red plastid receptor should respond to both blue and red light, the synthesis of mitochondrial enzymes in regreening cultures may be under the control of a new photoreceptor responding only to red light. In division-synchronized phototrophic cells the primary effector of synthesis of mitochondrial enzymes is not light but carbon nutrition.