In conclusion, these results show that PDHA2 tissue-specific expression is strongly controlled by epigenetic modification and demonstrate the possibility of activating its expression in somatic cells. Hereditary Methylmalonic Acidemias, MMAs, are severe autosomal recessive inborn errors of metabolism caused by the deficiency of methylmalonyl-CoA mutase (MUT). MUTconverts L-methylmalonyl-CoA into succinyl-CoA. A block at this enzymatic step results in elevated plasma levels of methylmalonic acid as well the accumulation of other propionyl-CoA-derived metabolites. The disorders are caused by mutations in the MUT apoenzyme or defective metabolism of the enzymatic cofactor, 5'-deoxyadenosylcobalamin. Two enzymatic phenotypes of apoenzyme deficiency are recognized: mut0 patients have no detectable residual enzyme activity while mut-patients have reduced enzyme activity that may be cobalamin-responsive. Mut0 patients can experience life-threatening metabolic instability. Liver specimens from six healthy donors and six mut0 patients were employed to identify deregulated proteins by using DIGE (Differential Gel Electrophoresis. We identified 150 deregulated spots, 80 down-regulated and 70 up-regulated. The spots were excised and identified by mass spectrometry. Proteomic results revealed decreased levels of proteins involved in energetic metabolism and cellular detoxification. A metabolic profiling is underway. The protein data set will be analyzed in the frame of metabolic networks to identify the most differentially altered cellular pathways. Defining altered protein profiles could lead to the identification of new therapeutic targets for MMA. The enzyme glutaryl-Coenzyme A-Dehydrogenase (GCDH) is a homotetrameric mitochondrial matrix protein, for which so far more than 150 diseases-causing mutations have been described, which lead to amino acid substitutions in different regions of the protein. Crosslink experiments show that distinct GCDH mutations with high residual enzymatic activity and amino acid substitutions localized on the surface of the protein affect the homotetramerization as well as the heteromeric complex formation with so far unidentified proteins. The electron-transfer flavoprotein (ETF) composed of the two subunits alpha-ETF and beta-ETF appeared as a promising candidate as interaction partners of GCDH. Pull down experiments with alpha-or beta-ETF immobilized to affinity columns and subsequent incubations with GCDHoverexpressing cell extracts revealed the first experimental evidence for a direct interaction of wildtype GCDH with alpha-as well as beta-ETF. The binding of alpha-or beta-ETF to distinct mutant GCDH proteins was reduced. The affinity of interactions between ETF and wildtype or mutant GCDH was characterized by surface plasmon resonance spectroscopy. Characterization of the interaction between alpha-and beta-ETF and different mutant GCDH proteins provided data on the site of the GCDH protein which is responsible for binding to ETF subunits. Herein we describe the use of aminoacid supplementation in restoring the in vitro folding and activity of natural arginine mutant proteins related to inborn errors of metabolism (IEM). Missense arginine mutations are the most frequent mutations in IEMlinked conformational disorders. Our group has observed that aminoacid supplementation reversed the clinical phenotypes of pyruvate dehydrogenase deficiency patients carrying mutations in arginine residues. Following this, we selected a range of arginine mutations in different proteins related to IEM, namely: E1 alpha subunit of pyruvate dehydrogenase complex (PDH-E1α), cystathionine-beta-synthase (CBS) and galactose-1-P uridyltransferase (GALT). The respective cDNA sequences were cloned into pET-based vectors, and the selected mutants generated by site-directed mutagenesis. By expressing the target proteins in E. coli BL21 (DE3) Rosetta cells, we evaluated the effect of aminoacid supplementation in the protein levels, folding and activity of WT and mutant proteins. The results on the in vitro restoration of protein function upon supplementation are currently being probed in a small set of patient fibroblasts. Altogether these results should provide clues as whether to employ aminoacid supplementation as a general therapeutic for IEM-conformational disorder patients carrying arginine missense mutations.