New developments in the diagnosis and investigation of mitochondrial fatty acid oxidation disorders

cis-4-decenoic acid provokes mitochondrial bioenergetic dysfunction in rat brain

AIMS

In the present work we investigated the in vitro effect of cis-4-decenoic acid, the pathognomonic metabolite of medium-chain acyl-CoA dehydrogenase deficiency, on various parameters of bioenergetic homeostasis in rat brain mitochondria.

MAIN METHODS

Respiratory parameters determined by oxygen consumption were evaluated, as well as membrane potential, NAD(P)H content, swelling and cytochrome c release in mitochondrial preparations from rat brain, using glutamate plus malate or succinate as substrates. The activities of citric acid cycle enzymes were also assessed.

KEY FINDINGS

cis-4-decenoic acid markedly increased state 4 respiration, whereas state 3 respiration and the respiratory control ratio were decreased. The ADP/O ratio, the mitochondrial membrane potential, the matrix NAD(P)H levels and aconitase activity were also diminished by cis-4-decenoic acid. These data indicate that this fatty acid acts as an uncoupler of oxidative phosphorylation and as a metabolic inhibitor. cis-4-decenoic acid also provoked a marked mitochondrial swelling when either KCl or sucrose was used in the incubation medium and also induced cytochrome c release from mitochondria, suggesting a non-selective permeabilization of the inner mitochondrial membrane.

SIGNIFICANCE

It is therefore presumed that impairment of mitochondrial homeostasis provoked by cis-4-decenoic acid may be involved in the brain dysfunction observed in medium-chain acyl-CoA dehydrogenase deficient patients.

Reye syndrome and reye-like syndrome

Reye syndrome is an acute metabolic encephalopathy, largely affecting children and adolescents. In Reye-like syndrome, because of inborn errors of metabolism, hypoglycemia, hypoketonemia, elevated ammonia, and organic aciduria are often evident. It is well-known that fatty-acid oxidation defects can present as Reye-like syndrome. The most commonly diagnosed metabolic disorder in association with Reye syndrome has been medium-chain acyl coenzyme A dehydrogenase deficiency. The present consensus seems to be that Reye syndrome is very rare, and that any child suspected of manifesting this disorder should undergo investigations for inborn errors of metabolism. We recently treated a child with "Reye-like illness" who possibly manifested a long-chain acyl dehydrogenase deficiency, and who had also ingested aspirin. We discuss the possible pathogenesis of the disorder in this child. The end results of mitochondrial dysfunction in Reye syndrome and Reye-like illness may be similar.

Oxidative stress induction by cis-4-decenoic acid: relevance for MCAD deficiency

Patients affected by medium-chain acyl-CoA dehydrogenase deficiency (MCADD) suffer from acute episodes of encephalopathy whose underlying mechanisms are poorly known. The present work investigated the in vitro effect of cis-4-decenoic acid (cDA), which accumulates in MCADD, on important parameters of oxidative stress in cerebral cortex of young rats. cDA markedly induced lipid peroxidation, as verified by the increased levels of spontaneous chemiluminescence and thiobarbituric acid-reactive substances. Furthermore, cDA significantly increased carbonyl formation and sulphydryl oxidation, which is indicative of protein oxidative damage, and promoted 2',7'-dihydrodichlorofluorescein oxidation. It was also observed that the non-enzymatic tissue antioxidant defenses were decreased by cDA, whereas the antioxidant enzyme activities catalase, superoxide dismutase and glutathione peroxidase were not altered. Moreover, cDA-induced lipid peroxidation and GSH reduction was totally blocked by free radical scavengers, suggesting that reactive species were involved in these effects. The data indicate that oxidative stress is induced by cDA in rat brain in vitro and that oxidative damage might be involved in the pathophysiology of the encephalopathy in MCADD.

Purification, characterization, and mass spectrometric sequencing of a medium chain acyl-CoA synthetase from mouse liver mitochondria and comparisons with the homologues of rat and bovine

Medium chain acyl-CoA synthetases catalyze the first reaction of amino acid conjugation of many xenobiotic carboxylic acids and fatty acid metabolism. This paper reports studies on purification, characterization, and the partial amino acid sequence of mouse liver enzyme. The medium chain acyl-CoA synthetase was isolated from mouse liver mitochondria. The purified enzyme catalyzes this reaction not only for straight medium chain fatty acids but also for aromatic and arylacetic acids. Maximal activity was found with hexanoic acid. High activities were obtained with benzoic acid having methyl, pentyl, and methoxy groups in the para- or meta-positions of the benzene ring. However, the enzyme was less active with valproic acid and ketoprofen. Salicylic acid exhibited no activity. The medium chain acyl-CoA synthetases from mouse and bovine liver mitochondria were subjected to in-gel tryptic digestion, followed by LC-MS/MS sequence analysis. The amino acid sequence of each tryptic peptide of mouse liver mitochondrial medium chain acyl-CoA synthetase differed from that from bovine liver mitochondria only in one or two amino acids. LC-MS/MS analysis provided the information about these differences in amino acid sequences. In addition, we compared the properties of this protein with the homologues from rat and bovine.

Analysis of medium-chain acyl-coenzyme A esters in mouse tissues by liquid chromatography-electrospray ionization mass spectrometry

Medium-chain acyl-coenzyme A (CoA) esters are key metabolites in lipid metabolism. Liquid chromatography-electrospray ionization mass spectrometry analysis of medium-chain acyl-CoA esters is described. Eight medium-chain acyl-CoA esters were well separated on a C(8)-MS reversed-phase column using a linear gradient of ammonium acetate buffer (pH 5.3)-acetonitrile. The positive-ion mass spectra of all the saturated and unsaturated medium-chain acyl-CoA esters gave dominant [M+H](+) ions, whereas their negative-ion mass spectra showed abundant [M-H](-) and [M-2H](2-) ions. The positive-ion mode of operation was slightly less sensitive than the negative-ion detection mode. Five medium-chain acyl-CoA esters of C(6:0), C(8:0), C(8:1), C(10:0), and C(10:1) in liver, heart, kidney, and brain from the mouse were identified. The predominant acyl-CoA peaks were C(6:0), C(8:0), and C(10:0). Small amounts of medium-chain acyl-CoAs of C(8:1) and C(10:1) were detected only in heart and kidney. The analytical method is very useful for the analysis of medium-chain acyl-CoA esters in the tissues.

Inhibition of energy metabolism in cerebral cortex of young rats by the medium-chain fatty acids accumulating in MCAD deficiency

Patients affected by medium-chain acyl CoA dehydrogenase (MCAD) deficiency, a frequent inborn error of metabolism, suffer from acute episodes of encephalopathy. However, the mechanisms underlying the neuropathology of this disease are poorly known. In the present study, we investigated the in vitro effect of the medium-chain fatty acids (MCFA), at concentrations varying from 0.01 to 3 mM, accumulating in MCAD deficiency on some parameters of energy metabolism in cerebral cortex of young rats. (14)CO(2) production from [U(14)] glucose, [1-(14)C] acetate and [1,5-(14)C] citrate was evaluated by incubating cerebral cortex homogenates from 30-day-old rats in the absence (controls) or presence of octanoic acid, decanoic acid or cis-4-decenoic acid. OA and DA significantly reduced (14)CO(2) production from acetate by around 30-40%, and from glucose by around 70%. DA significantly reduced (14)CO(2) production from citrate by around 40%, while OA did not affect this parameter. cDA inhibited (14)CO(2) production from all tested substrates by around 30-40%. The activities of the respiratory chain complexes and of creatine kinase were also tested in the presence of DA and cDA. Both metabolites significantly inhibited cytochrome c oxidase activity (by 30%) and complex II-III activity (DA, 25%; cDA, 80%). Furthermore, only cDA inhibited complex II activity (by 30%), while complex I-III and citrate synthase were not affected by these MCFA. On the other hand, only cDA reduced the activity of creatine kinase in total homogenates, as well as in mitochondrial and cytosolic fractions from cerebral cortex (by 50%). The data suggest that the major metabolites which accumulate in MCAD deficiency, with particular emphasis to cDA, compromise brain energy metabolism. We presume that these findings may contribute to the understanding of the pathophysiology of the neurological dysfunction of MCAD deficient patients.

Inhibitory effect of quinolone antimicrobial and nonsteroidal anti-inflammatory drugs on a medium chain acyl-CoA synthetase

The inhibitory effects of quinolone antimicrobial agents and nonsteroidal anti-inflammatory drugs on purified mouse liver mitochondrial medium chain acyl-CoA synthetase catalyzing the first reaction of glycine conjugation were examined, using hexanoic acid as a substrate. Enoxacin, ofloxacin, nalidixic acid, diflunisal, salicylic acid, 2-hydroxynaphthoic acid, and 2-hydroxydodecanoic acid, which do not act as substrates, were potent inhibitors. Diflunisal, nalidixic acid, salicylic acid, 2-hydroxynaphthoic acid, and 2-hydroxydodecanoic acid inhibited competitively this medium chain acyl-CoA synthetase with K(i) values of 0.6, 12.4, 19.6, 13.4, and 15.0 microM, respectively. Enoxacin and ofloxacin inhibited this medium chain acyl-CoA synthetase in a mixed-type manner with K(i) values of 23.7 and 38.2 microM, respectively. Felbinac, which is a substrate, inhibited the activity of this medium chain acyl-CoA synthetase for hexanoic acid (IC50 = 25 microM). The concomitant presence of enoxacin and felbinac strongly inhibited this medium chain acyl-CoA synthetase. These findings indicate that medium chain acyl-CoA synthetases may be influenced by quinolone antimicrobial and nonsteroidal anti-inflammatory drugs.

The mechanism of inhibition of beta-oxidation by aspirin metabolites in skin fibroblasts from Reye's syndrome patients and controls

The effects of aspirin metabolites on beta-oxidation were studied in skin fibroblasts from eight typical Reye's syndrome (RS) patients and controls. RS patients' cells did not differ from controls in rates of palmitate oxidation or in the three component activities of the mitochondrial trifunctional enzyme (MTE), indicating no inherited beta-oxidation defect. Aspirin metabolites salicylate, hydroxyhippurate and gentisate, but not aspirin, directly inhibited palmitate oxidation in control and RS cells. RS cells were significantly more sensitive to inhibition than controls at 0.5 to 5 mM salicylate. Inhibition was concentration-dependent and reversible. Inhibition did not occur in fibroblasts lacking activity of the long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) activity of MTE. Salicylate was therefore inhibiting beta-oxidation at this step. Hydroxyhippurate and salicylate reversibly inhibited HAD activities in extracts of control and RS cells. Studies with pure short-chain HAD and LCHAD (MTE) showed hydroxyhippurate and salicylate were competitive inhibitors of the former but mixed (not competitive) inhibitors of the latter. Both compounds inhibited the combined, three-step, MTE reaction measured in the physiological direction. We conclude that (1) salicylate and hydroxyhippurate decrease beta-oxidation in intact cells by reversible inhibition of LCHAD activity of the MTE, and (2) beta-oxidation in RS cells is inherently more sensitive to inhibition by low concentrations of salicylate than controls.

Evaluation of carnitine nutritional status in full-term newborn infants

Carnitine supplements may be advisable not only in premature but also in artificially-fed full-term babies. The acyl-carnitine/free carnitine (AC/FC) and FC/total carnitine (FC/TC) ratios have been considered markers of "carnitine insufficiency" and "carnitine deficiency", respectively. Values of AC/FC>0.40 are considered abnormal and mean that FC has a low bioavailability to the cells and so reflects a "carnitine insufficiency". Values of FC/TC<0.7 indicate "carnitine deficiency". We analyze the validity of such ratios and the limits for them in three groups of full-term neonates (n=66): 22 breast-fed (BF), 22 with formula (F); and 22 fed with carnitine-supplemented formula. Several studies have shown the need to give supplements of carnitine to the neonate because of its "essentiality", but no one has demonstrated the adequate dosages. We therefore propose to establish new limit levels for these ratios to control carnitine nutritional status in neonates, based on the control of percentile ranges for normal BF infants (in this study: 97th percentile of AC/FC>0.83; 3rd percentile of FC/TC<0.54) and on evaluating the needs of neonates and dosages required to supplement F. The supplement of 2.2 mg of L-carnitine/100 ml in the cow's milk formula used in the present study produces a similar biochemical pattern of plasma carnitine and ACs to that observed in BF infants, together with a lower risk of developing "carnitine deficiency" or "carnitine insufficiency" than those babies fed with nonenriched F. Considering that human milk is the best source of nutrition for full-term infants, the limit established for AC/FC and FC/TC ratios at other ages of life seems to be "inadequate" for neonates.

Fatty acid oxidation in fibroblasts from patients with defects in beta-oxidation and in the respiratory chain

Fatty acid oxidation has been studied with the tritium release assay in cultured fibroblasts from patients with defects in beta-oxidation and in the mitochondrial respiratory chain. Cells from all patients with beta-oxidation defects and cells from 10 of 16 patients with respiratory chain defects showed an impairment of fatty acid oxidation. The result of the tritium release assay is not only dependent on the proper function of the beta-oxidation cycle but is also influenced by the reoxidation of reduced cofactors. The assay can thus be used to study the expression of respiratory chain defects in cultured fibroblasts.

[Cardiac involvement in neuromuscular diseases]

Many neuromuscular disorders involve the heart, occasionally with overt clinical disease. Muscular dystrophies (dystrophinopathies, limb girdle muscular dystrophy, Emery-Dreifuss muscular dystrophy, Steinert's myotonic dystrophy), congenital myopathies, inflammatory myopathies and metabolic diseases (glycogenosis, periodic paralysis, mitochondrial diseases) may produce dilated or hypertrophic cardiomyopathy and heart rhythm or conduction disturbances. Furthermore the heart is commonly involved in some hereditary and degenerative diseases (Friedreich's ataxia and Kugelberg-Welander syndrome) and acquired (Guillain-Barré syndrome) or inherited (Refsum's disease and Charcot-Marie-Tooth syndrome) polyneuropathies. A cardiologist's high clinical suspicion and a simple but systematic skeletal muscle and peripheral nerve investigation, including muscle enzymes quantification, neurophysiological study and muscle biopsy, are necessary for an accurate diagnosis. In selected patients, more sophisticated biochemical and genetic analysis will be necessary. In most cases, endomyocardial biopsy is not essential for the diagnosis.

Metabolic function and liver histopathology in Reye-like illnesses

Forty children with Reye syndrome (RS) or Reye-like illnesses were investigated to elucidate the underlying aetiologies. Extensive biochemical studies including patterns of organic acids and amino acids, liver histopathology, and, if available, a DNA approach were performed. In addition to classical RS (n = 10), the causes of Reye-like conditions included hereditary organic acidaemias (n = 13), urea cycle defects (n = 4), mitochondrial disorders (n = 3), fulminant hepatitis (n = 2), tyrosinaemia (n = 1), valproate-associated hepatotoxicity (n = 1), and other non-specific generalized organic acid disorders (n = 6). It is important to collect specimens when encephalopathy with liver dysfunction of unknown causes is noted. When the underlying inherited metabolic disorders are confirmed, the prevention of the recurrence by adequate diet control and medications, and genetic counselling become possible.

Inhibition of oxidative phosphorylation by palmitoyl-CoA in digitonin permeabilized fibroblasts: implications for long-chain fatty acid beta-oxidation disorders

Long-chain fatty acid oxidation deficient patients present early in life with more severe features than patients with a medium-chain fatty acid oxidation deficiency. This may be related to the more toxic effect of long-chain fatty acid derivatives. In this paper we have studied the effect of different acyl-CoA esters, and palmitoyl-CoA in particular, on succinate-driven oxidative phosphorylation, using digitonin permeabilized human fibroblasts. Palmitoyl-CoA was found to inhibit the succinate-driven oxidative phosphorylation in a concentration dependent manner. If the inhibition of the oxidative phosphorylation system is also expressed under in vivo conditions this might explain some of the abnormalities found in patients with defects in long-chain fatty acid beta-oxidation.