Novel mitochondrial creatine transport activity. Implications for intracellular creatine compartments and bioenergetics

Immunoblotting of isolated mitochondria from rat heart, liver, kidney, and brain with antibodies made against N- and C-terminal peptide sequences of the creatine transporter, together with in situ immunofluorescence staining and immunogold electron microscopy of adult rat myocardium, revealed two highly related polypeptides with molecular masses of approximately 70 and approximately 55 kDa in mitochondria. These polypeptides were localized by immunoblotting of inner and outer mitochondrial membrane fractions, as well as by immunogold labeling in the mitochondrial inner membrane. In addition, a novel creatine uptake via a mitochondrial creatine transport activity was demonstrated by [(14)C]creatine uptake studies with isolated mitochondria from rat liver, heart, and kidney showing a saturable low affinity creatine transporter, which was largely inhibited in a concentration-dependent manner by the sulfhydryl-modifying reagent NEM, as well as by the addition of the above anti-creatine transporter antibodies to partially permeabilized mitochondria. Mitochondrial creatine transport was to a significant part dependent on the energetic state of mitochondria and was inhibited by arginine, and to some extent also by lysine, but not by other creatine analogues and related compounds. The existence of an active creatine uptake mechanism in mitochondria indicates that not only creatine kinase isoenzymes, but also creatine transporters and thus a certain proportion of the creatine kinase substrates, might be subcellularly compartmentalized. Our data suggest that mitochondria, shown here to possess creatine transport activity, may harbor such a creatine/phosphocreatine pool.     

Immunological analysis of plant mitochondrial NADH dehydrogenases.

Plant mitochondrial NADH dehydrogenases were analysed by two immunological strategies. The first exploited an antiserum raised to a preparation of SDS-solubilized mitochondrial-inner-membrane particles. By using a combination of activity-immunoprecipitation and crossed immunoelectrophoresis, it was shown that Triton X-100-solubilized membranes contain at least three immunologically distinct NADH dehydrogenases. Two of these were subsequently isolated by line immunoelectrophoresis and analysed for polypeptide composition: one contained three polypeptides with molecular masses of 75, 62 and 41 kDa; the other was a single polypeptide with a molecular mass of 53 kDa. The other approach was to probe plant mitochondrial membranes with antibodies raised to a purified preparation of ox heart rotenone-sensitive NADH dehydrogenase and subunits thereof. Cross-reactions were observed with the subunit-specific antisera against the 30 and 49 kDa ox heart proteins. However, the molecular masses of the equivalent polypeptides in plant mitochondria are slightly lower, at 27 and 46 kDa respectively.     

Creatine uptake and creatine transporter expression among rat skeletal muscle fiber types

Total creatine (Cr(total) = phosphocreatine + creatine) concentrations differ substantially among mammalian skeletal muscle. Because the primary means to add Cr(total) to muscle is uptake of creatine through the sodium-dependent creatine transporter (CrT), differences in creatine uptake and CrT expression could account for the variations in [Cr(total)] among muscle fiber types. To test this hypothesis, hindlimbs of adult rats were perfused with 0.05-1 mM [(14)C]creatine for up to 90 min. Creatine uptake rates at 1 mM creatine were greatest in the soleus (140 +/- 8.8 nmol x h(-1) x g(-1)), less in the red gastrocnemius (117 +/- 8.3), and least in the white gastrocnemius (97 +/- 10.7). These rates were unaltered by time, insulin concentration, or increased perfusate sodium concentration. Conversely, creatine uptake rates were correspondingly decreased among fiber types by lower creatine and sodium concentrations. The CrT protein content by Western blot analysis was similarly greatest in the soleus, less in the red gastrocnemius, and least in the white gastrocnemius, whereas CrT mRNA was not different. Creatine uptake rates differ among skeletal muscle fiber sections in a manner reasonably assigned to the 58-kDa band of the CrT. Furthermore, creatine uptake rates scale inversely with creatine content, with the lowest uptake rate in the fiber type with the highest Cr(total) and vice versa. This suggests that the creatine pool fractional turnover rate is not common across muscle phenotypes and, therefore, is differentially regulated.     

Rotenone-insensitive NAD(P)H dehydrogenases in plants: Immunodetection and distribution of native proteins in mitochondria

Antisera produced against peptides deduced from potato nda1 and ndb1, homologues of yeast genes for mitochondrial rotenone-insensitive NADH dehydrogenases, recognise respective proteins upon expression in Escherichia coli. In western blots of potato (Solanum tuberosum L.) mitochondrial proteins, the NDB and NDA antibodies specifically detect polypeptides of 61 and 48 kDa, respectively. The proteins are found in mitochondria of flowers, leaves and tubers. Different signal intensities are seen relative to other respiratory chain components when organs are compared, indicating variations in relative abundance of dehydrogenases within the plant. The antibodies detect single polypeptides, of similar size as in potato, in mitochondria from several plant species. No specific cross-reaction was found in chloroplasts, but a weak NDA signal of 50 kDa was found in microsomes, possibly associated with peroxisomes. Two-dimensional native/SDS-PAGE analyses indicate that both NDA and NDB proteins reside as higher molecular mass forms, possibly oligomeric. The NDB immunoreactive protein is released by sonication of mitochondria, but is resistant to extraction by digitonin and partially to Triton X-100. In comparison, the NDA protein remains bound to the inner membrane at sonication or digitonin treatment, but can be solubilised with Triton. Investigation of a beetroot (Beta vulgaris L.) induction system for external NADH dehydrogenase indicates that the NDB antibody does not recognise the induced external NADH dehydrogenase in this species, but possibly an external NADPH dehydrogenase.     

Muscle creatine uptake and creatine transporter expression in response to creatine supplementation and depletion.

The total creatine pool size [Cr(total); creatine (Cr) + phosphocreatine (PCr)] is crucial for optimal energy utilization in skeletal muscle, especially at the onset of exercise and during intense contractions. The Cr(total) likely is controlled by long-term modulation of Cr uptake via the sodium-dependent Cr transporter (CrT). To test this hypothesis, adult male Sprague-Dawley rats were fed 1% Cr, their muscle Cr(total) was reduced by approximately 85% [1% beta-guanidinoproprionic acid (beta-GPA)], or their muscle Cr(total) was repleted (1% Cr after beta-GPA depletion). Cr uptake was assessed by skeletal muscle (14)C-Cr accumulation to Cr and PCr by using hindlimb perfusion, and CrT protein content was assessed by Western blot. Cr uptake rate decreased with dietary Cr supplementation in the white gastrocnemius (WG; 45%) only. Depletion of muscle Cr(total) to approximately 15% of normal increased Cr uptake in the soleus (21%) and red gastrocnemius (22%), corresponding to 70-150% increases in muscle CrT content. In contrast, the inherently lower Cr uptake rate in the WG was unchanged with depletion of muscle Cr(total) even though CrT band density was increased by 230%. Thus there was no direct relationship between apparent muscle CrT abundance and Cr uptake rates. However, Cr uptake rates scaled inversely with decreases in muscle Cr(total) in the high-oxidative muscle types but not in the WG. This implies that factors controlling Cr uptake are different among fiber types. These observations may help explain the influence of initial muscle Cr(total), time dependency, and variations in muscle Cr(total) accumulation during Cr supplementation.     

Creatine transporter activity and content in the rat heart supplemented by and depleted of creatine

The intracellular creatine concentration is an important bioenergetic parameter in cardiac muscle. Although creatine uptake is known to be via a NaCl-dependent creatine transporter (CrT), its localization and regulation are poorly understood. We investigated CrT kinetics in isolated perfused hearts and, by using cardiomyocytes, measured CrT content at the plasma membrane or in total lysates. Rats were fed control diet or diet supplemented with creatine or the creatine analog beta-guanidinopropionic acid (beta-GPA). Creatine transport in control hearts followed saturation kinetics with a K(m) of 70 +/- 13 mM and a V(max) of 3.7 +/- 0.07 nmol x min(-1) x g wet wt(-1). Creatine supplementation significantly decreased the V(max) of the CrT (2.7 +/- 0.17 nmol x min(-1) x g wet wt(-1)). This was matched by an approximately 35% decrease in the plasma membrane CrT; the total CrT pool was unchanged. Rats fed beta-GPA exhibited a >80% decrease in tissue creatine and increase in beta-GPA(total). The V(max) of the CrT was increased (6.0 +/- 0.25 nmol x min(-1) x g wet wt(-1)) and the K(m) decreased (39.8 +/- 3.0 mM). The plasma membrane CrT increased about fivefold, whereas the total CrT pool remained unchanged. We conclude that, in heart, creatine transport is determined by the content of a plasma membrane isoform of the CrT but not by the total cellular CrT pool.     

Purification,Characterization, and Submitochondrial Localization of a 58-Kilodalton NAD(P)H Dehydrogenase

An NADH dehydrogenase activity from red beet (Beta vulgaris L.) root mitochondria was purified to a 58-kD protein doublet. An immunologically related dehydrogenase was partially purified from maize (Zea mays L. B73) mitochondria to a 58-kD protein doublet, a 45-kD protein, and a few other less prevalent proteins. Polyclonal antibodies prepared against the 58-kD protein of red beet roots were found to immunoprecipitate the NAD(P)H dehydrogenase activity. The antibodies cross-reacted to similar proteins in mitochondria from a number of plant species but not to rat liver mitochondrial proteins. The polyclonal antibodies were used in conjunction with maize mitochondrial fractionation to show that the 58-kD protein was likely part of a protein complex loosely associated with the membrane fraction. A membrane-impermeable protein cross-linking agent was used to further show that the majority of the 58-kD protein was located on the outer surface of the inner mitochondrial membrane or in the intermembrane space. Analysis of the cross-linked 58-kD NAD(P)H dehydrogenase indicated that specific proteins of 64, 48, and 45 kD were cross-linked to the 58-kD protein doublet. The NAD(P)H dehydrogenase activity was not affected by ethyleneglycol-bis([beta]-aminoethyl ether)-N,N[prime] -tetraacetic acid or CaCl2, was stimulated somewhat (21%) by flavin mononucleotide, was inhibited by p-chloromercuribenzoic acid (49%) and mersalyl (40%), and was inhibited by a bud scale extract of Platanus occidentalis L. containing platanetin (61%).     

Is E37, a major polypeptide of the inner membrane from plastid envelope,an S-adenosyl methionine-dependent methyltransferase?

Using antibodies raised against E37, one of the major polypeptides of the inner membrane from the chloroplast envelope, it has been demonstrated that a single immunologically related polypeptide was present in total protein extracts from various higher plants (monocots and dicots), in photosynthetic and non-photosynthetic tissues from young spinach plantlets, as well as in the cytoplasmic membrane from the cyanobacteria Synechococcus . This ubiquitous distribution of E37 strongly suggests that this protein plays an envelope-specific function common to all types of plastids. Comparison of tobacco and spinach E37 amino acid sequences deduced from the corresponding cDNA demonstrates that consensus motifs for S-adenosyl methionine-dependent methyltransferases are located in both sequences. This hypothesis was confirmed using a biochemical approach. It was demonstrated that E37, together with two minor spinach chloroplast envelope polypeptides of 32 and 39 kDa, can be specifically photolabeled with [³H]-S-adenosyl methionine upon UV-irradiation. Identification of E37 as a photolabeled polypeptide was established by immunoprecipitation. Furthermore, photolabeling of the three envelope polypeptides was specifically inhibited by very low concentration of S-adenosyl homocysteine, thus providing evidence for the presence within these proteins of S-adenosyl methionine- and S-adenosyl homocysteine-binding sites that were closely associated. Taken as a whole these results strongly suggest that E37 is an ubiquitous plastid envelope protein that probably has an S-adenosyl methionine-dependent methyltransferase activity. The 32 and 39 kDa envelope polypeptides probably have a similar methyltransferase activity.     

Specific inhibition of ATP-ADP translocase in cardiac mitoplasts by antibodies against mitochondrial creatine kinase

Mitochondrial creatine kinase was purified from rat hearts and used to produce antibodies in chicken and rabbits. Antibodies were purified to a high degree of homogeneity by an affinity chromatography method. Chicken antibodies against mitochondrial creatine kinase inhibited this enzyme in rat-heart mitochondrial inner membrane and matrix preparation, and simultaneously blocked oxidative phosphorylation. Under these conditions respiratory chain activities remained unchanged, but adenine nucleotide translocase was inhibited. Removal of mitochondrial creatine kinase from the membrane by pretreatment with 0.15 M KCl and 20 mM ADP completely abolished the effect of antibodies against mitochondrial creatine kinase on oxidative phosphorylation. Noninhibitory antibodies from rabbit with high affinity to rat mitochondrial creatine kinase inhibited neither creatine kinase activity nor oxidative phosphorylation. These data show close and specific spatial arrangement of mitochondrial creatine kinase and adenine nucleotide translocase in mitochondria. It is supposed that there is a fixed orientation of these proteins in the cardiolipin domain in the membrane and that their interaction may occur by a frequent collision due to their lateral movement.     

Creatine transporter (CrT; Slc6a8) knockout mice as a model of human CrT deficiency

Mutations in the creatine (Cr) transporter (CrT; Slc6a8) gene lead to absence of brain Cr and intellectual disabilities, loss of speech, and behavioral abnormalities. To date, no mouse model of CrT deficiency exists in which to understand and develop treatments for this condition. The purpose of this study was to generate a mouse model of human CrT deficiency. We created mice with exons 2–4 of Slc6a8 flanked by loxP sites and crossed these to Cre:CMV mice to create a line of ubiquitous CrT knockout expressing mice. Mice were tested for learning and memory deficits and assayed for Cr and neurotransmitter levels. Male CrT^−/y (affected) mice lack Cr in the brain and muscle with significant reductions of Cr in other tissues including heart and testes. CrT^−/y mice showed increased path length during acquisition and reversal learning in the Morris water maze. During probe trials, CrT^−/y mice showed increased average distance from the platform site. CrT^−/y mice showed reduced novel object recognition and conditioned fear memory compared to CrT^+/y. CrT^−/y mice had increased serotonin and 5-hydroxyindole acetic acid in the hippocampus and prefrontal cortex. Ubiquitous CrT knockout mice have learning and memory deficits resembling human CrT deficiency and this model should be useful in understanding this disorder.     

Chromium(VI) interaction with plant and animal mitochondrial bioenergetics: a comparative study

The mechanism of Cr(VI)-induced toxicity in plants and animals has been assessed for mitochondrial bioenergetics and membrane damage in turnip root and rat liver mitochondria. By using succinate as the respiratory substrate, ADP/O and respiratory control ratio (RCR) were depressed as a function of Cr(VI) concentration. State 3 and uncoupled respiration were also depressed by Cr(VI). Rat mitochondria revealed a higher sensitivity to Cr(VI), as compared to turnip mitochondria. Rat mitochondrial state 4 respiration rate triplicated in contrast to negligible stimulation of turnip state 4 respiration. Chromium(VI) inhibited the activity of the NADH-ubiquinone oxidoreductase (complex I) from rat liver mitochondria and succinate-dehydrogenases (complex II) from plant and animal mitochondria. In rat liver mitochondria, complex I was more sensitive to Cr(VI) than complex II. The activity of cytochrome c oxidase (complex IV) was not sensitive to Cr(VI). Unique for plant mitochondria, exogenous NADH uncoupled respiration was unaffected by Cr(VI), indicating that the NADH dehydrogenase of the outer leaflet of the plant inner membrane, in addition to complexes III and IV, were insensitive to Cr(VI). The ATPase activity (complex V) was stimulated in rat liver mitochondria, but inhibited in turnip root mitochondria. In both, turnip and rat mitochondria, Cr(VI) depressed mitochondrial succinate-dependent transmembrane potential (Deltapsi) and phosphorylation efficiency, but it neither affected mitochondrial membrane permeabilization to protons (H+) nor induced membrane lipid peroxidation. However, Cr(VI) induced mitochondrial membrane permeabilization to K+, an effect that was more pronounced in turnip root than in rat liver mitochondria. In conclusion, Cr(VI)-induced perturbations of mitochondrial bioenergetics compromises energy-dependent biochemical processes and, therefore, may contribute to the basal mechanism underlying its toxic effects in plant and animal cells.     

New creatine transporter assay and identification of distinct creatine transporter isoforms in muscle

Despite the pivotal role of creatine (Cr) and phosphocreatine (PCr) in muscle metabolism, relatively little is known about sarcolemmal creatine transport, creatine transporter (CRT) isoforms, and subcellular localization of the CRT proteins. To be able to quantify creatine transport across the sarcolemma, we have developed a new in vitro assay using rat sarcolemmal giant vesicles. The rat giant sarcolemmal vesicle assay reveals the presence of a specific high-affinity and saturable transport system for Cr in the sarcolemma (Michaelis-Menten constant 52.4 +/- 9.4 microM and maximal velocity value 17.3 +/- 3.1 pmol x min(-1) x mg vesicle protein(-1)), which cotransports Cr into skeletal muscle together with Na(+) and Cl(-) ions. The regulation of Cr transport in giant vesicles by substrates, analogs, and inhibitors, as well as by phorbol 12-myristate 13-acetate and insulin, was studied. Two antibodies raised against COOH- and NH(2)-terminal synthetic peptides of CRT sequences both recognize two major polypeptides on Western blots with apparent molecular masses of 70 and 55 kDa, respectively. The highest CRT expression occurs in heart, brain, and kidney, and although creatine kinase is absent in liver cells, CRT is also found in this tissue. Surprisingly, immunofluorescence staining of cultured adult rat heart cardiomyocytes with specific anti-CRT antibodies, as well as cell fractionation and cell surface biotinylation studies, revealed that only a minor CRT species with an intermediate molecular mass of approximately 58 kDa is present in the sarcolemma, whereas the previously identified major CRT-related protein species of 70 and 55 kDa are specifically located in mitochondria. Our studies indicate that mitochondria may represent a major compartment of CRT localization, thus providing a new aspect to the current debate about the existence and whereabouts of intracellular Cr and PCr compartments that have been inferred from [(14)C]PCr/Cr measurements in vivo as well as from recent in vivo NMR studies.     

Changes in creatine transporter function during cardiac maturation in the rat

Background  

It is well established that the immature myocardium preferentially utilises non-oxidative energy-generating pathways. It exhibits low energy-transfer capacity via the creatine kinase (CK) shuttle, reflected in phosphocreatine (PCr), total creatine and CK levels that are much lower than those of adult myocardium. The mechanisms leading to gradually increasing energy transfer capacity during maturation are poorly understood. Creatine is not synthesised in the heart, but taken up exclusively by the action of the creatine transporter protein (CrT). To determine whether this transporter is ontogenically regulated, the present study serially examined CrT gene expression pattern, together with creatine uptake kinetics and resulting myocardial creatine levels, in rats over the first 80 days of age.     

Translocation of proteins into rat liver mitochondria. The precursor polypeptides of a large subunit of succinate dehydrogenase and ornithine aminotransferase and their imports into their own locations of mitochondria

The precursor polypeptides of a large subunit of succinate dehydrogenase and ornithine aminotransferase (the enzymes which are located in the mitochondrial inner membrane and matrix respectively) were synthesized as a larger molecular mass than their mature subunits, when rat liver RNA was translated in vitro. These precursor polypeptides were also detected in vivo in ascites hepatoma cells (AH-130 cells). When the 35S-labeled precursor polypeptides were incubated with isolated rat liver mitochondria at 30 degrees C in the presence of an energy-generating system, these two precursors were converted to their mature size and the 35S-labeled mature-size polypeptides associated with mitochondria. Furthermore, these mature-size polypeptides were recovered from their own locations, the inner mitochondrial membrane and the matrix. The precursor of ornithine aminotransferase incubated with rat liver mitochondria at 0 degree C was specifically and tightly bound to the surface of the mitochondria even in the presence of an uncoupler of oxidative phosphorylation. This precursor, bound to the mitochondria, was imported into the matrix when the mitochondria were reisolated and incubated at 30 degrees C in the presence of an energy-generating system, suggesting that a specific receptor may be involved in the binding of the precursor. The processing enzyme for both precursor polypeptides seemed to be located in the mitochondrial matrix and was partially purified from the mitochondria. A metal-chelating agent strongly inhibited the processing enzyme and the inhibition was recovered by the addition of Mn2+ or Co2+.     

Microtubule-associated proteins bind to 30 kDa and 60 kDa proteins of rat brain mitochondria: visualization by ligand blotting

Axonal transport of mitochondria is a microtubule-associated movement. Microtubule-mitochondria interactions were studied in vitro using organelles isolated from rat brain. Thanks to the ligand blotting method we were able to show two mitochondrial membrane proteins with apparent molecular masses of 30 kDa and 60 kDa that bind microtubule-associated proteins. The binding of the 30 kDa protein has an apparent Kd of 8 x 10(-8) M. Digitonin fractionation of mitochondria reveals a bimodal localization of the 30 kDa and the 60 kDa proteins within the outer membrane. The data suggest that these polypeptides could participate to the interactions observed in situ between microtubules and mitochondria.     

Isolation and characterization of complex I, rotenone-sensitive NADH: ubiquinone oxidoreductase, from the procyclic forms of Trypanosoma brucei.

Additional characterization of complex I, rotenone-sensitive NADH:ubiquinone oxidoreductase, in the mitochondria of Trypanosoma brucei brucei has been obtained. Both proline:cytochrome c reductase and NADH:ubiquinone oxidoreductase of procyclic T. brucei were inhibited by the specific inhibitors of complex I rotenone, piericidin A, and capsaicin. These inhibitors had no effect on succinate: cytochrome c reductase activity. Antimycin A, a specific inhibitor of the cytochrome bc1 complex (ubiquinol:cytochrome c oxidoreductase), blocked almost completely cytochrome c reductase activity with either proline or succinate as electron donor, but had no inhibitory effect on NADH:ubiquinone oxidoreductase activity. The rotenone-sensitive NADH:ubiquinone oxidoreductase of procyclic T. brucei was partially purified by sucrose density centrifugation of mitochondria solubilized with dodecyl-beta-D-maltoside, with an approximately eightfold increase in specific activity compared to that of the mitochondrial membranes. Four polypeptides of the partially purified enzyme were identified as the homologous subunits of complex I (51 kDa, PSST, TYKY, and ND4) by immunoblotting with antibodies raised against subunits of Paracoccus denitrificans and against synthetic peptides predicted from putative complex I subunit genes encoded by mitochondrial and nuclear T. brucei DNA. Blue Native polyacrylamide gel electrophoresis of T. brucei mitochondrial membrane proteins followed by immunoblotting revealed the presence of a putative complex I with a molecular mass of 600 kDa, which contains a minimum of 11 polypeptides determined by second-dimensional Tricine-SDS/PAGE including the 51 kDa, PSST and TYKY subunits.     

Functional molecular aspects of the NADH dehydrogenases of plant mitochondria

There are multiple routes of NAD(P)H oxidation associated with the inner membrane of plant mitochondria. These are the phosphorylating NADH dehydrogenase, otherwise known as Complex I, and at least four other nonphosphorylating NAD(P)H dehydrogenases. Complex I has been isolated from beetroot, broad bean, and potato mitochondria. It has at least 32 polypeptides associated with it, contains FMN as its prosthetic group, and the purified enzyme is sensitive to inhibition by rotenone. In terms of subunit complexity it appears similar to the mammalian and fungal enzymes. Some polypeptides display antigenic similarity to subunits fromNeurospora crassa but little cross-reactivity to antisera raised against some beef heart complex I subunits. Plant complex I contains eight mitochondrial encoded subunits with the remainder being nuclear-encoded. Two of these mitochondrial-encoded subunits, nad7 and nad9, show homology to corresponding nuclear-encoded subunits inNeurospora crassa (49 and 30 kDa, respectively) and beef heart CI (49 and 31 kDa, respectively), suggesting a marked difference between the assembly of CI from plants and the fungal and mammalian enzymes. As well as complex I, plant mitochondria contain several type-II NAD(P)H dehydrogenases which mediate rotenone-insensitive oxidation of cytosolic and matrix NADH. We have isolated three of these dehydrogenases from beetroot mitochondria which are similar to enzymes isolated from potato mitochondria. Two of these enzymes are single polypeptides (32 and 55 kDa) and appear similar to those found in maize mitochondria, which have been localized to the outside of the inner membrane. The third enzyme appears to be a dimer comprised of two identical 43-kDa subunits. It is this enzyme that we believe contributes to rotenone-insensitive oxidation of matrix NADH. In addition to this type-II dehydrogenases, several observations suggest the presence of a smaller form of CI present in plant mitochondria which is insensitive to rotenone inhibition. We propose that this represents the peripheral arm of CI in plant mitochondria and may participate in nonphosphorylating matrix NADH oxidation.     

No unique mitochondrial translation products in respiratory chain-linked NADH dehydrogenase

Antibody prepared against beef heart mitochondrial NADH dehydrogenase immunoprecipitated 26 polypeptides from detergent solubilized beef heart mitochondria. All 26 polypeptides co-migrated with those present in the dehydrogenase antigen when resolved side by side on sodium dodecyl sulfateurea polyacrylamide gels. From mixed rat liver-[³⁵S]methionine pulsed hepatoma mitochondria the antibody immunoprecipitated 24 stained liver polypeptides and 19 radio-labelled hepatoma polypeptides. The translation of three of the labelled polypeptides was resistent to inhibition by cycloheximide, indicating these are translated on mitochondrial ribosomes. These same polypeptides, however, wre previously identified as cytochrome c oxidase subunits; and, apparently, non-specifically co-precipitate with dehydrogenase associated polypeptides. We conclude that there are no mitochondrially translated polypeptides specifically associated with NADH dehydrogenase.     

Dual Electron Microscopic Localization of Mitochondrial Creatine Kinase in Brain Mitochondria

Mitochondrial creatine kinase in brain mitochondria appears to be located at two different intramitochondrial sites. By using immunogold-labeling techniques, a peripheral immunoreactivity was localized between the two boundary membranes, while an additional, central immunoreactivity was found at the crista surface. The peripheral enzyme was accessible to the antibodies after treatment of the brain mitochondria with 100-300 μg digitonin/mg mitochondrial protein, which left 75% of the activity bound to the membranes. Electron microscopic analyses revealed that 43% of the labeled, peripheral creatine kinase was bound at those places where outer membrane vesicles remained attached to the inner envelope membrane, suggesting that the enzyme is in involved in contact formation between outer and inner mitochondrial membranes. Postembedding staining of mitochondria on thin sections of brain tissue or in the isolated state led to the observation of a second location of creatine kinase inside the mitochondria, along the cristae, which was not accessible to the antibodies in isolated, digitonin-treated mitochondria.     

MCT1 confirmed in rat striated muscle mitochondria.

We sought to test the hypothesis that monocarboxylate transporter isoform 1 (MCT1) is the inner mitochondrial membrane lactate/pyruvate transporter, and, as such, contributes to functioning of the intracellular lactate shuttle. However, presence of a mammalian mitochondrially localized MCT1 (mMCT1) has been contested. We sought to confirm by Western blotting the mitochondrial localization of MCT1 in rat cardiac, soleus, and extensor digitorum longus muscles utilizing three different cell fractionation methods and three different antibodies. We performed Western blotting using antibodies to cell membrane glucose transporter isoform GLUT1, inner mitochondrial constituent cytochrome oxidase, the monocarboxylate transporter protein chaperone CD147, as well as custom and commercially available MCT1 antibodies. Western blots demonstrated similar results with each MCT1 antibody and two of three methods of fractionation. MCT1 was found in the mitochondria, as well as in the sarcolemmal membrane and whole muscle homogenates. Probing with GLUT1 and CD147 demonstrated that mitochondrial fractions were not contaminated with sarcolemmal remnants. Probing with cytochrome oxidase showed mitochondrial localization of MCT1. Comparison of these results to the findings of others indicates that the most likely source of discrepancy is the cell fractionation procedure utilized.