gamma-Hydroxybutyric acid (GHB) is a naturally occurring compound in the central nervous system (CNS) whose tissue concentration are highly increased in the neurometabolic-inherited deficiency of succinic semialdehyde dehydrogenase (SSADH) activity or due to intoxication. SSADH deficiency is biochemically characterized by increased concentrations of GHB in tissues, cerebrospinal fluid, blood and urine of affected patients. Clinical manifestations are variable and include retardation of mental, motor, and language development along with other neurological symptoms, such as hypotonia, ataxia and seizures, whose underlying mechanisms are practically unknown. The precursor of GHB, 1,4-butanediol (1,4-BD) has been used to study the mechanisms of in vivo GHB neurotoxicity. Therefore, in the present work, the effect of acute administration of 20 or 120 mg/Kg 1,4-BD was investigated on various parameters of oxidative stress, such as spontaneous chemiluminescence, thiobarbituric acid-reactive substances (TBA-RS), total antioxidant reactivity (TAR), sulfhydryl and protein carbonyl contents, as well as the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) in homogenates from cerebral cortex of 14-day-old Wistar rats. Acute administration of 120 mg/Kg 1,4-BD significantly increased spontaneous chemiluminescence and TBA-RS levels, while TAR measurement was markedly diminished, whereas injection of a lower dose (20 mg/Kg) did not change the parameters examined. Other parameters of oxidative stress evaluated were not affected by administration of 1,4-BD. These results indicate that 1,4-BD induces in vivo oxidative stress by stimulating lipid peroxidation and decreasing the non-enzymatic antioxidant defenses in cerebral cortex of young rats. If these effects also occur in humans, it is possible that they might contribute to the brain damage found in SSADH-deficient patients and possibly in individuals intoxicated by GHB or its prodrugs (gamma-butyrolactone or 1,4-BD).

Tyrosine accumulates in inborn errors of tyrosine catabolism, especially in tyrosinemia type II, where tyrosine levels are highly elevated in tissues and physiological fluids of affected patients. In tyrosinemia type II, high levels of tyrosine are correlated with eyes, skin and central nervous system disturbances. Considering that the mechanisms of brain damage in these disorders are poorly known, in the present study, we investigated whether oxidative stress is elicited by l-tyrosine in cerebral cortex homogenates of 14-day-old Wistar rats. The in vitro effect of 0.1–4.0mM l-tyrosine was studied on the following oxidative stress parameters: total radical-trapping antioxidant potential (TRAP), total antioxidant reactivity (TAR), ascorbic acid content, reduced glutathione (GSH) content, spontaneous chemiluminescence, thiobarbituric acid-reactive substances (TBA-RS), thiol-disulfide redox state (SH/SS ratio), protein carbonyl content, formation of DNA-protein cross-links, and the activities of the enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glucose-6-phosphate dehydrogenase (G6PDH). TRAP, TAR, ascorbic acid content, SH/SS ratio and CAT activity were significantly diminished, while formation of DNA-protein cross-link was significantly enhanced by l-tyrosine in vitro. In contrast, l-tyrosine did not affect the other parameters of oxidative stress evaluated. These results indicate that l-tyrosine decreases enzymatic and non-enzymatic antioxidant defenses, changes the redox state and stimulates DNA damage in cerebral cortex of young rats in vitro. This suggests that oxidative stress may represent a pathophysiological mechanism in tyrosinemic patients, in which this amino acid accumulates.

5-Oxoproline (pyroglutamic acid) accumulates in glutathione synthetase deficiency, an inborn metabolic defect of the gamma-glutamyl cycle. This disorder is clinically characterized by hemolytic anemia, metabolic acidosis and severe neurological disorders. Considering that the mechanisms of brain damage in this disease are poorly known, in the present study we investigated whether oxidative stress is elicited by 5-oxoproline. The in vitro effect of (0.5-3.0 mM) 5-oxoproline was studied on various parameters of oxidative stress, such as total radical-trapping antioxidant potential, total antioxidant reactivity, chemiluminescence, thiobarbituric acid-reactive substances, sulfhydryl content, carbonyl content, and 2',7'-dichlorofluorescein fluorescence, as well as on the activities of the antioxidant enzymes catalase, superoxide dismutase and glutathione peroxidase in cerebral cortex and cerebellum of 14-day-old rats. Total radical-trapping antioxidant potential and total antioxidant reactivity were significantly reduced in both cerebral structures. Carbonyl content and 2',7'-dichlorofluorescein fluorescence were significantly enhanced, while sulfhydryl content was significantly diminished. In contrast, chemiluminescence and thiobarbituric acid-reactive substances were not affected by 5-oxoproline. The activities of catalase, superoxide dismutase and glutathione peroxidase were also not altered by 5-oxoproline. These results indicate that 5-oxoproline causes protein oxidation and reactive species production and decrease the non-enzymatic antioxidant defenses in rat brain, but does not cause lipid peroxidation. Taken together, it may be presumed that 5-oxoproline elicits oxidative stress that may represent a pathophysiological mechanism in the disorder in which this metabolite accumulates.

Citrullinemia is an inborn error of the urea cycle caused by deficient argininosuccinate synthetase, which leads to accumulation of L-citrulline and ammonia in tissues and body fluids. The main symptoms include convulsions, tremor, seizures, coma, and brain edema. The pathophysiology of the neurological signs of citrullinemia remains unclear. In this context, we investigated the in vitro effects of L-citrulline and ammonia in cerebral cortex from 30-day-old rats on oxidative stress parameters, namely thiobarbituric acid-reactive substances (TBA-RS), chemiluminescence, mitochondrial membrane protein thiol content, intracellular content of hydrogen peroxide, total radical-trapping antioxidant potential (TRAP), total antioxidant reactivity (TAR) as well as on the activities of the antioxidant enzymes (catalase, superoxide dismutase, and glutathione peroxidase). L-Citrulline significantly diminished TRAP (26%) and TAR (37%), while ammonia decreased TAR (30%). Ammonia increased SOD activity (65%) and L-citrulline did not affect the activities of any antioxidant enzymes. We also observed that L-citrulline and ammonia did not alter lipid peroxidation parameters, levels of hydrogen peroxide, and mitochondrial membrane protein thiol content. Taken together, these results may indicate that L-citrulline and ammonia decreased the antioxidant capacity of the brain, which may reflect a possible involvement of oxidative stress in the neuropathology of citrullinemia.

GHB is a naturally occurring compound in the central nervous system (CNS) whose tissue concentration are highly increased during drug abuse and in the inherited deficiency of succinic semialdehyde dehydrogenase (SSADH) activity. SSADH deficiency is a neurometabolic-inherited disorder of the degradation pathway of γ-aminobutyric acid (GABA). It is biochemically characterized by increased concentrations of γ-hydroxybutyric acid (GHB) in tissues, cerebrospinal fluid (CSF), blood and urine of affected patients. Clinical manifestations are variable, ranging from mild retardation of mental, motor, and language development to more severe neurological symptoms, such as hypotonia, ataxia and seizures, whose underlying mechanisms are practically unknown. In the present study, the in vitro and in vivo effects of GHB was investigated on some parameters of oxidative stress, such as chemiluminescence, thiobarbituric acid-reactive substances (TBA-RS), total radical-trapping antioxidant potential (TRAP), total antioxidant reactivity (TAR), as well as the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) in homogenates from cerebral cortex of 15-day-old Wistar rats. In vitro, GHB significantly increased chemiluminescence and TBA-RS levels, while TRAP and TAR measurements were markedly diminished. In contrast, the activities of the antioxidant enzymes SOD, CAT and GPX were not altered by GHB in vitro. Acute administration of GHB provoked a significant enhance of TBA-RS levels and a decrease of TRAP and TAR measurements. These results indicate that GHB induces oxidative stress by stimulating lipid peroxidation and decreasing the non-enzymatic antioxidant defenses in cerebral cortex of young rats. If these effects also occur in humans, it is possible that they might contribute to the brain damage found in SSADH-deficient patients and possibly in individuals who consume GHB or its prodrug γ-butyrolactone.

Maple syrup urine disease (MSUD) is an inherited disorder caused by deficiency of branched-chain L-2-keto acid dehydrogenase complex activity. Affected patients present severe brain dysfunction manifested as convulsions, coma, psychomotor delay and mental retardation. However, the underlying mechanisms of these neurological findings are virtually unknown. In this study, we tested the in vitro effect of L-leucine, L-isoleucine and L-valine, the amino acids accumulating in MSUD, on the lipid peroxidation parameters chemiluminescence and thiobarbituric acid-reactive substances (TBA-RS), as well as on total radical-trapping antioxidant potential (TRAP) and total antioxidant reactivity (TAR) in cerebral cortex from 30-day-old rats. L-Leucine significantly increased chemiluminescence and TBA-RS measurements and markedly decreased TRAP and TAR values. L-Isoleucine increased chemiluminescence and decreased TRAP measurements, but TAR and TBA-RS levels were not altered by the amino acid. Finally, TRAP measurement was diminished by L-valine. The results indicate a stimulation of lipid peroxidation and a reduction of brain capacity to efficiently modulate the damage associated with an increased production of free radicals by the branched-chain amino acids (BCAAs) accumulated in MSUD. It is therefore tempting to speculate that oxidative stress may be implicated in the brain damage found in MSUD patients.