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Mitochondria are thought to play a crucial role in the etiology of muscle insulin resistance (IR). The aim of this study was to gain more insight into the timing and nature of mitochondrial adaptations during the development of high-fat-diet (HFD)-induced IR. Adult Wistar rats were fed HFD or normal chow for 2.5 and 25 wk. Intramyocellular lipids (IMCLs) were quantified in vivo using H-1 magnetic resonance spectroscopy (MRS). Muscle oxidative capacity was assessed in vivo using P-31 MRS and in vitro by measuring mitochondrial DNA copy number and oxygen consumption in isolated mitochondria. MRS in tibialis anterior muscle revealed 3.3-fold higher IMCL content and 1.2-fold increased oxidative capacity after 2.5 wk of HFD feeding. The latter result could be fully accounted for by increased mitochondrial content. After 25 wk of HFD, maximal ADP-stimulated oxygen consumption in isolated mitochondria oxidizing pyruvate plus malate remained unaffected, while IMCL and mitochondrial content had further increased compared to controls (5.1-fold and 1.4-fold, respectively). Interestingly, in vivo oxidative capacity at this time point was identical to controls. These results show that skeletal muscle in HFD-induced IR accompanied by IMCL accumulation requires a progressively larger mitochondrial pool size to maintain normal oxidative capacity in vivo.-Van den Broek, N. M. A., Ciapaite, J., De Feyter, H. M. M. L., Houten, S. M., Wanders, R. J. A., Jeneson, J. A. L., Nicolay, K., Prompers, J. J. Increased mitochondrial content rescues in vivo muscle oxidative capacity in long-term high-fat-diet-fed rats. FASEB J. 24, 1354-1364 (2010). www.fasebj.org

The permeability properties of the rat-liver mitochondrial membrane for N-acetylglutamate, the activator of carbamoyl-phosphate synthetase (ammonia), were studied. 1 Transport of N-acetylglutamate into the mitochondria was only observed in partially or fully de-energized mitochondria and when the extramitochondrial concentration was unphysiologically high (in the mM range). However, even under these conditions the intramitochondrial concentration of N-acetylglutamate was much lower than that outside. 2 Mitochondria1 N-acetylglutamate efflux only occurs when the mitochondria are in an energized state. At 25 °C, at an intramitochondrial N-acetylglutamate concentration of 0.7–1.0 nmol/mg protein, efflux proceeds at a rate of about 0.05 nmol × min−1× mg protein-′. This is 10-fold lower than the maximal rate of N-acetyl- glutamate synthesis in the mitochondria. 3 Homologous exchange between intramitochondrial N-[14C]acetylglutamate and extrainitochondrial un- labelled N-acetylglutamate could not be demonstrated. 4 It is concluded that transport of N-acetylglutamate in vivo is effectively unidirectional, out of the mitochondria. This behaviour is in accordance with the physiological requirement for efflux of N-acetylglutamate from the mitochondria in order to be degraded in the cytosol.

Systemic carnitine deficiency (CDSP) (McKusick 212140) is a rare autosomal recessive disease caused by defective plasma membrane uptake of carnitine. The disease is characterized by Reye syndrome, progressive cardiomyopathy, skeletal myopathy, hypoglycaemia and hyperammonaemia. CDSP is a treatable disease provided an early diagnosis is made and prompt treatment with L-carnitine is initiated. The biochemical diagnosis of the disease is based on the findings of very low plasma and tissue carnitine concentrations. Recently, a human gene, SLC22A5, encoding a sodium-dependent high-affinity carnitine transporter OCTN2 was cloned from human kidney and shown to be mutated in systemic carnitine deficiency. Here we report two unrelated Saudi CDSP patients who were detected by tandem mass spectrometric analysis (MS/MS) of blood spots. Studies in skin fibroblasts from the two patients showed a severely reduced carnitine uptake. Subsequent molecular studies led to the identification of two novel missense mutations in the OCTN2 gene in the two patients.

Two siblings presented with a new phenotype consisting of fatal progressive macrocephaly and hypertrophic cardiomyopathy. Onset of symptoms started in both patients at the end of the first month of life with massive brain swelling causing macrocephaly and evolving to extensive brain destruction. Light microscopy of the lesions showed extensive small-vessel proliferation and gliosis. A distinct deficiency of complex I of mitochondrial respiratory chain was established in cultured fibroblasts, skeletal muscle, and heart muscle. Specific lack of complex I protein was demonstrated by two-dimensional gel electrophoresis.

A boy aged 21 months who was being investigated for developmental delay and failure to thrive was found to have punctate epiphyseal calcification. He had no evidence of rhizomelic shortening of the limbs or cataracts. Investigation revealed defective plasmalogen synthesis due to isolated deficiency of dihydroxyacetonephosphate acyltransferase (DHAP-AT). The parents were consanguineous and a sister was similarly affected, suggesting autosomal recessive inheritance. Hitherto, recessively inherited isolated DHAP-AT deficiency has only been described in patients with a phenotype similar to that of rhizomelic chondrodysplasia punctata. This report indicates that the same biochemical disorder can be associated with a less severe phenotype.

Studies concerning peroxisomal β-oxidation have mainly been performed using radiolabelled fatty acids, after which the end products, 14 CO 2 and water-soluble radiolabelled products were measured (Wanders et al 1995). These studies only enable the investigation of a complete β-oxidation cycle and do not distinguish between the individual steps of βoxidation. We have developed a method for investigation of the first two steps of peroxisomal β-oxidation of pristanic acid in human liver. After incubation of human liver homogenates with pristanic acid or pristanoyl-CoA, the intermediates formed were hydrolysed to give their free acids, derivatized, and quantified by stable-isotop e dilution gas chromatography ‐mass fragmentography with negative chemical ionization. The applicability of the method described in this paper is demonstrated by showing the deficient oxidation of 2,3-pristanic acid to pristenic acid and 3-hydroxypristanic acid in liver from a Zellweger (McKusick 214100) patient.