2. Neurometabolic disorders
One of our aims is to elucidate the biochemical basis of neurometabolic disorders. This we do by combining biochemical characterization of enzymes, database searches, mutation search in candidate genes in patients DNA and expression and characterization of putative enzymes and of their mutated forms. One example of such research is the identification of the gene mutated in L-2-hydroxyglutaric aciduria, a disorder characterized by the accumulation of L-2-hydroxyglutaric acid. Biochemical investigations performed on rat tissues indicated the presence of a mitochondrial, membrane-bound enzyme, using FAD as a cofactor and converting L-2-hydroxyglutarate to alpha-ketoglutarate. Based on this information we searched database for a mammalian homologue of a bacterial enzyme catalyzing a similar reaction, which would be membrane-bound and targeted to the mitochondria. One such homologue was found in the human genome. Further investigations showed that its gene was indeed mutated in patients with L-2-hydroxyglutaric aciduria. Overexpression studies in mammalian cells confirmed that it indeed encoded L-2-hydroxyglutarate dehydrogenase.
Studies aimed at determining the way L-2-hydroxyglutarate is formed led to the conclusion that it is made by mitochondrial L-malate dehydrogenase, an extremely active enzyme (on L-malate/oxaloacetate) in tissues. This enzyme also converts alpha-ketoglutarate to L-2-hydroxyglutarate at a slow rate. Yet this rate is sufficient to account for all the L-2-hydroxyglutarate that is formed everyday and is excreted by the patients with L-2-hydroxyglutaric aciduria in their urine. L-2-hydroxyglutarate dehydrogenase therefore appears to catalyze a ‘metabolite repair’ reaction. The elucidation of the mechanism of toxicity of L-2-hydroxyglutarate is one of our present goals.
Formation of L-2hydroxyglutarate and its destruction by L-2-hydroxyglutarate dehydrogenase (FAD)
Our group is also interested by elucidating other metabolic defects. Among these are the serine deficiencies. L-Serine, a major amino acid, is biosynthesized in mammals through a simple-three step pathway. Patients with L-serine defects have major neurological problems, which may be prevented (at least to some extent) by the administration of L-serine and glycine. Mutations have been found in all three enzymes. However, there seems to be cases where low serine levels have been found in the cerebrospinal fluid and where no enzyme defect has been identified. Identifying the defect in these patients is one of our present goals.
Pathway of L-serine biosynthesis
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