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Organic acidurias represent a group of inherited disorders resulting from deficient activity of specific enzymes of the catabolism of amino acids, carbohydrates or lipids, leading to tissue accumulation of one or more carboxylic (organic) acids. Patients affected by organic acidurias predominantly present neurological symptoms and structural brain abnormalities, of which the aetiopathogenesis is poorly understood. However, in recent years increasing evidence has emerged suggesting that oxidative stress is possibly involved in the pathology of some organic acidurias and other inborn errors of metabolism. This review addresses some of the recent developments obtained mainly from animal studies indicating oxidative damage as an important determinant of the neuropathophysiology of some organic acidurias. Recent data showing that various organic acids are capable of inducing free radical generation and decreasing brain antioxidant defences is presented. The discussion focuses on the relatively low antioxidant defences of the brain and the vulnerability of this tissue to reactive species. This offers new perspectives for potential therapeutic strategies for these disorders, which may include the early use of appropriate antioxidants as a novel adjuvant therapy, besides the usual treatment based on removing toxic compounds and using special diets and pharmacological agents, such as cofactors and L-carnitine.

Large amounts of d-2-hydroxyglutaric acid (DGA) accumulate in d-2-hydroxyglutaric aciduria (D-2-OHGA), an inherited neurometabolic disorder characterized by severe neurological dysfunction and cerebral atrophy. Despite the significant brain abnormalities, the neurotoxic mechanisms of brain injury in this disease are virtually unknown. In this work, the in vitro effect of DGA on various parameters of oxidative stress was investigated; namely chemiluminescence, thiobarbituric acid-reactive substances (TBA-RS), total radical-trapping antioxidant potential (TRAP), total antioxidant reactivity (TAR) and the activities of the antioxidant enzymes catalase, glutathione peroxidase and superoxide dismutase in cerebral cortex from 30-day-old-rats. DGA significantly increased chemiluminescence and TBA-RS and decreased TAR values in the cortical supernatants. In contrast, TRAP and the antioxidant enzyme activities were not altered by the metabolite. Furthermore, the DGA-induced increase of TBA-RS was fully prevented by the free radical scavengers ascorbic acid plus Trolox (water-soluble alpha-tocopherol) and attenuated by the inhibitor of nitric oxide synthase Nomega-nitro-L-arginine methyl ester (L-NAME), suggesting the role of superoxide, hydroxyl and nitric oxide radicals in this action. The data indicate a stimulation of lipid peroxidation through the production of free radicals and a reduction of the brain capacity to efficiently modulate the damage associated with the enhanced generation of free radicals by DGA. In the case that these findings also occur in human D-2-OHGA, it is feasible that oxidative stress may be involved in the pathophysiology of the brain injury observed in patients with this disease.

Phenylketonuria (PKU) is an autossomal recessive disease caused by phenylalanine-4-hydroxylase deficiency, which is a liver-specific enzyme that catalyzes the hydroxylation of l-phenylalanine (Phe) to l-tyrosine (Tyr). The deficiency of this enzyme leads to the accumulation of Phe in the tissues and plasma of patients. The clinical characterization of this disease is mental retardation and other neurological features. The mechanisms of brain damage are poorly understood. Oxidative stress is observed in some inborn errors of intermediary metabolism owing to the accumulation of toxic metabolites leading to excessive free radical production and may be a result of restricted diets on the antioxidant status. In the present study we evaluated various oxidative stress parameters, namely thiobarbituric acid-reactive species (TBA-RS) and total antioxidant reactivity (TAR) in the plasma of PKU patients. The activities of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) were also measured in erythrocytes from these patients. It was observed that phenylketonuric patients present a significant increase of plasma TBA-RS measurement, indicating a stimulation of lipoperoxidation, as well as a decrease of plasma TAR, reflecting a deficient capacity to rapidly handle an increase of reactive species. The results also showed a decrease of erythrocyte GSH-Px activity. Therefore, it is presumed that oxidative stress is involved in the pathophysiology of the tissue damage found in PKU.

The in vitro effects of propionic and L-methylmalonic acids on some parameters of oxidative stress were investigated in the cerebral cortex of 21-day-old rats. Chemiluminescence, thiobarbituric acid-reactive substances (TBA-RS) and total radical-trapping antioxidant capacity (TRAP) were measured in brain tissue homogenates in the presence of propionic or L-methylmalonic acids at concentrations ranging from 1 to 10mM. Both acids significantly increased chemiluminescence and TBA-RS and decreased TRAP, indicating a simulation of lipid peroxidation and a reduction of tissue antioxidant potential. Other organic acids tested which accumulate in some organic acidemias (suberic, sebacic, adipic, 3-methylglutaric and 4-hydroxybutyric acids) did not affect these parameters. This study provides evidence that free radical generation may participate in the neurological dysfunction of propionic and methylmalonic acidemias.

The effects of methylmalonate (MMA) on succinate dehydrogenase (SDH) and beta-hydroxybutyrate dehydrogenase (HBDH) activities in brain and liver of 15-day-old rats were studied. The apparent Km of SDH for succinate was 0.45 mmol/L in brain and 0.34 mmol/L in liver. MMA inhibited the enzyme activity in both tissues with Ki values of 4.5 mmol/L and 2.3 mmol/L in brain and liver, respectively, and the inhibition was of the reversible competitive type. The calculated Km for HBDH with beta-hydroxybutyrate as substrate was 1.26 mmol/L in brain and 0.36 mmol/L in liver. MMA inhibited the enzyme with a Ki value of 0.015 mmol/L in brain and 0.275 mmol/L in liver. These results are probably relevant to our understanding of cerebral metabolism in methylmalonic acidaemic children, especially during ketoacidotic and hypoglycaemic crises, and may be related to the pathogenesis of cerebral dysfunction of methylmalonic acidaemia.

Hyperhomocysteinemia has been related to various diseases, including homocystinuria, neurodegenerative and hepatic diseases. In the present study we initially investigated the effect of chronic homocysteine administration on some parameters of oxidative stress, named total radical-trapping antioxidant potential, total antioxidant reactivity, catalase activity, chemiluminescence, thiobarbituric acid-reactive substances, and total thiol content in liver of rats. We also performed histological analysis, evaluating steatosis, inflammatory infiltration, fibrosis, and glycogen/glycoprotein content in liver tissue sections from hyperhomocysteinemic rats. Finally, we evaluated the activities of aminotransferases in liver and plasma of hyperhomocysteinemic rats. Wistar rats received daily subcutaneous injection of Hcy from their 6th to their 28th day of life. Twelve hours after the last injection the rats were sacrificed, liver and plasma were collected. Hyperhomocysteinemia decreased antioxidant defenses and total thiol content, and increased lipid peroxidation in liver of rats, characterizing a reliable oxidative stress. Histological analysis indicated the presence of inflammatory infiltrate, fibrosis and reduced content of glycogen/glycoprotein in liver tissue sections from hyperhomocysteinemic rats. Aminotransferases activities were not altered by homocysteine. Our data showed a consistent profile of liver injury elicited by homocysteine, which could contribute to explain, at least in part, the mechanisms involved in human liver diseases associated to hyperhomocysteinemia.

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