Background: Many enzyme defects show tissue-specificity in mitochondrial respiratory deficiency (MRCD), which sometimes make difficult for diagnosis. . Subject/Method: 158 patients were diagnosed to have MRCD out of 429 candidate patients. We classified clinically the following groups, Leigh's disease (LD), mitochondrial cytopathy (MC), neurodegenerative diseases (ND), hepatic disease(HD), cardiomyopathy (CM) and infantile mitochondrial disorder (IMD). We investigated the tissue specific pattern of enzyme defect to contribute correct diagnosis for MRCD. Results: Clinical diagnosis of lethal infantile mitochondrial disease was the greatest in number. The number of clinical LD, MC, ND, HD, CM and non-lethal IMD was 29, 23, 9, 21, 11 and 10. In LD, 14/18, 12/15, 3/3 and 0/0 were diagnosed in fibroblasts, muscle, liver and heart, respectively. In MC, 4/11, 11/16, 4/5 and 2/4, in ND, 3/3, 5/5, 1/1 and 0/0, in HD, 1/7, 0/0, 20/20 and 1/1, in CM, 2/6, 3/5, 4/9 and 10/10, in IMD, 14/25, 17/22, 22/24 and 8/13 were diagnosed in each specimen. Discussion: Almost HD and CM can be diagnosed only with affected organs. In other MRCD, enzyme defects can be detected in wide variety of organs and tissues, which support from many specimens can lead to greater confidence in the diagnosis achieved. Mitochondrial disorders (MD) may manifest in neonates, but the early diagnosis is difficult. The aims of the study were to analyse clinical and laboratory characteristics of MD with neonatal onset and identify possible association between clinical findings and specific mitochondrial diseases. Retrospective clinical and laboratory data were evaluated in 461 patients (331 families) with confirmed MD from two metabolic centers Prague and Salzburg. The neonatal onset of MD was reported in 129 patients (28%). Prematurity, intrauterine growth retardation, hypotonia necessitating ventilatory support were present in one third, cardiomyopathy in 40%, Leigh syndrome in 15% and profound lactic acidosis in half of neonates. 19 patients died in neonatal period. PDH complex deficiency was identified in 6, complex I in 15, complex III in one, complex IV in 23, complex V in 31, combined deficiency of several complexes in 53 patients. Background: Mitochondrial disorders affect~1 in 5000 births, and frequently present with combined deficiency of multiple respiratory chain (RC) enzymes. 3-hydroxy-isobutyryl-CoA hydrolase (HIBCH) deficiency is a rare disorder of valine catabolism. Objective: To report the association of multiple RC deficiencies with HIBCH deficiency. Patients and Methods: Two brothers, born to healthy distantly-related parents, presented with neonatal-onset neurodegenerative disease including seizures, dystonia and developmental regression. Relentless progression led to early death in both siblings. Investigation for mitochondrial disease revealed multiple RC defects (affecting complexes I, II+III and IV) in muscle from the older brother but normal activities in his sibling. The observation of persistently elevated hydroxy-butyrylcarnitine levels in the younger sibling led to investigation of HIBCH activity. Results: HIBCH activity was markedly reduced in cultured skin fibroblasts from both brothers. Subsequently a homozygous missense mutation affecting a highly conserved amino acid residue in the HIBCH gene was identified in both brothers. This mutation segregated with disease in the family. Conclusion: Multiple RC deficiencies are usually thought to arise from impaired maintenance and/or expression of the mitochondrial genome. We now show that HIBCH deficiency may lead to multiple RC defects. Objective: to study association of folate cycle polymorphisms (FCP) with mitochondrial dysfunction. Materials and Methods: 54 patients with a primary mitochondriopathy (PM) (Kearns-Sayre, MELAS, MNGIE, MERRF, organic acidurias) and 22 patients with secondary mitochondriopathy (SM). In both groups study of polymorphisms of MTHFR C677T and A66G MTRR by method of allelespecific PCR. Results: In 47 patients (87%) with PM were identified, compounds of allele frequencies MTHFR/MTRR as follows: N/N-7(13%); hmzg/hmzg-1 (1,9%); htrzg/htrzg-9(16.7%); htrzg/hmzg-17(31.5% ); hmzg/htrzg 2 (3.7%); N/hmzg-6(11.1%); N/htrzg-6(11.1%); hmzg/N-1(1,9%); htrzg/ N-5(9.3 %). In 19 (86.4%) patients with SM identified FCP, distribution of allele frequencies was as follows: N/N-3(13.6%); hmzg/hmzg-0(0%) ; htrzg/htrzg-2(9.1%); htrzg/hmzg-10(45.5%); hmzg/htrzg-0(0%); N/ hmzg-3(13.6%); N/htrzg-3(13.6%) ; hmzg/N-0(0%); htrzg/N-1 (4.5%). Conclusions: 66 (86.8%) patients with mitochondrial dysfunction had FCP and more specific to them was a polymorphism A66G MTRR-in 59 patients (77.6%) than the C677T MTHFR-in 48 patients (63.2%). This should be considered when developing an individual tactical correction energy deficit and conduct of prevention of complications associated with the presence of these polymorphisms. Background: Pristanic acid (Prist) concentrations are increased in peroxisomal disorders characterized by neurologic dysfunction and brain abnormalities, whose pathogenesis is poorly known. Objectives: Prist effects on important parameters of energy homeostasis were investigated in rat brain mitochondria. Material and methods: Purified mitochondrial preparations were obtained from brain of 30-day-old rats. Various parameters of energy homeostasis were then determined in the presence of 20-100 μM Prist. Results: Prist markedly increased state 4 respiration and diminished the RCR using both glutamate plus malate or succinate as substrates. In addition, Prist decreased state 3 respiring mitochondria, the ADP/O ratio, the mitochondrial membrane potential and NAD(P)H levels. Prist also induced mitochondrial swelling, probably through oxidative attack to the permeability transition pore (PTP) since cyclosporine A (PTP inhibitor) and N-acetylcysteine were able to prevent this effect. Conclusion/Discussion: The data indicate that Prist acts as an uncoupler of oxidative phosphorylation and as a metabolic inhibitor, besides causing mitochondrial swelling. It is presumed that impairment of mitochondrial homeostasis may contribute to the neurological abnormalities presented by patients affected by peroxisomal diseases in which brain Prist concentrations are increased. Background: Peroxisomal abnormalities are biochemically characterized by tissue accumulation of branched chain fatty acids, including pristanic acid (Prist). Affected patients usually present brain abnormalities, however the pathomechanisms of cerebral injury in these diseases are still unknown. Objectives: We investigated the in vitro effects of Prist on Na+,K±ATPase and respiratory chain complex activities. Material and methods: Brain of young rats was dissected and the biochemical test performed after tissue preparation. Results: Prist markedly decreased the activities of Na+,K±ATPase (80 %) and of complexes I (65%), II (40%) and II-III (95%), without affecting complex IV activity. Conclusion/discussion: Considering the importance of Na+,K±ATPase for the maintenance of the membrane potential necessary to normal neuronal excitability and cellular cell volume control and of oxidative phosphorylation for brain energy production, the present data indicate that Prist compromises neurotransmission and brain bioenergetics. It is presumed that these pathomechanisms may be involved in the brain damage occurring in disorders in which Prist accumulates. Background: Pristanic acid (Prist) is a branched-chain fatty acid that accumulates in a variety of peroxisomal disorders. Although these disorders are characterized by neurological symptoms, the mechanisms involved in the pathophysiology are poorly known. Objectives: We studied the in vitro effects of Prist on important parameters of oxidative stress in cerebral cortex from young rats. Methods: Thiobarbituric acid-reactive substances (TBA-RS), carbonyl formation, sulfhydryl content, reduced glutathione (GSH) levels and nitric oxide production were measured in cerebral cortex from 30-day-old rats. Results: Prist increased TBA-RS levels, reflecting an increase of lipid peroxidation. This effect was totally prevented by the free radical scavenger melatonin, suggesting the involvement of reactive species. Prist also provoked protein oxidative damage, as determined by increased carbonyl formation and sulfhydryl oxidation. Otherwise, it did not alter nitric oxide production. Furthermore, the concentration of GSH was significantly decreased by Prist and this decrease was prevented by melatonin and alfa-tocopherol. Conclusions: It is therefore presumed that Prist elicits oxidative stress in the brain and that this pathomechanism may possibly be involved in the brain damage found in patients affected by peroxisomal disorders where Prist accumulates. Financial support: CNPq, FAPERGS, PRONEX and the FINEP research grant Rede IBN-Net and INCT-EN.