Uptake of aspartate aminotransferase into mitochondria in vitro causes efflux of malate dehydrogenase and vice versa

Proteomic analysis reveals that iron availability alters the metabolic status of the pathogenic fungus Paracoccidioides brasiliensis

Paracoccidioides brasiliensis is a thermodimorphic fungus and the causative agent of paracoccidioidomycosis (PCM). The ability of P. brasiliensis to uptake nutrients is fundamental for growth, but a reduction in the availability of iron and other nutrients is a host defense mechanism many pathogenic fungi must overcome. Thus, fungal mechanisms that scavenge iron from host may contribute to P. brasiliensis virulence. In order to better understand how P. brasiliensis adapts to iron starvation in the host we compared the two-dimensional (2D) gel protein profile of yeast cells during iron starvation to that of iron rich condition. Protein spots were selected for comparative analysis based on the protein staining intensity as determined by image analysis. A total of 1752 protein spots were selected for comparison, and a total of 274 out of the 1752 protein spots were determined to have changed significantly in abundance due to iron depletion. Ninety six of the 274 proteins were grouped into the following functional categories; energy, metabolism, cell rescue, virulence, cell cycle, protein synthesis, protein fate, transcription, cellular communication, and cell fate. A correlation between protein and transcript levels was also discovered using quantitative RT-PCR analysis from RNA obtained from P. brasiliensis under iron restricting conditions and from yeast cells isolated from infected mouse spleens. In addition, western blot analysis and enzyme activity assays validated the differential regulation of proteins identified by 2-D gel analysis. We observed an increase in glycolytic pathway protein regulation while tricarboxylic acid cycle, glyoxylate and methylcitrate cycles, and electron transport chain proteins decreased in abundance under iron limiting conditions. These data suggest a remodeling of P. brasiliensis metabolism by prioritizing iron independent pathways.

Localization of mitochondrial DNA encoded cytochrome c oxidase subunits I and II in rat pancreatic zymogen granules and pituitary growth hormone granules

Cytochrome c oxidase (COX) complex is an integral part of the electron transport chain. Three subunits of this complex (COX I, COX II and COX III) are encoded by mitochondrial (mit-) DNA. High-resolution immunogold electron microscopy has been used to study the subcellular localization of COX I and COX II in rat tissue sections, embedded in LR Gold resin, using monoclonal antibodies for these proteins. Immunofluorescence labeling of BS-C-1 monkey kidney cells with these antibodies showed characteristic mitochondrial labeling. In immunogold labeling studies, the COX I and COX II antibodies showed strong and specific mitochondrial labeling in the liver, kidney, heart and pancreas. However, in rat pancreatic acinar tissue, in addition to mitochondrial labeling, strong and specific labeling was also observed in the zymogen granules (ZGs). In the anterior pituitary, strong labeling with these antibodies was seen in the growth hormone secretory granules. In contrast to these compartments, the COX I or COX II antibodies showed only minimal labeling (five- to tenfold lower) of the cytoplasm, endoplasmic reticulum and the nucleus. Strong labeling with the COX I or COX II antibodies was also observed in highly purified ZGs from bovine pancreas. The observed labeling, in all cases, was completely abolished upon omission of the primary antibodies. These results provide evidence that, similar to a number of other recently studied mit-proteins, COX I and COX II are also present outside the mitochondria. The presence of mit-DNA encoded COX I and COX II in extramitochondrial compartments, provides strong evidence that proteins can exit, or are exported, from the mitochondria. Although the mechanisms responsible for protein exit/export remain to be elucidated, these results raise fundamental questions concerning the roles of mitochondria and mitochondrial proteins in diverse cellular processes in different compartments.

Mitochondrial proteins at unexpected cellular locations: export of proteins from mitochondria from an evolutionary perspective

Researchers in a wide variety of unrelated areas studying functions of different proteins are unexpectedly finding that their proteins of interest are actually mitochondrial proteins, although functions would appear to be extramitochondrial. We review the leading current examples of mitochondrial macromolecules indicated to be also present outside of mitochondria that apparently exit from mitochondria to arrive at their destinations. Mitochondrial chaperones, which have been implicated in growth and development, autoimmune diseases, cell mortality, antigen presentation, apoptosis, and resistance to antimitotic drugs, provide some of the best studied examples pointing to roles for mitochondria and mitochondrial proteins in diverse cellular phenomena. To explain the observations, we propose that specific export mechanisms exist by which certain proteins exit mitochondria, allowing these proteins to have additional functions at specific extramitochondrial sites. Several possible mechanisms by which mitochondrial proteins could be exported are discussed. Gram-negative proteobacteria, from which mitochondria evolved, contain a number of different mechanisms for protein export. It is likely that mitochondria either retained or evolved export mechanisms for certain specific proteins.

Changes in liver enzyme activity in the teleost Sparus aurata in response to cadmium intoxication

Enzyme activity modulation by cadmium in the liver of the teleost fish Sparus aurata was investigated in vivo following 3 and 6 days of CdCl2 administration (2.5 mg/kg body wt). The specific activities of the mitochondrial enzymes NAD-isocitrate dehydrogenase, succinate dehydrogenase, and malate dehydrogenase were stimulated by approximately 20% after 3 days administration and were further increased (by about 40%) after 6 days treatment. In comparison with these enzymes, the activities of glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) in mitochondria were less stimulated after the two indicated intervals of treatment. Cadmium significantly reduced the activities of liver cytoplasmic GOT and GPT while a simultaneous increase occurred in the serum activities of these same enzymes. The activity of liver NADPH-cytochrome P450 reductase was stimulated by 25 and 40% after 3 and 6 days cadmium intoxication, respectively. Lastly, the antioxidant enzymes glutathione peroxidase and glutathione reductase in liver and catalase in both liver and blood were strongly reduced after 3 and 6 days cadmium administration. These data suggest that cadmium in fish hepatocytes alters cell membrane structure and concomitantly induces some perturbation in the integrity of the mitochondrial membrane.

Ethanol up-regulates fatty acid uptake and plasma membrane expression and export of mitochondrial aspartate aminotransferase in HepG2 cells

To explain the increased plasma mitochondrial aspartate aminotransferase (mAspAT) observed in alcoholics, we cultured HepG2 hepatoma cells in ethanol. Acute (24 hour) exposure to 0, 20, 40, or 80 mmol/L ethanol produced a dose-dependent (r = .98) increase in mAspAT messenger RNA (mRNA) of < or = thirteen-fold, with no significant change in the cellular content of mAspAT or of several other enzymes. The recovery of mAspAT in the medium over 24 hours of ethanol exposure correlated with both ethanol concentration and with mAspAT mRNA (r = .90), reaching 808% of cellular enzyme content/24 hours at 80 mmol/L. Recovery of all other enzymes studied was < or = 20% of cellular content and unaffected by ethanol. Plasma membrane mAspAT content also correlated with mAspAT mRNA (r = .96) and mitochondrial levels were unchanged. No mitochondrial morphologic abnormalities were observed at any ethanol concentration studied. In cells cultured chronically at 0 to 80 mmol/L ethanol, fatty acid uptake Vmax increased in parallel with plasma membrane expression of mAspAT (r = .98). Cellular triglyceride content was highly correlated with Vmax. Thus, the data suggest that: 1) the increased plasma mAspAT observed in alcoholics may reflect pharmacologic upregulation of mAspAT mRNA and of mAspAT synthesis by ethanol; and 2) increased mAspAT-mediated fatty acid uptake may contribute to alcoholic fatty liver.

Flavin adenine dinucleotide and flavin mononucleotide metabolism in rat liver--the occurrence of FAD pyrophosphatase and FMN phosphohydrolase in isolated mitochondria

In order to gain some insight into mitochondrial flavin biochemistry, rat liver mitochondria essentially free of lysosomal and microsomal contamination were prepared and their capability to metabolise externally added and endogenous FAD and FMN tested both spectroscopically and via HPLC. The existence of two novel mitochondrial enzymes, namely FAD pyrophosphatase (EC 3.6.1.18) and FMN phosphohydrolase (EC 3.1.3.2), which catalyse FAD-->FMN and FMN-->riboflavin conversion, respectively, is shown. They differ from each other and from extramitochondrial enzymes, as judged by their pH profile and inhibitor sensitivity, and can be separated in a partial FAD pyrophosphatase purification. Digitonin titration and subfractionation experiments show that FAD pyrophosphatase is located in the outer mitochondrial membrane and FMN phosphohydrolase in the intermembrane space. Since these enzymes can metabolise endogenous FAD and FMN, which are made available by using both Triton X-100 and the effector oxaloacetate, a proposal is made that FAD pyrophosphatase and FMN phosphohydrolase play a major role in mitochondrial flavoprotein turnover.

The in vitro-synthesized precursor and mature mitochondrial aspartate aminotransferase share the same import pathway in isolated mitochondria

Both the precursor and the mature form of mitochondrial aspartate aminotransferase were synthesized in a cell-free coupled transcription/translation system directed by the recombinant expression plasmid pOTS-pmAspAT and pOTS-mAspAT, respectively. Both newly synthesized forms of the protein were imported into isolated mitochondria, with the precursor correctly processed to the mature form. In both cases the import process showed resistance to externally added pronase and was abolished in mitochondria treated with the uncoupler carbonyl cyanide m-chlorophenylhydrazone. Moreover the imported products showed the same intramitochondrial localization as judged by a subfractionation procedure. In both cases import was time dependent and was completed in about 15 min. Finally a competitive inhibition of the import of the precursor of aspartate aminotransferase was found due to externally added purified aspartate aminotransferase.

Certain N-terminal peptides inhibit uptake of mature aspartate aminotransferase by isolated mitochondria

To gain insight into the uptake of mature aspartate aminotransferase by isolated mitochondria, the capability of certain cyanogen bromide peptides from mature beef heart mitochondrial aspartate aminotransferase to inhibit enzyme uptake was kinetically tested. N-terminal peptides (1-9 and 10-31) proved to inhibit the rate of aspartate aminotransferase uptake respectively in purely competitive and non-competitive ways, whereas other peptides distal from the N-terminus (203-217, 321-327 and 328-353) were found to be completely ineffective.