Succinate transport inhibition by valproate in rat renal mitochondria

Energy substrate metabolism and mitochondrial oxidative stress in cardiac ischemia/reperfusion injury

The heart is the most metabolically flexible organ with respect to the use of substrates available in different states of energy metabolism. Cardiac mitochondria sense substrate availability and ensure the efficiency of oxidative phosphorylation and heart function. Mitochondria also play a critical role in cardiac ischemia/reperfusion injury, during which they are directly involved in ROS-producing pathophysiological mechanisms. This review explores the mechanisms of ROS production within the energy metabolism pathways and focuses on the impact of different substrates. We describe the main metabolites accumulating during ischemia in the glucose, fatty acid, and Krebs cycle pathways. Hyperglycemia, often present in the acute stress condition of ischemia/reperfusion, increases cytosolic ROS concentrations through the activation of NADPH oxidase 2 and increases mitochondrial ROS through the metabolic overloading and decreased binding of hexokinase II to mitochondria. Fatty acid-linked ROS production is related to the increased fatty acid flux and corresponding accumulation of long-chain acylcarnitines. Succinate that accumulates during anoxia/ischemia is suggested to be the main source of ROS, and the role of itaconate as an inhibitor of succinate dehydrogenase is emerging. We discuss the strategies to modulate and counteract the accumulation of substrates that yield ROS and the therapeutic implications of this concept.

Lactic acidosis: recognition, kinetics, and associated prognosis

Lactic acidosis is a common condition encountered by critical care providers. Elevated lactate and decreased lactate clearance are important for prognostication. Not all lactate in the intensive care unit is due to tissue hypoxia or ischemia and other sources should be evaluated. Lactate, in and of itself, is unlikely to be harmful and is a preferred fuel for many cells. Treatment of lactic acidosis continues to be aimed the underlying source.

Methylmalonate inhibits succinate-supported oxygen consumption by interfering with mitochondrial succinate uptake

The effect of methylmalonate (MMA) on mitochondrial succinate oxidation has received great attention since it could present an important role in energy metabolism impairment in methylmalonic acidaemia. In the present work, we show that while millimolar concentrations of MMA inhibit succinate-supported oxygen consumption by isolated rat brain or muscle mitochondria, there is no effect when either a pool of NADH-linked substrates or N,N,N',N'-tetramethyl-p-phenylendiamine (TMPD)/ascorbate were used as electron donors. Interestingly, the inhibitory effect of MMA, but not of malonate, on succinate-supported brain mitochondrial oxygen consumption was minimized when nonselective permeabilization of mitochondrial membranes was induced by alamethicin. In addition, only a slight inhibitory effect of MMA was observed on succinate-supported oxygen consumption by inside-out submitochondrial particles. In agreement with these observations, brain mitochondrial swelling experiments indicate that MMA is an important inhibitor of succinate transport by the dicarboxylate carrier. Under our experimental conditions, there was no evidence of malonate production in MMA-treated mitochondria. We conclude that MMA inhibits succinate-supported mitochondrial oxygen consumption by interfering with the uptake of this substrate. Although succinate generated outside the mitochondria is probably not a sig-nificant contributor to mitochondrial energy generation, the physiopathological implications of MMA-induced inhibition of substrate transport by the mitochondrial dicarboxylate carrier are discussed.

Altered cognitive functioning in children with idiopathic epilepsy receiving valproate monotherapy

Once a child is diagnosed with epilepsy, a primary concern is whether or not the child's behavior and cognitive abilities will be affected by the disease, by the drug prescribed for seizure control, or both. Direct cognitive effects by the epileptic condition have been described. On the other hand, cognitive effects in epilepsy have been attributed to antiepileptic drug therapy. Valproate is an antiepileptic drug of choice in managing the commonest childhood epilepsy syndromes. Although frequently prescribed in pediatric neurology practice, there have been relatively few studies investigating the cognitive effects of valproate therapy in children. Cognitive effects of valproate reported in normal adult volunteers and in adults with epilepsy cannot be generalized to the pediatric population. The results of investigations on children are less conclusive. Guidelines for antiepileptic drug trials in children have recently been formulated. Carefully designed studies are required in determining the cognitive effects of valproate in the pediatric population. Neuropsychological measures that are likely to assess subtle changes in higher brain functions crucial to learning in children should be employed. We propose a test battery to assess for cognitive changes associated with anticonvulsant therapy in children.

Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity

Severe and prolonged impairment of mitochondrial beta-oxidation leads to microvesicular steatosis, and, in severe forms, to liver failure, coma and death. Impairment of mitochondrial beta-oxidation may be either genetic or acquired, and different causes may add their effects to inhibit beta-oxidation severely and trigger the syndrome. Drugs and some endogenous compounds can sequester coenzyme A and/or inhibit mitochondrial beta-oxidation enzymes (aspirin, valproic acid, tetracyclines, several 2-arylpropionate anti-inflammatory drugs, amineptine and tianeptine); they may inhibit both mitochondrial beta-oxidation and oxidative phosphorylation (endogenous bile acids, amiodarone, perhexiline and diethylaminoethoxyhexestrol), or they may impair mitochondrial DNA transcription (interferon-alpha), or decrease mitochondrial DNA replication (dideoxynucleoside analogues), while other compounds (ethanol, female sex hormones) act through a combination of different mechanisms. Any investigational molecule should be screened for such effects.

In vitro effects of valproate and valproate metabolites on mitochondrial oxidations. Relevance of CoA sequestration to the observed inhibitions

The inhibitory effects of valproate (VPA) and nine of its metabolites on mitochondrial oxidations have been investigated. Valproate, 4-ene-VPE, 2,4-diene-VPA and 2-propylglutaric acid inhibited the rate of oxygen consumption by rat liver mitochondrial fractions with long- and medium-chain fatty acids, glutamate (+/- malate), succinate, alpha-ketoglutarate (+ malate) and pyruvate (+ malate) as substrates. Sequestration of intramitochondrial free CoA by valproate and these three metabolites has been demonstrated and quantified. However, CoA trapping could not account for all the inhibitions observed. 2-ene-VPA and 3-oxo-VPA, metabolites formed during the beta-oxidation of valproate, were not capable of trapping intramitochondrial CoA although they were inhibitors of the beta-oxidation of decanoate, probably by inhibition of the medium-chain acyl-CoA synthetase.

In vitro effects of eight-carbon fatty acids on oxidations in rat liver mitochondria

Sodium valproate, a commonly used anticonvulsant agent, is a simple branched-chain fatty acid which interferes with beta-oxidation and ammonia metabolism in most patients, with hepatotoxic consequences in some cases. Rat liver mitochondria incubated with valproate displayed time-dependent inhibitions of state 3 oxidation rates with all the substrates tested, but most markedly with glutamate, pyruvate, alpha-ketoglutarate and acylcarnitines (Ki = 125 microM with glutamate and palmitoylcarnitine, and 24 microM with pyruvate). The inhibition of glutamate appeared to be specifically directed against the glutamate dehydrogenase pathway of this oxidation. Valproate was less effective when added to uncoupled mitochondria, suggesting the formation of an inhibitory species by an ATP-dependent mechanism. Mitochondria from clofibrate-treated rats were less sensitive to valproate inhibition. Neither fasting nor the presence of 1 mM L-carnitine affected the inhibition of beta-oxidation. The branched-chain isomer, 2-ethylhexanoic acid, had similar effects to valproate, but the straight-chain octanoic acid was totally different in its spectrum of actions on mitochondria. The data support the theory that valproate may inhibit by sequestration of CoA as valproyl-CoA, but also suggest that there are other mechanisms responsible for some of the inhibitions. Furthermore, it argued that while mitochondrial respiration is decreased, valproate is not an inhibitor of oxidative phosphorylation per se.

Valproate-induced hyperammonemia of renal origin. Effects of valproate on glutamine transport in rat kidney mitochondria

The antiepileptic sodium valproate (VPA) systematically induces an asymptomatic hyperammonemia of renal origin in fasting normal human volunteers and in fasting rats, accompanied by an increased renal glutamine uptake. Fasting rats were injected with VPA and their mitochondria isolated, or isolated mitochondria of fasting rats were incubated with VPA. Transmembranal mitochondrial glutamine uptake and activities for five mitochondrial and three cytosolic enzymes involved in ammoniagenesis were measured. In VPA-incubated mitochondria, glutamine transport increased for VPA concentrations between 10(-3) and 10(-5) M; enzyme activities did not change. In mitochondria of VPA-treated rats, Km and Vmax were unaffected. These findings reflect membrane effects of VPA observed in other experimental settings.