Muskelmitochondrien

Wanderings in bioenergetics and biomembranes

Having worked for 55 years in the center and at the fringe of bioenergetics, my major research stations are reviewed in the following wanderings: from microsomes to mitochondria, from NAD to CoQ, from reversed electron transport to reversed oxidative phosphorylation, from mitochondrial hydrogen transfer to phosphate transfer pathways, from endogenous nucleotides to mitochondrial compartmentation, from transport to mechanism, from carrier to structure, from coupling by AAC to uncoupling by UCP, and from specific to general transport laws. These wanderings are recalled with varying emphasis paid to the covered science stations.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Mitochondrial oxidation of p-phenylenediamine derivatives in vitro: structure-activity relationships and correlation with myotoxic activity in vivo

A number of p-phenylenediamine derivatives are known to cause necrosis of skeletal and/or cardiac muscle when administered to experimental animals. Compounds of this type are oxidized to semiquinonedi-imines or quinonedi-imines by mitochondria in vitro, establishing alternative pathways for electron transport in the respiratory chain with concomitant decreases in respiratory control and ADP:O ratios. Muscle mitochondria were found to be particularly effective in promoting p-phenylenediamine oxidation in vitro and the magnitude of the mitochondrial effects of the various compounds tested correlated well with their ability to cause muscle necrosis in vivo. It is suggested that mitochondrial oxidation may be involved in the initiation of the myotoxic effects of these compounds and account for their target-site specificity.

Structure-activity relationships in the myotoxicity of ring-methylated p-phenylenediamines in rats and correlation with autoxidation rates in vitro

A number of N-methylated p-phenylenediamines are known to cause necrosis of skeletal and cardiac muscle in rats. The severity of the muscle damage induced by these compounds in vivo was found to be directly proportional to their autoxidation rates in vitro, suggesting that reactive species formed during oxidation may be involved in the initiation of this toxic effect. In the present study, the in vitro oxidation rates and in vivo toxicities of a number of ring-methylated p-phenylenediamines have been evaluated. 2,3,5,6-Tetramethyl p-phenylenediamine readily autoxidized at neutral pH. Hydrogen peroxide was formed in this reaction, while oxidation in the presence of glutathione or reduced pyridine nucleotides led to the production of both hydrogen peroxide and superoxide radical. Less highly methylated derivatives oxidized more slowly, with rates decreasing in the order 2,3,5,6-tetramethyl greater than 2,5-dimethyl greater than 2,6-dimethyl greater than 2-methyl. All these compounds were myotoxic in rats, with damage being largely confined to skeletal muscle. Toxicity was again proportional to oxidation rate. Myotoxicity appears to be a general property of certain substituted p-phenylenediamines and the structure-activity relationships identified may permit an estimate to be made of the potential toxicity of other compounds of this type.

Transaminative pathway of glutamate oxidation in adrenal-cortex mitochondria

Mitochondria isolated from adrenal cortex of beef do oxidize glutamate if the amino group acceptor-oxaloacetate (or its precursor-malate) is present in the incubation medium. The glutamate (plus oxaloacetate) oxidation was enhanced by ADP or deoxycorticosterone, indicating that this respiration can support both oxidative phosphorylation and 11 beta-hydroxylation of deoxycorticosterone to corticosterone. Avenaciolide (inhibitor of glutamate entry into the mitochondria), aminooxyacetate (inhibitor of aspartate aminotransferase activity) and arsenite (inhibitor of 2-oxoglutarate dehydrogenase) when introduced into the incubation media before respirating substrates, inhibited the ability of ADP or deoxycorticosterone to stimulate the rate of glutamate (plus oxaloacetate) oxidation.

Different yield and properties of mitochondria from skeletal muscle of normal hamsters

Yield and properties of mitochondria, isolated from hind leg muscles of normal hamsters, are different, when 4 preparation procedures are applied simultaneously: 1. mechanical homogenization of muscle with a glass-Teflon homogenizer followed by rehomogenization of the isolated nuclear fraction for 1--6 times. The procedure yields impure mitochondria showing the highest amounts of mitochondrial enzymes (50--65% of total), protein and DNA; the isolated mitochondria exhibit disrupted outer membranes and loosened inner membranes causing a better supply with substrates of enzymes located at the inner membrane and in the matrix. 2. Single mechanical homogenization of muscle for increasing periods (2--40 min) yields lower amounts of all constituents (25--40% of total). 3. The yields of Nagarse treatment with one step of 2 min mechanical homogenization range between 20--35% of total. Mitochondria isolated with the latter procedure exhibit compact membranes and are very pure, but still contaminated with Nagarse, causing destructions of mitochondrial membranes already after 24 h storage in buffer with sucrose and mannitol at 2 degrees C. 4. One single Nagarse treatment of muscle for 40 min produces a minor yield of these these constituents. Therefore, at least two different isolation procedures are proposed for optimum investigation of skeletal muscle mitochondria.

Characterization of frog muscle mitochondria

Studies on oxidative phosphorylation revealed that, in frog skeletal muscle mitochondria (SKMM) from the thigh, the adenosine diphosphate/oxygen ratio (ADP/O) was 2.8 +/- 0.1 SE, and the respiratory control ratio was 9.5 +/- 0.9, with pyruvate/malate as the substrate. Oxygen uptake rate (Qo2) was 225 mumol O2 per minute per gram mitochondrial protein +/- 13; phosphorylation rate (ADP/O X Qo2 X 2) was 1,230 mumol ADP phosphorylation per minute per gram mitochondrial protein +/- 77; and the phosphorylation capacity (phosphorylation rate times tissue mitochondrial protein content) was 3.6 mumol ADP phosphorylated per gram wet weight of muscle +/- 0.2. Tissue mitochondrial protein content was determined by the measurement of nicotinamide adenine dinucleotide (NADH) oxidase activity. Electron microscopy (EM) revealed intact, isolated, energized twisted mitochondria of a condensed form. Frog sartorius muscle mitochondria gave similar oxidative phosphorylation parameters when investigated independently of the rest of the thigh. These values of SKMM respiration from the frog are similar to those values obtained from pigeon and rabbit heart and rat skeletal muscles. However, because of the low NADH-oxidase activity indicating reduced mitochondrial content (this was verified in low-magnification EM pictures), phosphorylation capacity was significantly reduced in frog skeletal muscle mitochondria.

Localization in cardiac muscle of some enzymes related to glutamate metabolism

A tetrazolium staining medium incorporated in a gel has been used in a histochemical study of enzymes in thin sections of heart muscle. Formazan distribution patterns given by mitochondrial enzymes were inconsistent with the location of these enzymes revealed by the extraction of whole tissue. Similar stain distributions were given by lactate dehydrogenase, glutamate oxaloacetate transaminase and glutamate dehydrogenase. The distribution given by succinate dehydrogenase was not the same as that given by cytochrome oxidase stained by a different technique. Alcohol dehydrogenase added to the tissue assumed a distribution which suggested some adsorption of the enzyme to the tissue. But experiments suggested that this enzyme was not firmly bound to muscle proteins in the manner of some glycolytic enzymes.