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Concentrations of glutarate (GA) and its derivatives such as 3-hydroxyglutarate (3OHGA), D- (D-2OHGA) and L-2-hydroxyglutarate (L-2OHGA) are increased in plasma, cerebrospinal fluid (CSF) and urine of patients suffering from different forms of organic acidurias. It has been proposed that these derivatives cause neuronal damage in these patients, leading to dystonic and dyskinetic movement disorders. We have recently shown that these compounds are eliminated by the kidneys via the human organic anion transporters, OAT1 and OAT4, and the sodium-dependent dicarboxylate transporter 3, NaDC3. In neurons, where most of the damage occurs, a sodium-dependent citrate transporter, NaCT, has been identified. Therefore, we investigated the impact of GA derivatives on hNaCT by two-electrode voltage clamp and tracer uptake studies. None of these compounds induced substrate-associated currents in hNaCT-expressing Xenopus laevis oocytes nor did GA derivatives inhibit the uptake of citrate, the prototypical substrate of hNaCT. In contrast, D- and L-2OHGA, but not 3OHGA, showed affinities to NaDC3, indicating that D- and L-2OHGA impair the uptake of dicarboxylates into astrocytes thereby possibly interfering with their feeding of tricarboxylic acid cycle intermediates to neurons.

Glutaric aciduria type I (GA I) is a recessive disorder caused by a deficiency of glutaryl-CoA dehydrogenase (GCDH). The biochemical hallmark of the disease is the accumulation of glutaric acid and, to a lesser degree, of 3-hydroxyglutaric acid and glutaconic acid in body fluids and tissues. A substantial number of patients show only slightly, intermittently elevated or even normal urinary excretion of glutaric acid, which makes early diagnosis and treatment to prevent the severe neurological sequelae difficult. Furthermore, elevated urinary excretion of glutaric acid can also be found in a number of other disease states, mostly related to mitochondrial dysfunction. Stable-isotope dilution assays were designed for both glutaric acid and 3-hydroxyglutaric acid and their diagnostic sensitivity and specificity were evaluated. Control ranges of glutaric acid in urine were 1.1-9.7 mmol/mol creatinine before and 4.1-32 after hydrolysis. The respective values of 3-hydroxyglutaric acid were 1.4-8.0 and 2.6-11.7 mmol/mol creatnine. For other body fluids, control ranges in mumol/l/L were: for glutaric acid 0.55-2.9 (plasma), 0.18-0.63 (cerebrospinal fluid) and 0.19-0.7 (amniotic fluid); and for 3-hydroxyglutaric acid, 0.2-1.36 (plasma), < 0.2 (cerebrospinal fluid) and 0.22-0.41 (amniotic fluid). Twenty-five patients with GCDH deficiency were studied. Low excretors (12 patients) were defined by a urinary glutaric acid below 100 mmol/mol creatinine down into the normal range, while high excretors (13 patients) had glutaric acid excretions well above this value. With and without hydrolysis there was an overlap of glutaric acid values between patients and controls. Diagnostic sensitivity and specificity of 100% could only be achieved by the quantitative determination of 3-hydroxyglutaric acid with the newly developed stable-isotope dilution assay, allowing an accurate diagnosis of all patients, regardless of the amount of glutaric acid excreted in urine.

L-2-Hydroxyglutaric acidaemia represents a newly defined inborn error of metabolism, with increased levels of L-2-hydroxyglutaric acid in urine, plasma and cerebrospinal fluid. The concentration in cerebrospinal fluid is higher than in plasma. The other consistent biochemical finding is an increase of lysine in blood and cerebrospinal fluid, but lysine loading does not increase L-2-hydroxyglutaric acid concentration in plasma. This autosomal recessively inherited disease is expressed as progressive ataxia, mental deficiency with subcortical leukoencephalopathy and cerebellar atrophy on magnetic resonance imaging. Since these features were described in 8 patients by Barth and co-workers in 1992, 4 more patients with similar findings have been diagnosed and added to the present series. L-2-Hydroxyglutaric acid is found in only trace amounts on routine gas chromatographic screening in normal persons, and its origin, its fate and even its relevance to normal metabolism are unknown. Therefore its catabolism was studied in normal liver. Incubation of rat liver with L-2-hydroxyglutaric acid did not produce H2O2, which excluded (peroxisomal) L-2-hydroxyacid oxidase as the main route of catabolism. However, L-2-hydroxyglutaric acid is rapidly dehydrogenated if NAD+ is added as a co-factor to the standard reaction medium. This could also be demonstrated in human liver. The preliminary evidence for this enzyme activity in rats and humans, L-2-hydroxyglutaric acid dehydrogenase, is given. Further investigations are required to clarify the possible relevance to the metabolic defect in L-2-hydroxyglutaric acidaemia.