Pyruvate metabolism in Friedreich's ataxia

Drug Repositioning in Friedreich Ataxia

Friedreich ataxia is a rare neurodegenerative disorder caused by insufficient levels of the essential mitochondrial protein frataxin. It is a severely debilitating disease that significantly impacts the quality of life of affected patients and reduces their life expectancy, however, an adequate cure is not yet available for patients. Frataxin function, although not thoroughly elucidated, is associated with assembly of iron-sulfur cluster and iron metabolism, therefore insufficient frataxin levels lead to reduced activity of many mitochondrial enzymes involved in the electron transport chain, impaired mitochondrial metabolism, reduced ATP production and inefficient anti-oxidant response. As a consequence, neurons progressively die and patients progressively lose their ability to coordinate movement and perform daily activities. Therapeutic strategies aim at restoring sufficient frataxin levels or at correcting some of the downstream consequences of frataxin deficiency. However, the classical pathways of drug discovery are challenging, require a significant amount of resources and time to reach the final approval, and present a high failure rate. Drug repositioning represents a viable alternative to boost the identification of a therapy, particularly for rare diseases where resources are often limited. In this review we will describe recent efforts aimed at the identification of a therapy for Friedreich ataxia through drug repositioning, and discuss the limitation of such strategies.

Long-term treatment with thiamine as possible medical therapy for Friedreich ataxia

Thiamine (vitamin B1) is a cofactor of fundamental enzymes of cell energetic metabolism; its deficiency causes disorders affecting both the peripheral and central nervous system. Previous studies reported low thiamine levels in cerebrospinal fluid and pyruvate dehydrogenase dysfunction in Friedreich ataxia (FRDA). We investigated the effect of long-term treatment with thiamine in FRDA, evaluating changes in neurological symptoms, echocardiographic parameters, and plasma FXN mRNA levels. Thirty-four consecutive FRDA patients have been continuously treated with intramuscular thiamine 100 mg twice a week and have been assessed with the Scale for the Assessment and Rating of Ataxia (SARA) at baseline, after 1 month, and then every 3 months during treatment. Thiamine administration ranged from 80 to 930 days and was effective in improving total SARA scores from 26.6 ± 7.7 to 21.5 ± 6.2 (p < 0.02). Moreover, deep tendon reflexes reappeared in 57 % of patients with areflexia at baseline, and swallowing improved in 63 % of dysphagic patients. Clinical improvement was stable in all patients, who did not show worsening even after 2 years of treatment. In a subgroup of 13 patients who performed echocardiogram before and during treatment, interventricular septum thickness reduced significantly (p < 0.02). Frataxin mRNA blood levels were modestly increased in one-half of treated patients. We suppose that a focal thiamine deficiency may contribute to a selective neuronal damage in the areas involved in FRDA. Further studies are mandatory to evaluate thiamine role on FXN regulation, to exclude placebo effect, to verify our clinical results, and to confirm restorative and neuroprotective action of thiamine in FRDA.

Emerging Treatments: Replacement Therapy with Choline or Lecithin in Neurological Diseases

This review evaluates the theoretical background and experimental data behind a new development: the replacement therapy of deficient central cholinergic systems with the dietary precursors choline or lecithin. Cholinergic deficiency states are possibly present in five neurological entities: Huntington's chorea, Tardive Dyskinesia, Gilles de la Tourette's disease, Friedreich's ataxia and presenile dementia. Preliminary data from various laboratories, including our own, in each of these disorders indicate that some clinical improvement can occasionally be seen, and that this approach deserves further investigation.

Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay

A new syndrome of autosomal recessive spastic ataxia has been isolated in the Charlevoix-Saguenay region of Quebec. This syndrome is remarkably homogeneous and includes: spasticity, dysarthria, distal muscle wasting, foot deformities, truncal ataxia, absence of sensory evoked potentials in the lower limbs, retinal striation reminiscent of early Leber's atrophy and the frequent presence (57%) of a prolapse of the mitral valve. Biochemically, many cases show impaired pyruvate oxidation, others have hyperbilirubinaemia and some have low serum β-lipoproteins and HDL apoproteins. These features are similar to those found in trypical Friedreich's ataxia.

Friedreich's Ataxia 1978 — An Overview

In the present overview an attempt is made to summarize the investigations carried out during the first part of Phase Two of the Quebec Cooperative Study of Friedreich's Ataxia. These investigations delineated the relative importance of various biochemical leads uncovered during the preliminary survey. It is possible to indicate some findings that may be primary and which should be pursued in subsequent investigations. Among these, the observation of an abnormal composition of high density lipoproteins in Friedreich's Ataxia appears to be the most important.

Serum and Platelet Lipoamide Dehydrogenase in Friedreich's Ataxia

Pyruvate dehydrogenase (PDH), α -keto gluturate dehydrogenase (α -KGDH) and lipoamide dehydrogenase (LAD) were measured in platelets of II patients with typical Friedreich's ataxia and 10 normal control subjects. Serum LAD was also evaluated in the same patients. No statistically significant changes were found in platelets for the group as a whole, although some patients had low values (more than one standard deviation below control mean). Serum LAD was significantly reduced in the patients with Friedreich's ataxia. This was not due to associated diabetes.

Interactions of endoplasmic reticulum and mitochondria Ca(2+) stores with capacitative calcium entry

Thiamine dependent enzymes are diminished in Alzheimer's disease (AD). Thiamine deficiency in vitro and in rodents is a useful model of this reduction. Thiamine interacts with cellular calcium stores. To directly test the relevance of the thiamine dependent changes to dynamic processes in AD, the interactions must be studied in cells from patients with AD. These studies employed fibroblasts. Mitochondrial dysfunction including reductions in thiamine dependent enzymes and abnormalities in calcium homeostasis and oxidative processes occur in fibroblasts from Alzheimer's Disease (AD) patients. Bombesin-releasable calcium stores (BRCS) from the endoplasmic reticulum (ER) are exaggerated in fibroblasts from patients with AD bearing a presenilin-1 (PS-1) mutation and in control fibroblasts treated with oxidants. ER calcium regulates calcium entry into the cell through capacitative calcium entry (CCE), which is reduced in fibroblasts and neurons from mice bearing PS-1 mutations. Under physiological conditions, mitochondria and ER play important and interactive roles in the regulation of Ca(2+) homeostasis. Thus, the interactions of mitochondria and oxidants with CCE were tested. Inhibition of ER Ca(2+)-ATPase by cyclopiazonic acid (CPA) stimulates CCE. CPA-induced CCE was diminished by inhibition of mitochondrial Ca(2+) export (-60%) or import (-40%). Different aspects of mitochondrial Ca(2+) coupled to CPA-induced-CCE were sensitive to select oxidants. The effects were very different when CCE was examined in the presence of InsP3, a physiological regulator of ER calcium release, and subsequent CCE. CCE under these conditions was only mildly reduced (20-25%) by inhibition of mitochondrial Ca(2+) export, and inhibition of mitochondrial Ca(2+) uptake exaggerated CCE (+53%). However, t-BHP reversed both abnormalities. The results suggest that in the presence of InsP3, mitochondria buffer the local Ca(2+) released from ER following rapid activation of InsP3R and serve as a negative feedback to the CCE. The results suggest that mitochondrial Ca(2+) modifies the depletion and refilling mechanism of ER Ca(2+) stores.

High-dose thiamine improves the symptoms of Friedreich's ataxia

Friedreich's ataxia (FRDA) is an autosomal recessive inherited disorder characterised by progressive gait and limb ataxia, dysarthria, areflexia, loss of position sense and a progressive motor weakness of central origin. Some observations indicate that all symptoms of FRDA ataxia could be the manifestation of a thiamine deficiency because of enzymatic abnormalities. Two patients with FRDA were under rehabilitative treatment from February 2012 to February 2013. The scale for assessment and rating of ataxia was performed. The patient began an intramuscular therapy with 100 mg of thiamine every 3-5 days. Injection of high-dose thiamine was effective in reversing the motor failure. From this clinical observation, it is reasonable to infer that a thiamine deficiency due to enzymatic abnormalities could cause a selective neuronal damage in the centres that are typically affected by this disease.

Problems in the congenital lactic acidoses

The congenital lactic acidosis form a heterogeneous group of inborn errors that includes defects of gluconeogenesis, the pyruvate dehydrogenase complex, the Krebs cycle and the respiratory chain. These disorders are not easily classified because of the absence of specific metabolites, difficulties in providing suitable tissue specimens and technical problems with the enzyme assays. The commonest causes of lactic acidosis due to inborn errors are the deficiencies of glucose-6-phosphatase and fructose bisphosphatase, which present with hypoglycaemia, lactic acidosis and hepatomegaly. Pyruvate carboxylase and phosphoenolpyruvate deficiencies vary considerably in both clinical expression and biochemical findings. Neurological symptoms predominate in defects of the pyruvate dehydrogenase complex, and some cases of the spinocerebellar ataxias may be due to partial defects of the pyruvate and 2-oxoglutarate dehydrogenase complexes.

Friedreich's ataxia in the elderly

Friedreich's ataxia is one of the best defined and most common forms of hereditary ataxia of unknown aetiology. It is transmitted in an autosomal recessive manner, appearing sporadically, usually in childhood or adolescence. The case of an elderly patient with a possible diagnosis of late-onset Friedreich's ataxia is reported; this is thought to be the only such case in the literature. The 91-year-old Anglo female presented with ataxia that had been progressive over the last 5 years. Magnetic resonance imaging scans of the head revealed mild peripheral cerebellar atrophy and moderate cerebral atrophy. The patient's parents were unaffected but two of her six siblings had had Friedreich's ataxia starting in childhood, and four of her grandfather's siblings had had an undiagnosed illness that left them in wheelchairs early in life. Friedreich's ataxia was diagnosed in view of the strong family history and non-revealing magnetic resonance imaging of the brain.

Cerebral glucose hypermetabolism in Friedreich's ataxia detected with positron emission tomography

Local cerebral metabolic rate for glucose was studied with 18F-2-fluoro-2-deoxy-D-glucose and positron emission tomography (PET) in 22 patients with Friedreich's ataxia and 23 age-matched normal control subjects. The diagnosis of Friedreich's ataxia was established by the history and physical findings and by excluding other diseases through laboratory investigations. PET studies revealed a statistically significant widespread increase of local cerebral metabolic rate for glucose in the brains of patients with Friedreich's ataxia who were still ambulatory, in comparison with normal control subjects. Nonambulatory patients with Friedreich's ataxia, in comparison with normal control subjects, had significantly increased local cerebral metabolic rates for glucose in the caudate and lenticular nuclei, but not in the other structures studied. The rate was significantly greater in ambulatory patients with Friedreich's ataxia than in nonambulatory patients in all structures studied except the caudate and lenticular nuclei. The data suggest that early in the course of Friedreich's ataxia, the local cerebral metabolic rate for glucose is increased extensively in the central nervous system, and as the disease progresses, it decreases in a regionally specific manner.

Mitochondrial enzymes in hereditary ataxias

As a test of the hypothesis that mitochondrial abnormalities are common in patients with hereditary ataxias, the activities of two mitochondrial enzymes were studied in platelets from an unselected series of patients. For the group of ataxics, the activity of the pyruvate dehydrogenase complex (PDHC) was 68% of the control (P less than 0.01) and that of glutamate dehydrogenase (GDH) was 81% of the control (P less than 0.05). Of the ataxics studied, 30% had activities of either or both mitochondrial enzymes more than 2 SD below the control mean. Immunoblots of PDHC revealed antibody cross-reacting material in platelets and fibroblasts very similar to those in human brain and appeared normal in platelets from patients with ataxias. Immunoblots of GDH showed a single antibody cross-reacting material in brain but at least two species in normal fibroblasts and platelets. The pathophysiology of hereditary ataxias may often involve mitochondrial damage associated with secondary decreases in the activities of mitochondrial enzymes.

Acetazolamide responsive hereditary paroxysmal ataxia

The fourteenth family with an apparently autosomal dominant paroxysmal ataxia is described. As in the seven families described since 1978, acetazolamide completely cured the attacks. A possible mechanism of action of acetazolamide is discussed.

Mitochondrial dysfunction and spinocerebellar degenerations

A simplified classification of the spinocerebellar degenerations is proposed. Axonal ataxias include Friedreich's ataxia and other conditions involving, primarily, neurons with very long axons. Multiple system degenerations include the various olivopontocerebellar atrophies and related disorders. Ataxic encephalopathies are diffuse diseases of the nervous system in which ataxia is a prominent clinical feature. Several lines of data suggest that mitochondrial damage is a common mechanism in the spinocerebellar degenerations. Reasonable pathophysiological mechanisms can be invoked, linking mitochondrial damage to the observed pathologies (including the many cases of intermediate on variant forms).

Quantitative evaluation of the effects of acetazolamide in Friedreich's ataxia: a pilot study

We evaluated the effects of acetazolamide in 4 young patients with Friedreich's ataxia by clinical and quantitative laboratory methods. Dynamic muscle function of the lower extremity was measured during isokinetic knee movements and gait. The acetazolamide trial was terminated at 7 to 11 weeks because of reported side effects or increased ataxia in 3 of the patients. The quantitative evaluations revealed lower dynamic strength values and alterations in the gait movement pattern in all patients. These changes, which were interpreted as deterioration, were partially reversible with cessation of acetazolamide. The advantages of such quantitative evaluations of dynamic muscle function in the evaluation of therapy in Friedreich's ataxia are discussed.

Plasma cholesteryl sulfate in Friedreich's ataxia

Alteration of membrane fluidity and anomalies of membrane structural proteins have been suspected in Friedreich's ataxia. Plasma lecithin:cholesterol acyltransferase (LCAT) activity is also lowered in this disease, presumably because of a substrate effect. The membrane-stabilizing effect of cholesteryl sulfate (CS) and its inhibitory effect on LCAT activity prompted us to measure this substance in the plasma of Friedreich's ataxia patients as well as in normal subjects and in patients with Charlevoix-Saguenay disease. Plasma cholesteryl sulfate concentrations were significantly higher in Friedreich's ataxia, with levels above the upper limit of normal in nearly half of the cases. This increase was unrelated to age, sex or plasma cholesterol levels, but closely associated with the severity of the disease and thus considered to be secondary. A similar phenomenon (except the association with severity) was observed in Charlevoix-Saguenay ataxia. Levels also tended to be higher in first-degree relatives of Friedreich cases. The significance of these findings is discussed in the light of recent knowledge and experimental data obtained in this laboratory on rats made deficient in essential fatty acids. The highest concentrations of CS observed in Friedreich's ataxia (1097 micrograms/dL, 6 times the normal mean) was only 25% as high as the concentrations reported to inhibit LCAT activity.

Friedreich's ataxia: malic enzyme activity in cellular fractions of cultured skin fibroblasts

We have measured the activity of malic enzyme NADP+ dependent in the nuclear, mitochondrial, lysosomal and cytosolic fractions of cultured skin fibroblasts from twelve patients with Friedreich's ataxia and nine control subjects. Hexosaminidase, cytochrome-C-oxidase, lactate dehydrogenase and malic enzyme NAD+ dependent were used as marker enzymes. The activity of malic enzyme NADP+ dependent was not significantly reduced in the mitochondrial fraction of patients with Friedreich's ataxia as compared with controls. When corrected for possible contamination between mitochondrial and cytosolic fractions, malic enzyme NADP+ dependent activity was still not significantly reduced in patients with Friedreich's ataxia. Unless critical methodological differences were overlooked in this or previously published studies, we conclude that mitochondrial malic enzyme deficiency is not the primary genetic defect underlying Friedreich's ataxia.

Low dose choline chloride in cerebellar degeneration

Sixteen patients with cerebellar degeneration were treated with oral choline chloride for six weeks at doses of 3 and 6 g daily. Two patients improved with choline but another four improved with placebo. Choline chloride in a dose of 3-6 g daily is no better than placebo in improving ataxia due to cerebellar degeneration.

Normal mitochondrial malic enzyme levels in Friedreich's ataxia fibroblasts

Normal levels of mitochondrial malic enzyme were found in fibroblasts from three patients with Friedreich's ataxia.

Friedreich ataxia and low pyruvate carboxylase activity in liver and fibroblasts

Biochemical studies in liver, muscle, and cultured fibroblasts were carried out in seven patients with Friedreich ataxia. Lowered activity of pyruvate carboxylase was shown in liver and cultured fibroblasts in all instances.

[Glutamate and GABA concentration in the brain and cerebrospinal fluid of rats treated with phosphatidylcholine]

Phosphatidylcholine increases CNS concentrations of acetylcholine. In rats we investigated whether or not phosphatidylcholine also influences the neurotransmitters glutamate and GABA. In 17 rats 1.5 gram/kg Lethicon perorally was administered daily for 2 weeks, 15 rats served as controls. In tissue from frontal cortex, striatum, substantia nigra, cerebellar cortex no significant differences between treated and untreated animals were found in glutamate or GABA concentrations. A central nervous interaction between the cholinergic system and the neurotransmitters glutamate and GABA, therefore, could not be demonstrated after 2 weeks of phosphatidylcholine intake.

Leukocyte valine dehydrogenase activity in Friedreich's ataxia

We studied the activity of valine dehydrogenase (VDH) in leukocytes of 14 Friedreich's ataxia patients and of 14 normal control subjects. There was a significant 26% mean decrease in enzyme activity in the patients, a finding which could be responsible for the chronic accumulation of some alpha-keto acids with toxic metabolic consequences in that disease. However the deficiency was not present in all patients with the typical symptoms, nor was its magnitude sufficient to be considered the primary genetic defect in Friedreich's Ataxia.

Oral lecithin and linoleic acid in Friedreich's ataxia: I. Design of the study, material and methods

A clinical and biochemical evaluation of twenty-two patients with Friedreich's Ataxia and ten normal controls was undertaken in 1980 to assess the effect of lecithin and linoleic acid supplements on the course of the disease. The trial consisted of two consecutive six months periods on either supplements in a double-blind crossover fashion. Clinical appraisal was performed with regards to the following parameters: joints mobility, muscle strength, equilibrium, coordination, motor accuracy, speech and numerous day to day activities. Blood samples were obtained at the beginning and in the course of the trial for enzymatic determinations. This paper describes the methodology of the study.

Biochemical and clinical studies of Friedreich's ataxia

A series of biochemical tests aimed at elucidating the fundamental cause has been applied to 20 patients with Friedreich's ataxia. Special emphasis was placed upon pyruvate metabolism. The results demonstrate no precisely identifiable defect in the metabolism of pyruvate but indicate an abnormality in glucose uptake and metabolism.

Choline chloride in the treatment of cerebellar and spinocerebellar ataxia

The use of orally administered choline chloride in the treatment of cerebellar and spinocerebellar ataxia was investigated by a short-term double-blind crossover trial in 20 patients with ataxia. These patients, who include 7 with Friedreich's ataxia, 7 with mixed spinocerebellar ataxia and 6 will primary cerebellar degeneration received placebo and 6g/day or 12g/day of choline with crossover at 6 weeks. Serum choline levels were measured 1 h after the first daily dose. Mild but significant improvement in upper limb co-ordination was noted in 3 patients with Friedreich's ataxia, 3 with mixed ataxia and 4 patients with primary cerebellar degeneration. Improvement in gait and lower limb co-ordination was observed in only 2 patients (one with cerebellar ataxia and 1 with mixed ataxia) There was no correlation between serum choline levels and clinical response to choline. Choline chloride produces a mild but functionally significant improvement in motor co-ordination in some patients with cerebellar and spinocerebellar ataxia.

Friedreich's ataxia 1980. An overview of the physiopathology

Phase three of the Quebec Cooperative Study of Friedreich's Ataxia was devoted to an understanding of the physiopathology of individual symptoms on the basis of previously discovered biochemical leads. The present paper attempts to pull these results together by presenting, as a hypothesis, a unifying scheme of possible interactions and relationships. The central core of this hypothesis is the demonstration in Friedreich's ataxia of a state of mitochondrial energy deprivation. This is indirectly responsible for such associated and important symptoms as muscle weakness, dying-back neuropathy, scoliosis and hypertrophic cardiomyopathy. Secondarily, and possibly as an independent but linked-event, the entry of glucose into cells and pyruvate oxidation, are slowed down, favoring the development of diabetes. As a consequence, tissue concentrations of glutamic acid and aspartic acid are decreased, particularly in more vulnerable areas such as the cerebellum, brain stem and dorsal root ganglia. This tissue deficiency in putative excitatory neurotransmitters is directly responsible for the symptom of ataxia. This conclusion is reinforced by the correction of the ataxia in experimental animals, by the intraventricular injection of the same amino acids, and not by the injection of other stimulants of motricity. The observed mitochondrial energy deprivation could be the metabolic consequence of major changes in the linoleic acid (18.2) composition of inner mitochondrial membrane phospholipids, such as cardiolipin. Such decreases in membrane 18:2 could be the result of interference with the normal incorporation of this fatty acid to lipoproteins and/or cell membranes. It is at this level that the search for the specific enzyme defect in Friedreich's ataxia is continuing.

Friedreich's ataxia in northern Italy. II. Biochemical studies in cultured cells

Pyruvate and palmitate oxidations by cultured fibroblasts suspensions were measured in optimized conditions and proved to be within normal range in the cells from Friedreich's patients. However, when pyruvate oxidation was measured by direct assay of the pyruvate dehydrogenase complex, this enzyme activity proved to be significantly lower in Friedreich's than in controls' cells. These abnormalities were not observed when the cells were sonicated. Moreover, lipoamide dehydrogenase activity. Km and Vmax were within the normal range in Friedreich's cells. These data suggest that the low activities of the PDH complex are not a primary defect in Friedreich's ataxia, but are more likely related to membrane abnormalities in Friedreich's cells.

Friedreich's ataxia in northern Italy: I. Clinical, neurophysiological and in vivo biochemical studies

Eighteen patients with the presumptive diagnosis of Friedreich's ataxia were studied. Clinical, neurophysiological and biochemical data were concordant in 14 patients and led to the diagnosis of typical Friedreich's ataxia in this group of patients. The remaining 4 patients differed from the typical patients in several respects, but mainly in the cardiological findings. It is concluded that no single clinical or laboratory finding is typical of F.A. Multidisciplinary approaches are essential to the diagnosis of Friedreich's ataxia.

Effect of asparagine, glutamine and insulin on cerebral amino acid neurotransmitters

Treatment of rats with asparagine or glutamine caused substantial increases in glutamine concentrations in cerebellum and medulla oblongata. Insulin treatment caused a diminution of glutamate and GABA in these regions of brain. Since it is now well-established that glutamine is a very efficient precursor of the neurotransmitter pool of glutamate in mammalian brain, treatment with asparagine or glutamine could be of therapeutic (replacement) value in the treatment of neurological disorders such as Friedreich's ataxia, in which cerebral glutamate concentrations have been found to be diminished.

Friedreich's ataxia II. Biochemical studies in cultured cells

Pyruvate and palmitate oxidations by cultured fibroblast suspensions were measured in optimized conditions and proved to be within normal range in the cells from Friedreich's patients. But when pyruvate oxidation was measured by direct assay of the pyruvate dehydrogenase complex, this enzyme activity proved to be significantly lower in Friedreich's than in controls' cells. These abnormalities were not observed when the cells were sonicated. Moreover, lipoamide dehydrogenase activity Km and Vmax were within the normal range in Friedreich's cells. These data suggest that the low activities of the PDH complex are not a primary defect in Friedreich's ataxia but are more likely to be related to membrane abnormalities in Friedreich's cells.

Friedreich's ataxia. I. Clinical, neurophysiological and in vivo biochemical studies

Eighteen patients with the presumptive diagnosis of Friedreich's ataxia were studied. Clinical, neurophysiological and biochemical data were concordant in 14 patients and led to the diagnosis of typical Friedreich's ataxia in this group of patients: the remaining 4 patients differed from the typical patients in several respects but mainly in the cardiological findings. It is concluded that so far no single clinical or laboratory finding is typical of F.A.. Multidisciplinary approaches are essential to the diagnosis of Friedreich's ataxia.

An investigation of pyruvate metabolism in patients with cerebellar and spinocerebellar degeneration

This study extends previous observations of pyruvate metabolism in the spino-cerebellar degenerations by screening for abnormalities of pyruvate oxidation using the rise in blood pyruvate after an oral glucose load and examining the activity of the lipoamide dehydrogenase (LAD) moeity of the pyruvate dehydrogenase complex in the serum of 31 patients with Friedreich's ataxia, hereditary spastic ataxia and primary cerebellar degeneration. Serum LAD activity was significantly reduced in 10 Friedreich's ataxia patients when compared to controls and to 10 patients with spastic ataxia, thus confirming previous studies. Two patients with Friedreich's ataxia and 2 with primary cerebellar degeneration had abnormal blood pyruvate curves after oral glucose loading. The findings suggest that abnormal pyruvate oxidation occurs in some cases of Friedreich's ataxia and primary cerebellar degeneration and that the abnormality of pyruvate metabolism is not necessarily reflected in the serum LAD activity of these patients. The relevance of these findings to the heterogeneity of the hereditary ataxias is discussed.

Amino acid changes in thiamine-deficient encephalopathy: some implications for the pathogenesis of Friedreich's ataxia

Thiamine-deficient encephalopathy in the rat is characterized by ataxic gait, loss of righting reflex and curvature of the spine. Neurochemical changes include a diminished activity of cerebral pyruvate decarboxylase leading to abnormal pyruvate oxidation. The present study shows that this defective pyruvate oxidation produces a significant depletion of three important amino acid neurotransmitters, namely gamma aminobutyric acid (GABA), glutamic acid, and aspartic acid. Such changes could lead to severe neuronal dysfunction and the observed neurological symptoms of thiamine deficiency. Some implications for the pathogenesis of Friedreich's ataxia are discussed.

Lipoamide dehydrogenase in cultured human skin fibroblasts

Lipoamide dehydrogenase was identified in cultured skin fibroblasts of normal individuals and patients with Friedreich's ataxia. The optimum conditions for its assay were defined. Data disclosed a normal range of 36--122 mumol/min/mg protein in control fibroblasts and 61--112 mumol/min/mg protein in patients fibroblasts. Numerous precautions should be taken in handling fibroblast cultures for lipoamide dehydrogenase determination.

Regulation of brain pyruvate dehydrogenase multienzyme complex

A number of excellent and comprehensive reviews on various aspects of pyruvate dehydrogenase multienzyme complex have been written recently. The purpose of the present review is to summarize briefly the reaction mechanism and the regulation of this enzyme. Emphasis is put on the most recent literature not covered by previous reviews. Particular attention is also paid to the regulation of brain pyruvate dehydrogenase multienzyme complex, since a number of patients with neuromuscular diseases, such as Friedreich's ataxia, show a decreased rate of pyruvate oxidation.

Lipoamide Dehydrogenase Regulation in Rat Brain

The Pyruvate dehydrogenase multienzyme complex (PDHC) purified from rat brain is phosphorylated in the presence of low concentrations of ATP and MgCl2. The phosphorylated PDHC is incapable of catalyzing the oxidative decarboxylation of pyruvate. In the presence of high concentrations (10 mM) of MgCl2, the phosphorylated (inactive) PDHC is converted back to the dephospho-form of PDHC which is catalytically active.

The dihydrolipoyl dehydrogenase (LAD) component, E3, of PDHC is inactivated by pyridoxal phosphate (PLP) and the PLP-inactivated LAD can be reactivated by an amino acid, taurine. These results indicate the reversible formation of Schiff base between PLP and LAD. They also provide clear evidence for the involvement of LAD (E3) in the previously reported inactivation of PDHC by PLP.