Mitochondrial phosphate transport. The Saccharomyces cerevisiae (threonine 43 to cysteine) mutant protein explicitly identifies transport with genomic sequence

The yeast mitochondrial phosphate transport protein (PTP) has only 38% sequence similarity to the bovine heart protein, and it has recently been postulated to code for a mitochondrial import receptor. Since the reconstitutively active protein is not completely pure, it is important to demonstrate explicitly that the yeast gene codes for PTP. We have replaced Thr43 with Cys (T43C) and show that its unidirectional and pH gradient-dependent inorganic phosphate transport activity becomes highly sensitive to N-ethylmaleimide. This new PTP/T43C catalyzes less than 10% of the wild type transport activity (1 mM [Pi]e, pHe (6.80); 0 mM [Pi]i, pHi (8.07); 30 s [Pi] uptake) suggesting that Thr43 occupies an important position in the PTP.     

Mitochondrial phosphate transport: Correlation between alkylation of membrane proteins with N-[3H]ethylmaleimide and inhibition of transport

N-ethylmaleimide inhibits the mitochondrial phosphate carrier. Mitochondria were titrated with N-[³H]ethylmaleimide, dissolved in dodecylsulfate-mercaptoethanol, and their proteins separated on dodecylsulfate-polyacrylamide gels. While the phosphate transport is essentially insensitive to low concentrations of N-ethylmaleimide, the six primary N-ethylmaleimide reactive inner membrane proteins are labeled in direct proportion to the amount of inhibitor added. The reaction of N-[³H]ethylmaleimide with proteins I and III is independent of the preincubation of the mitochondria with p-mercuribenzoic acid, a membrane impermeable inhibitor of the transport. Comparing the alkylation of proteins II, IV, V and VI with the inhibition of phosphate transport, it is found that only proteins IV (45,000 daltons) and V (32,000 daltons) are maximally labeled at the same N-[³H]ethylmaleimide concentration that maximally inhibits the transport.     

Identification of the N-ethylmaleimide reactive protein of the mitochondrial phosphate transporter.

The mitochondrial phosphate carrier is inhibited by the SH reagents p-(hydroxymercuri)benzoate and N-ethylmaleimide. Based on an analysis utilizing dodecyl sulfate-polyacrylamide gels, an SH-containing 32 000-dalton protein has been identified as a component of the phosphate carrier system. Two other N-[3H]ethylmaleimide-labeled proteins of the inner mitochondrial membrane have been eliminated from this role [Wholrab, H., & Greaney, J., Jr. (1978) Biochim. Biophys. Acta 503, 425] on the basis that band IV (45,000 daltons) is absent from heart sonic submitochondrial particles and band VII (6 500 daltons) does not react with p-(hydroxymercuri)benzoate. The mobility of the 32 000-dalton protein (0.43) is lower than that of the gamma subunit of the mitochondrial ATPase (0.46) and the carboxyatractyloside binding protein (0.48) on 12.5% dodecyl sulfate-polyacrylamide gels. In these flight muscle mitochondria, 0.87 nmol of N-[3H]ethylmaleimide per nmol of cytochrome a is bound to the 32,000-dalton protein.     

Energy-linked anion transport. Cloning, sequencing, and characterization of a full length cDNA encoding the rat liver mitochondrial proton/phosphate symporter

A full length cDNA clone encoding the precursor of the rat liver mitochondrial phosphate transporter (H+/Pi symporter) has been isolated from a cDNA library using a bovine heart partial length phosphate transporter clone as a hybridization probe. The entire clone is 1263 base pairs in length with 5'- and 3'-untranslated regions of 16 and 168 base pairs, respectively. The open reading frame encodes for the mature protein (312 amino acids) preceded by a presequence of 44 amino acids enriched in basic residues. The polypeptide sequence predicted from the DNA sequence was confirmed by analyzing the first 17 amino-terminal amino acids of the pure phosphate transporter protein. The rat liver phosphate transporter differs from the bovine heart transporter in 32 amino acids (i.e. approximately 10%). It contains a region from amino acid 139 to 159 which is 37% identical with the beta-subunit of the liver mitochondrial ATP synthase. Amino acid sequence comparisons of the Pi transporter with Pi binding proteins, other H+-linked symporters, and the human glucose transporter did not reveal significant sequence homology. Analysis of genomic DNA from both rat and S. cerevisiae by Southern blots using the rat liver mitochondrial Pi carrier cDNA as a probe revealed remarkably similar restriction patterns, a finding consistent with the presence in lower and higher eukaryotes of homologous Pi carrier proteins. This is the first report of the isolation, sequencing, and characterization of a full length cDNA coding for a protein involved in energy-coupled Pi transport.     

Characterization of SH groups in porin of bovine heart mitochondria. Porin cysteines are localized in the channel walls.

Porin from bovine heart mitochondria contains probably two cysteines (Cys126 and Cys230 in human porin, Kayser, H., Kratzin, H. D., Thinnes, F. P., G?tz, H., Schmidt, W. E., Eckart, K. & Hilschmann, N. (1989) Biol. Chem. Hoppe-Seyler 370, 1265-1278). Reduced and oxidized forms of these cysteines were investigated in purified protein and in intact mitochondria using the agents dithioerythritol, cuprous(II) phenantroline, diamide and performic acid. Furthermore, intact mitochondria were labelled with the sulfhydryl-alkylating agents N-[14C]ethylmaleimide, eosin-5-maleimide and N-(1-pyrenyl)-maleimide. Affinity chromatography of bovine heart porin was performed with cysteine-specific material. The results can be summarized as follows: (1) Porin has one reduced and two oxidized forms of apparent molecular masses between 30 and 35 kDa. The native form of porin is the reduced 33 kDa form. The oxidized forms only appear after denaturation with SDS. (2) The 35-kDa reduced and the 33.5-kDa oxidized forms of porin show the same pore-forming properties after reconstitution of the protein into lipid bilayer membranes. (3) Labelling of cysteines by eosin-5-maleimide and N-(1-pyrenyl)-maleimide suggested their location at a boundary between the water-phase and the lipid-phase. Incubation of intact mitochondria with N-ethylmaleimide prior to eosin-5-maleimide and N-(1-pyrenyl)maleimide treatment resulted in the inhibition of the fluorescent labelling. Among the cysteines present in the primary structure, Cys126 is the most sensitive to N-ethylmaleimide binding. (4) Bovine heart mitochondrial porin covalently bound to Affi-Gel 501 (with a 1.75 nm long spacer), but not to Thiopropyl-Sepharose 6B (with a 0.51 nm spacer). This suggests that at least one of the cysteines is localized between 0.51 nm and 1.75 nm deep in the protein micelle.     

Revised mechanism for inactivation of mitochondrial monoamine oxidase by N-cyclopropylbenzylamine

A mechanism previously proposed for inactivation of monoamine oxidase (MAO) by N-cyclopropylbenzylamine (N-CBA) [Silverman, R. B., & Hoffman, S. J. (1980) J. Am. Chem. Soc. 102, 884-886] is revised. Inactivation of MAO by N-[1-3H]CBA results in incorporation of about 3 equiv of tritium into the enzyme and release of [3H]acrolein. Treatment of inactivated enzyme with benzylamine, a reactivator for N-CBA-inactivated MAO, releases only 1 equiv of tritium as [3H]acrolein concomitant with reactivation of the enzyme. Even after MAO is inactivated by N-[1-3H]CBA, the reaction continues. At pH 7.2, a linear release of [3H]acrolein is observed for 70 h, which produces 55 equiv of [3H]acrolein while 2.3 equiv of tritium is incorporated into the enzyme. At pH 9, only 3.5 equiv of [3H]acrolein is detected in solution after 96 h, but 40 equiv of tritium is incorporated into the enzyme, presumably as a result of greater ionization of protein nucleophiles at the higher pH. N-[1-3H]Cyclopropyl-alpha-methylbenzylamine (N-C alpha MBA) produces the same adduct as N-CBA but gives only 1-1.35 equiv of tritium bound after inactivation of the enzyme. Denaturation of labeled enzyme results in reoxidation of the flavin without release of tritium, indicating attachment is not to the flavin but rather to an amino acid residue. Enzyme inactivated with N-[1-3H]C alpha MBA is reactivated by benzylamine with the release of 1 equiv of [3H]acrolein, which must have come from an adduct attached to an active site amino acid residue.(ABSTRACT TRUNCATED AT 250 WORDS)     

The tritium isotope effect of sn-glycerol 3-phosphate oxidase and the effects of clofenapate and N-(2-benzoyloxyethyl)norfenfluramine on the esterification of glycerol phosphate and dihydroxyacetone phosphate by rat liver mitochondria

1. Owing to a (3)H isotope effect, the mitochondrial sn-glycerol 3-phosphate oxidase (EC 1.1.99.5) had a mean activity which was 8.4 times less with sn-[2-(3)H]-rather than with sn-[1-(14)C]glycerol 3-phosphate as a substrate. 2. A method for measuring the simultaneous synthesis of lipid from glycerol phosphate and dihydroxyacetone phosphate in rat liver mitochondria is described. 3. The lipid synthesized by rat liver mitochondria from sn-[1-(14)C]glycerol 3-phosphate was mainly phosphatidate and lysophosphatidate, whereas that synthesized from dihydroxy[1-(14)C]acetone phosphate was mainly acyldihydroxyacetone phosphate. 4. Additions of NADPH facilitated the conversion of acyldihydroxyacetone phosphate into lysophosphatidate and phosphatidate. 5. Hydrazine (1.4mm) or KCN (1.4mm) inhibited the synthesis of lipids from dihydroxyacetone phosphate but not from glycerol phosphate. 6. Clofenapate (1-2.5mm) inhibited the synthesis of lipids from dihydroxyacetone phosphate but slightly stimulated synthesis from glycerol phosphate. 7. The methanesulphonate of N-(2-benzoyloxyethyl)norfenfluramine, at 0.25-0.75mm, inhibited lipid synthesis from both glycerol phosphate and dihydroxyacetone phosphate.     

Response of adrenal tumor cells to adrenocorticotropin: site of inhibition by cytochalasin B.

The ability of cytochalasin B to inhibit the steroidogenic response of mouse adrenal tumor cells (Y-1) to adrenocorticotropin (ACTH) was examined with two aims: to consider the specificity of the inhibitor and to determine at what point(s) in the steroidogenic pathway it acts. Cytochalasin B did not inhibit protein synthesis or transport of [3H]-cholesterol into the cells nor did it alter total cell concentration of ATP. Together with previous evidence, this suggests that the effects of cytochalasin observed are relatively specific in these cells. Cytochalasin inhibits the increase in conversion of [3H]cholesterol to 20alpha-[3H]dihydroprogesterone (20alpha-hydroxypregn-4-en-3-one: a major product of the steroid pathway in Y-1 cells) produced by ACTH but does not inhibit conversion of cholesterol to pregnenolone by mitochondrial and purified enzyme preparations from Y-1 cells and bovine adrenal, respectively. Cytochalasin does not inhibit the conversion of pregnenolone to 20alpha-dihydroprogesterone but was shown to inhibit increased transport of [3H]cholesterol to mitochondria resulting from the action of ACTH. These findings indicate that cytochalasin acts after cholesterol has entered the cells and before it is subjected to side-chain cleavage in mitochondria. In view of the known action of cytochalasin on microfilaments, it is proposed that these organelles are necessary for the transport of cholesterol to the mitochondrial cleavage enzyme and that at least one effect of ACTH (and cyclic AMP) is exerted upon this transport process. The specificity of the effects of cytochalasin is considered in relation to this conclusion.     

Purification and characterization of the phosphate/hydroxyl ion antiport protein from rat liver mitochondria.

The phosphate transport protein was purified from rat liver mitochondria by extraction in an 8% (v/v) Triton X-100 buffer followed by adsorption chromatography on hydroxyapatite and Celite. SDS/polyacrylamide-gel electrophoresis (10%, w/v) demonstrated that the purified polypeptide was apparently homogeneous when stained with Coomassie Blue and had a subunit Mr of 34,000. However, lectin overlay analysis of this gel with 125I-labelled concanavalin A demonstrated the presence of several low- and high-Mr glycoprotein contaminants. To overcome this problem, mitochondria were pre-extracted with a 0.5% (v/v) Triton X-100 buffer as an additional step in the purification of phosphate transport protein. SDS/polyacrylamide gradient gel electrophoresis (14-20%, w/v) of the hydroxyapatite and Celite eluates revealed one major band of Mr 34,000 when stained with Coomassie Blue. The known thiol group sensitivity of the phosphate transporter was employed to characterize the isolated polypeptide further. Labelling studies with N-[2-3H]ethylmaleimide showed that only the 34,000-Mr band was labelled in both the hydroxyapatite and Celite fractions, when purified from rat liver mitochondria. Further confirmation of its identity has been provided with an antiserum directed against the 34,000-Mr protein. Specific partial inhibition of phosphate uptake, as measured by iso-osmotic swelling in the presence of (NH4)2HPO4, was achieved when mitoplasts (mitochondria minus outer membrane) were incubated with this antiserum. Finally, amino acid analysis of the rat liver mitochondrial phosphate/hydroxyl ion antiport protein indicates that it is similar in composition to the equivalent protein isolated from ox heart.     

Reversible and irreversible inhibition of phosphate transport in human erythrocytes by a membrane impermeant carbodiimide

Phosphate entry into chloride-loaded human erythrocytes is inhibited by treatment of cells with the water-soluble carbodiimide 1-ethyl-3-(4-azonia-4,4-dimethylpentyl)carbodiimide (EAC) in the absence of added nucleophile. EAC does not penetrate the erythrocyte membrane or lead to significant intermolecular cross-linking of membrane proteins. At neutral extracellular pH in chloride-free medium, only about 50% of transport is rapidly and irreversibly inhibited, but at alkaline pH, inhibition is more rapid and complete. Inhibition by EAC was reversible in the presence of extracellular NaCl. Modification of membrane sulfhydryl groups does not prevent inhibition of phosphate transport by EAC but almost complete protection is afforded by 4,4-dinitrostilbene-2,2-disulfonic acid, a reversible competitive inhibitor of anion transport. N-(4-Azido-2-nitrophenyl)-2-aminoethylsulfonate, a reversible noncompetitive inhibitor of anion transport did not protect against EAC inhibition of transport but prevented reversal of inhibition in saline medium. Transport inhibition by [3H]EAC did not lead to specific incorporation of radioactivity into Band 3, the anion transport protein. These results suggest that inhibition of anion transport by EAC is due to modification of a carboxylic acid residue in or near the transport site accessible from the external face of the membrane. The subsequent fate of the modified carboxyl residue appears to be sensitive to the orientation of the anion transport site.     

An intrinsic tonoplast protein of protein storage vacuoles in seeds is structurally related to a bacterial solute transporter (GIpF).

The tonoplast mediates the transport of various ions and metabolites between the vacuole and cytosol by mechanisms that remain to be elucidated at the molecular level. The primary structure of only one tonoplast protein, the H(+)-ATPase, has been reported to date. Here we report the primary structure of tonoplast intrinsic protein (TIP), a 27-kilodalton intrinsic membrane protein that occurs widely in the tonoplasts of the protein storage vacuoles (protein bodies) of seeds [Johnson, K.D., et al. (1989). Plant Physiol. 91, 1006-1013]. Hydropathy plots and secondary structure analysis of the polypeptide predict six membrane-spanning domains connected by short loops and hydrophilic, cytoplasmically oriented N- and C-terminal regions. TIP displays significant homology with several other membrane proteins from diverse sources: major intrinsic polypeptide from bovine lens fiber plasma membrane; NOD 26, a peribacteroid membrane protein in the nitrogen-fixing root nodules of soybean; and interestingly, GIpF, the glycerol facilitator transport protein in the cytoplasmic membrane of Escherichia coli. Based on the homology between TIP and GIpF and the knowledge that the protein storage vacuolar membrane and the peribacteroid membrane are active in solute transport, we propose that TIP transports small metabolites between the storage vacuoles and cytoplasm of seed storage tissues.     

Proton-translocating adenosine triphosphatase of chromaffin-granule membranes. The active site is in the largest (70 kDa) subunit.

The proton-translocating adenosine triphosphatase (ATPase) of bovine chromaffin granules contains up to five different polypeptides. Its activity is inhibited by N-ethylmaleimide, and ATP protects the enzyme from inhibition. After treatment of membranes with N-[2-3H]ethylmaleimide, only one polypeptide is strongly radiolabelled: this is the largest (70 kDa) subunit of the proton-translocating ATPase. This subunit therefore contains the ATP-hydrolysing site. Two-dimensional electrophoresis reveals heterogeneity in this polypeptide.     

Functional relevance of carnitine transporter OCTN2 to brain distribution of l-carnitine and acetyl-l-carnitine across the blood–brain barrier

Transport of L-[3H]carnitine and acetyl-L-[3H]carnitine at the blood-brain barrier (BBB) was examined by using in vivo and in vitro models. In vivo brain uptake of acetyl-L-[3H]carnitine, determined by a rat brain perfusion technique, was decreased in the presence of unlabeled acetyl-L-carnitine and in the absence of sodium ions. Similar transport properties for L-[3H]carnitine and/or acetyl-L-[3H]carnitine were observed in primary cultured brain capillary endothelial cells (BCECs) of rat, mouse, human, porcine and bovine, and immortalized rat BCECs, RBEC1. Uptakes of L-[3H]carnitine and acetyl-L-[3H]carnitine by RBEC1 were sodium ion-dependent, saturable with K(m) values of 33.1 +/- 11.4 microM and 31.3 +/- 11.6 microM, respectively, and inhibited by carnitine analogs. These transport properties are consistent with those of carnitine transport by OCTN2. OCTN2 was confirmed to be expressed in rat and human BCECs by an RT-PCR method. Furthermore, the uptake of acetyl-L-[3H]carnitine by the BCECs of juvenile visceral steatosis (jvs) mouse, in which OCTN2 is functionally defective owing to a genetical missense mutation of one amino acid residue, was reduced. The brain distributions of L-[3H]carnitine and acetyl-L-[3H]carnitine in jvs mice were slightly lower than those of wild-type mice at 4 h after intravenous administration. These results suggest that OCTN2 is involved in transport of L-carnitine and acetyl-L-carnitine from the circulating blood to the brain across the BBB.     

Structure and expression of the overlapping ND4L and ND5 genes of Neurospora crassa mitochondria

Summary Genes homologous to the mammalian mitochondrial NADH dehydrogenase subunit genes ND4L and ND5 were identified in the mitochondrial genome of the filamentous fungus Neurospora crassa, and the structure and expression of these genes was examined. The ND4L gene (interrupted by one intervening sequence) potentially encodes an 89 residue long hydrophobic protein that shares about 26% homology (or 41% homology if conservative amino acid substitutions are allowed) with the analogous human mitochondrial protein. The ND5 gene (which contains two introns) encodes a 715 residue polypeptide that shares 23% homology with the human analogue; a 300 amino acid long region is highly conserved (50% homology) in the two ND5 proteins. The stop codon of the ND4L gene overlaps the initiation codon of the downstream ND5 gene, and the two genes are contranscribed and probably cotranslated. A presumed mature dicistronic (ND4L plus ND5) RNA was detected. The postulated mRNA (about 3.2 kb) contains 5 and 3 non-coding regions of about 86 and 730 nucleotides, respectively; this species is generated from very large precursor RNAs by a complex processing pathway. The ND4L and ND5 introns are all stable after their excision from the precursor species.Abbreviations bp base pairs - rRNA ribosomal RNA - ND NADH dehydrogenase - URF unidentified reading frame - kDal kilodaltons; a.a., amino acid     

Homodimeric mitochondrial phosphate transport protein. Transient subunit/subunit contact site between the transport relevant transmembrane helices A

The three Cys of the yeast (Saccharomyces cerevisiae) mitochondrial phosphate transport protein (PTP) subunit were replaced with Ser. The seven mutants (single, double, and complete Cys replacements) were expressed in yeast, and the homodimeric mutant PTPs were purified from the mitochondria and reconstituted. The pH gradient-dependent net phosphate (Pi) transport uptake rates (initial conditions: 1 mM [Pi]e, pHe 6.80; 0 mM [Pi]i, pHi 8.07) catalyzed by these reconstituted mutants are similar to those of the wild-type protein and range from 15 to 80 micromol Pi/min mg PTP protein. Aerobic media inhibit only the Pi uptake rates catalyzed by PTPs with the conserved (yeast and bovine) Cys28. This inhibition in the proteoliposomes is 84-95% and can be completely reversed by dithiothreitol. Transport by the wild type as well as by all mutant proteins with Cys28 is more than 90% inhibited by mersalyl. Transport catalyzed by mutant proteins with only Cys300 or only Cys134 is less sensitive, and that catalyzed by the no Cys mutant shows 40% inhibition by mersalyl. When dithiothreitol is removed from purified single Cys mutant proteins, only the mutant protein with Cys28 appears as a homodimer in a nonreducing SDS polyacrylamide gel. Thus, the function relevant transmembrane helix A, with Cys 28 about equidistant from the two inner membrane surfaces, is in close contact with parts of transmembrane helix A of the other subunit in the functional homodimeric PTP. The results identify for the first time not only a transmembrane helix contact site between the two subunits of a homodimeric mitochondrial transport protein but also a contact site that if locked into position blocks transport. The results are related to two available secondary transporter structures (lactose permease, glycerol-3-phosphate transporter) as well as to a low resolution projection structure and a high resolution structure of monomers of inhibitor ADP/ATP carrier complexes.     

Interaction of mitochondrial phosphate carrier with fatty acids and hydrophobic phosphate analogs

Mitochondrial transporters, in particular uncoupling proteins and the ADP/ATP carrier, are known to mediate uniport of anionic fatty acids (FAs), allowing FA cycling which is completed by the passive movement of FAs across the membrane in their protonated form. This study investigated the ability of the mitochondrial phosphate carrier to catalyze such a mechanism and, furthermore, how this putative activity is related to the previously observed HgCl(2)-induced uniport mode. The yeast mitochondrial phosphate carrier was expressed in Escherichia coli and then reconstituted into lipid vesicles. The FA-induced H(+) uniport or Cl(-) uniport were monitored fluorometrically after HgCl(2) addition. These transport activities were further characterized by testing various inhibitors of the two different transport modes. The phosphate carrier was found to mediate FA cycling, which led to H(+) efflux in proteoliposomes. This activity was insensitive to ATP, mersalyl or N-ethylmaleimide and was inhibited by methylenediphosphonate and iminodi(methylenephosphonate), which are new inhibitors of mitochondrial phosphate transport. Also, the HgCl(2) induced Cl(-) uniport mediated by the reconstituted yeast PIC, was found to be inhibited by these reagents. Both methylenediphosphonate and iminodi(methylenephosphonate) blocked unidirectional Cl(-) uptake, whereas Cl(-) efflux was inhibited by iminodi(methylenephosphonate) and phosphonoformic acid only. These results suggest that a hydrophobic domain, interacting with FAs, exists in the mitochondrial phosphate carrier, which is distinct from the phosphate transport pathway. This domain allows for FA anion uniport via the phosphate carrier and consequently, FA cycling that should lead to uncoupling in mitochondria. This might be considered as a side function of this carrier.     

Identification of the gene encoding the mitochondrial elongation factor G in mammals.

Protein synthesis in cytosolic and rough endoplasmic reticulum associated ribosomes is directed by factors, many of which have been well characterized. Although these factors have been the subject of intense study, most of the corresponding factors regulating protein synthesis in the mitochondrial ribosomes remain unknown. In this report we present the cloning and initial characterization of the gene encoding the rat mitochondrial elongation factor-G (rEF-Gmt). The rat gene encoding EF-Gmt (rMef-g) maps to rat chromosome 2 and it is expressed in all tissues with highest levels in liver, thymus and brain. Its DNA sequence predicts a 752 amino acid protein exhibiting 72% homology to the yeast Saccharomyces cerevisiae mitochondrial elongation factor-G (YMEF-G), 62% and 61% homology to the Thermus thermophilus and E. coli elongation factor-G (EF-G) respectively and 52% homology to the rat elongation factor-2 (EF-2). The deduced amino acid sequence of EF-G contains characteristic motifs shared by all GTP binding proteins. Therefore, similarly to other elongation factors, the enzymatic function of EF-Gmt is predicted to depend on GTP binding and hydrolysis. EF-Gmt differs from its cytoplasmic homolog, EF-2, in that it contains an aspartic acid residue at amino acid position 621 which corresponds to the EF-2 histidine residue at position 715. Since this histidine residue, following posttranslational modification into diphthamide, appears to be the sole cellular target of diphtheria toxin and Pseudomonas aeruginosa endotoxin A, we conclude that EF-Gmt will not be inactivated by these toxins. The severe effects of these toxins on protein elongation in tissues expressing EF-Gmt suggest that EF-Gmt and EF-2 exhibit nonoverlapping functions. The cloning and characterization of the mammalian mitochondrial elongation factor G will permit us to address its role in the regulation of normal mitochondrial function and in disease states attributed to mitochondrial dysfunction.