*8.1.1. Glutamate carrier*

Except for decisive interconnection between lipogenesis, gluconeogenesis, and glycolysis, CiC has been proposed to play a role in the maintenance of chromosome integrity and in the regulation of autophagy [161]. A particularly important role played by the CiC is in the regulation of insulin secretion by providing isocitrate for NADP-dependent isocitrate dehydrogenase [162]. The studies have shown that the CiC activity and properties could be changed under specific conditions: starvation-induced decrease of CiC activity and considerable reduction of CiC mRNA in starved rats. The reduced CiC mRNA levels were ascribed to shortened half-life and accelerated degradation of CiC mRNA. Refeeding, however, leads to renewal of mRNA and increased activity of CiC [163]. The polyunsaturated fatty acid (PUFA) administration also dramatically affects CiC gene expression by transcriptional and posttranscriptional mechanisms. Rat liver mitochondria showed more decreased CiC activity and reduced transcriptional rate of CiC mRNA when affected by n-3 PUFA than by n-6 [164]. During the inflammation, CiC gene expression is activated by NF-κB, which causes an increased availability of cytosolic acetyl-CoA and NADPH + H+ for synthesis and activity of compounds and enzymes involved in inflammatory response (e.g., COX2, iNOS, and NADPH oxidase) [156]. The human SLC25A1 gene is localized on chromosome 22, within the region associated with allelic losses in DiGeorge/22q11 syndrome, velo-cardio-facial syndrome, and a subtype of schizofrenia [165]. To date, recessive mutations of CiC gene in 20 persons with combined D,L-hydroxyglutaric aciduria were described. The clinical phenotype of disorder is characterized by severe developmental delay, hypotonia, seizures, secondary microcephaly, hypoplasia or agenesis of the corpus callosum, optic nerve hypoplasia, dysmorphic feature, lactic acidosis, and recurrent apneic crises

[166]. CIC has also been reported to be upregulated in ovarian and colon cancer [167].

Dicarboxylate carrier protein ((DIC) SLC25A10) transporting malate and succinate out of

provides a cytosolic source of malate for CiC and therefore plays a significant role in fatty acid synthesis [168]. DIC interacts with malate dehydrogenase by acting as an oxaloacetate shuttle, thus improving functional coupling of citric acid cycle with shuttle. In the cytosol, malate is converted into oxaloacetate following conversion into phosphoenolpyruvate by carboxykinase-catalyzed reaction. The reaction is rate-limiting for gluconeogenesis. No less important role of the carrier is the transport of reduced glutathione into the mitochondria shared with OGC. Therefore, limiting protein expression results in significantly reduced levels of glutathione in the mitochondria and subsequent altered redox conditions [169]. It was found that the activity of DIC is increased in type I diabetes, in contrast to decreased activities of CiC [170].

The OGC mediates transfer of oxoglutarate across MIM in exchange for dicarboxylate. OGC is a component of malate-aspartate shuttle; thus, dicarboxylate is usually malate. After export, malate is converted into oxaloacetate, which is in transamination reaction with glutamate converted into

is ubiquitously expressed in mammalian mitochondria. The

and other phosphate and substrate analogues. Malate exchange for Pi

**7.2. Dicarboxylate carrier**

266 Mitochondrial Diseases

carrier is inhibited by Pi

**8. Oxoglutarate carrier**

mitochondria in exchange for Pi

Another one from SLC25 family is a glutamate carrier. Two glutamate-transfer isoforms are known for the glutamate carrier: GC1 (SLC25A22) and GC2 (SLC25A18). They transport glutamate across the MIM in symport with a proton or in exchange for hydroxyl ions. In mitochondria, glutamate is converted by glutamate dehydrogenase into α-ketoglutarate while reducing NAD (P)+ . Ammonia is released and reduced coenzyme enters the complex I of respiratory chain. The mRNAs of GC1 have been found to be highly expressed than that of GC2 in liver, pancreas, and kidney but are similar in the brain. Moreover, the *K*m and *V*max values are higher than those of GC2 (5.2 vs. 0.26 mM; 12.2 vs. 3.9 μmol/min/g of proteins). Therefore, an acceptable explanation, for the expression pattern is that GC2 is responsible for the basic function, whereas GC1 functions in tissues with increased demands [174].

GC1 has been demonstrated to have an important physiological function in the control of glucose-stimulated insulin secretion in pancreatic β cells [96]. The signaling mechanism leads to adjustment of insulin release to levels greater than the sole contribution of Ca2+-induced triggering pathway in rats. Stimulation of β cells with high glucose might result in rapid saturation of the respiratory chain [175], favored by glucokinase properties (as mentioned in Section 4.1.) and low lactate release. Saturated electron transport chain would then promote export of metabolites (GTP, citrate, NADH, and glutamate) out of the mitochondria compensated by activity of anaplerotic pathways [176]. Energetic sufficiency favors the glutamate dehydrogenase reaction from α-ketoglutarate toward glutamate formation. Glutamate is taken up by secretory granules, which are consistent with the expression of vesicular glutamate transporters (VGLUT1 and 2) in insulin-secreting cells [177]. Inside the secretory granule, glutamate could induce pH changes and activate metabotropic receptors mGlu5, thereby mediating insulin release [178]. Alternative mechanisms include activation of acetyl-CoA carboxylase and inhibition of phosphatase activities involved in insulin exocytosis [179].

GC1 is highly expressed in astrocytes from different structures (retina, spinal cord, and cortex) [180] and represents the principal gate for glutamate entry into the mitochondria of astrocytes. Restricting glutamate access to mitochondria results in reduced ATP and NAD(P)H formation. A defective glutamate carrier may lead to glutamate accumulation in the astrocytes cytosol and then to glutamate liberation in the synaptic cleft. The release could result in neuronal synchronicity, which may contribute to the generation of epileptic-like discharges in the brain [181]. Mutations in the human *GC1* gene (localized on chromosome 11p15.5) are responsible for the autosomal recessive form of early infantile epileptic encephalopathy caused by complete loss of transport and uniport activity of the protein [182–184]. Clinical manifestations are similar to epileptic spasms and focal seizures associated with suppression bursts beginning in the first days of life, microcephaly, hypotonia, abnormal retinogram recording, and psychomotor retardation [183].

Activity of ORC1 and 2 is enhanced by Pi

highest importance in urea cycle continuance [193].

**9.2. Mitochondrial uncoupling proteins**

, malate, and dicarboxylates and inhibited by pyr-

http://dx.doi.org/10.5772/intechopen.72937

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Nuclear Encoded Mitochondrial Proteins in Metabolite Transport and Oxidation Pathway…

idoxal 5′-phosphate (PLP), mercurials, spermine, and spermidine. The affinity of ORC2 to lysine and arginine is lower and to ornithine and citrulline is higher in comparison to ORC1. Moreover, ORC2 has been reported to be about three times less active than ORC1. The dispositions are also related to protein expression. The ORC1 is expressed in most tissues, with the highest levels in the liver, pancreas, lungs, kidney, and testis, unlike the ORC2 being more restricted to these organs [193]. ORC3 is expressed in heart, brain, liver, and kidney and is induced after partial hepatectomy or fasting [195, 196]. The import of arginine, lysine, and histidine allows for protein synthesis in mitochondria and that for ornithine enables degradation of arginine surplus. Transfer of ornithine out of the mitochondria allows for synthesis of polyamines reversibly inhibiting ORC activity. Ornithine is synthesized in mitochondria from glutamate in tissues with low arginase activity (except for the liver), from glutamine in intestinal mitochondria or when deficient in the diet [193]. Considering ornithine and citrulline transport efficiency and level of protein expression in the liver, the ORC1 isoform is of

Mutation in the gene encoding ORC1 isoform (localized on 13q14.1 chromosome) causes hyperornithinemia-hyperammonemia-homocitrullinuria (HHH syndrome), characterized by early-onset neurological deficits. Hyperammonemia results from impaired urea cycle due to ORC1 malfunction. Ornithine accumulates in the cytosol leading to hyperornithinemia and increases polyamine synthesis. Carbamoyl phosphate condensates with lysine in the absence of ornithine inside the mitochondria, leading to homocitrullinuria, or enters pyrimidine synthesis, thus increasing excretion of orotic acid and uracil [97, 197]. Overexpression of ORC2 might only partially compensate defective function of ORC1 due to lower affinity for ornithine and citrulline [196, 198]. ORC3 has not been found to compensate lack of ORC1 function but is probably responsible for lysine transport in patients with HHH syndrome [194].

Uncoupling proteins (UCP) sharing the same tripartite structure belongs to the family of the mitochondrial anion carriers. Six families of UCP members encoding by 45 genes have been described [199]. In mammals, UCPs consist of five homologs: UCP1 (SLC25A7), UCP2 (SLC25A8), UCP3 (SLC25A9), UCP4 (SLC25A27), and UCP5 (SLC25A14, BMCP1). *UCP1* genes are localized on human chromosome 4. The human and mouse *UCP2* genes are located 7–20 kb downstream of the *UCP3* stop codon, as the result of a duplication; the *UCP3-UCP2* locus is located on human chromosome 11q13 (between the genetic markers D11S916 and D11S911). The UCP5 homolog *Bmcp1* is located on Xq25–26 chromosome (between the markers DXS1206 and DXS1047), and *UCP4* on 6p11.2-q12 (close to the genetic marker SHGC-34952) [200].

UCPs are ubiquitous, except for UCP2 [201], however, exhibiting tissue-specific expression pattern. As reviewed in Gutérrez-Aquilar and Baines [202], UCP1 is unique to brown adipose tissue, UCP3 to heart and skeletal muscle, and UCP4 and 5 are typical to the brain. The general designation of this carrier family is derived from observed function of the first member, UCP1 in brown fat tissue—the heat production in the nonshivering thermogenesis [199]. According to Mitchell's theory, any proton leak not coupled with ATP synthesis would provoke uncoupling of respiration and thermogenesis. The discharge of proton gradient formed
