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H Nohl

Publications and source records attributed to H Nohl.

86 records · Page 5Linked to original sources
PubMed1978

Kinetics of ADP, ATP transport in mitochondria as studied by the quench-flow method.

The kinetics in the millisecond range of ADP, ATP counterexchange in rat liver mitochondria were investigated using a quench-flow apparatus. The exchange was stopped with atractylate and the mitochondria were separated by centrifugation. 1. After correcting for leakage due to flow stress, apparent biphasic exchange kinetics were observed, with a more rapid phase within 100--200 ms, which extends to 10% of the total exchange. In general, the extent of the rapid phase increased when the translocation rate was changed under various conditions, in agreement with the model of the quench mechanism by atractylate. 2. The nature of the "rapid" phase was analyzed in the "steady" and "transient" state of the translocation and was shown to be caused by a delayed binding of atractylate due to competition with ADP. This quench delay results in a residual exchange which could explain the rapid part of the kinetics. 3. Deenergization of mitochondria by valinomycin or by uncoupler largely abolishes the rapid kinetic phase. This is explained by an increased availability of carrier sites at the outer face of the membrane to atractylate in the deenergized state, resulting in a more rapid quench. 4. The interpretation of the rapid exchange phase as a function of carrier sites accessible to atractylate quenching at the outer membrane face was simulated by a computer program based on the reorientating carrier model. With a set of rate constants, an approximate fit for the extent of quench delay with the experimental data is obtained.

Adenosine Diphosphate↗
PubMed1976

Mechanism of the stimulatory effect of fructose on ethanol oxidation in perfused rat liver.

The stimulatory effect of fructose on ethanol oxidation was studied in livers from fasted rats perfused with Krebs-Henseleit-bicarbonate buffer in a non-recirculating system. Two series of experiments were performed: (A) ethanol was infused with stepwise increasing concentrations (0.1-20 mM) in the presence of 4 mM fructose; (B) fructose was infused with stepwise increasing concentrations (0.5-10 mM) in the presence of 2 mM ethanol. From measured metabolic rates the following parameters were calculated: energy-rich phosphates consumed for fructose metabolism which were provided from oxidative phosphorylation (delta approximately P); reducing equivalents derived from stimulated ethanol utilization which were disposed by mitochondrial oxidation (delta2H). Under the various conditions studied a linear relationship between these parameters was observed. The ratio delta approximately P/delta2H was about 2.0. It is suggested that fructose stimulates ethanol oxidation indirectly by increasing the energy consumption of the liver due to the production of glucose from fructose. Consequetnly, the rate of oxidative phosphorylation is increased and, therefore, the capacity of the respiratory chain for oxidizing reducing equivalents derived from ethanol is enhanced. The data support the more general hypothesis that the rate of ethanol oxidation depend upon the rate of hepatic energy consumption in a given metabolic state.

Alcohol Oxidoreductases↗
PubMedDate not supplied

Molecular basis of age-dependent changes in the activity of adenine nucleotide translocase.

(1) Rat-heart mitochondria from 30-month-old animals are 40% less active in translocating adenine nucleotides across the inner membrane than 3-month-old rats. (2) The number of sites available for binding the specific ligands to the adenine nucleotide carrier remains unchanged during aging. (3) The endogenous pool of the adenine nucleotides exhibits an age-dependent fall by more than 25%, essentially at the expense of ATP. The amount of ATP + ADP representing the exchangeable pool for adenine nucleotide translocation is decreased to the same extent. (4) Negatively charged phospholipids as well as polyunsaturated fatty acids of membrane lipids were found to be reduced with aging. (5) The results are discussed in terms of changes in the phospholipid-protein interactions due to the observed alterations in the physical state of the bulk phase of membrane lipids.

Adenosine Diphosphate↗
PubMedDate not supplied

Imbalance of oxygen activation and energy metabolism as a consequence or mediator of aging.

Ever increasing numbers of aging theories suggest that free radicals are only one factor among others that may initiate stochastic disorders finally terminating life. It is therefore compelling not only to demonstrate the existence of increasing steady-state concentrations of free oxygen radicals during senescence, but it is essential to show that they act in concert with other postulated triggering factors of aging. We have recently shown that various factors may have a life-long influence and challenge oxygen homeostasis of cell respiration. Among these factors are environmental pollutants, therapeutics, and transient hypoxia. Although the nature of these "hits" is different, mitochondrial respiration was found to respond in a similar manner to each of them. The major derangement was an univalent electron leak to oxygen giving rise to the establishment of oxidative stress. Associated with this transformation, oxidative phosphorylation was impaired with the resultant reduction of cellular ATP. Mitochondria from senescent rats exhibited similar alterations of all cell parameters found when adult animals were exposed to "environmental stress" or transient ischemia. Age-related stimulation of mitochondrial oxygen radical generation is therefore suggested to result from accumulation of minihits during life. Based on our data, together with those from other laboratories, it is possible to assess the ranking order of oxygen radicals in the development of stochastic events associated with (or causing) aging.

Aging↗
PubMedDate not supplied

Age-dependent changes in the structure-function correlation of ADP/ATP-translocating mitochondrial membranes.

Morphological changes in rat heart mitochondria following mobilization of the adenine nucleotide binding sites from the matrix to the cytosolic face of the inner membrane have been documented in this paper by means of electron microscopy. Significant differences in the shape of cristae membranes could be established when rat heart mitochondria from 3- and 30-month-old animals were compared. Changes in structural organization of the inner membrane - an event associated with the function of the adenine nucleotide carrier - were found to be distinctly less marked in mitochondria from the aged group. Kinetic analysis of this process revealed that both the velocity and the extent of conformational membrane changes were reduced as the animal ages. These observations are discussed in relation to the finding, also described in this paper, that aging is linked to a drastic decrease in the activity of adenosine diphosphate/adenosine triphosphate exchange.

Adenosine Diphosphate↗
PubMedDate not supplied

Evaluation of the antioxidant capacity of ubiquinol and dihydrolipoic acid.

Ubiquinone and alpha-lipoic acid are natural constituents which are involved in mitochondrial energy metabolism. Their bioenergetic activities require redox-cycling. In the case of alpha-lipoic acid redox-cycling leads to dihydrolipoic acid which occurs in multienzyme complexes involved in the citric acid cycle while UQ recycles through semi- and divalently reduced ubiquinones in the respiratory chain. We have proved the validity of the concept about the antioxidant function of these natural compounds in their reduced form. Ubiquinol was found to interfere with lipid peroxidation of liposomal membranes being itself degradated by two consecutive oxidation steps. Dihydrolipoic acid was found to totally recycle ubiquinone to the antioxidant active divalently reduced form. In contrast to the antioxidative derived reaction products of ubiquinols which in turn promoted lipid peroxidation, the antioxidant derived reaction product of dihydrolipoic acid was the unreactive two electron oxidation product alpha-lipoic acid. Our experiments demonstrate the existence of an dihydrolipoic acid driven recycling of UQ to the antioxidative-active UQH2. The efficiency of the antioxidative capacity of the latter was found to be diminished through prooxidant activities of the antioxidant-derived metabolites.

Antioxidants↗
PubMedDate not supplied

The exogenous NADH dehydrogenase of heart mitochondria is the key enzyme responsible for selective cardiotoxicity of anthracyclines.

The molecular mechanism of the anthracycline-dependent development of cardiotoxicity is still far from being clear. However, it is generally accepted, that mitochondria play a significant role in triggering this organ specific injury. The results presented in this study demonstrate that, in contrast to liver mitochondria, isolated heart mitochondria shuttle single electrons to adriamycin, giving rise to oxygen radical formation via autoxidation of adriamycin semiquinones. This one electron reduction of anthracyclines is catalyzed by the exogenous NADH dehydrogenase associated with complex I of heart mitochondria, an enzyme which is lacking in liver mitochondria. Upon addition of NADH heart mitochondria generate significant amounts of adriamycin semiquinones while liver mitochondria were ineffective. Adriamycin semiquinones undergo both autoxidation leading to superoxide radical release and complex reactions under formation of adriamycin aglycone. Due to the high lipophilicity adriamycin aglycones accumulate in the inner mitochondrial membrane where they interfere with electron carriers of the respiratory chain. Adriamycin aglycone semiquinones emerging from an interaction with complex I were found to trigger homolytic cleavage of H2O2 which results in the formation of hydroxyl radicals. As demonstrated in this study the activation of adriamycin by the exogenous NADH dehydrogenase of cardiac mitochondria initiates a cascade of reaction steps leading to the establishment of oxidative stress. Our experiments suggest the exogenous NADH dehydrogenase of heart mitochondria to play a key role in the cardiotoxicity of adriamycin. This organ-specific enzyme initiates a sequence of one electron transfer reactions ending up in the establishment of oxidative stress.

Animals↗
PubMedDate not supplied

Influence of age on thermotropic kinetics of enzymes involved in mitochondrial energy-metabolism.

Aging has recently been shown to promote lipid peroxidation of mitochondrial membranes by a mechanism involving chaotropic oxidants (5). The present paper reports on the relationship between these membrane alterations and the activities of lipid-dependent enzymes of isolated heart mitochondria from 3 month and 24 month old rats. 1. Temperature breaks of Arrhenius plots reveal age-dependent shifts to higher temperatures for the succinate oxidase system (delta t = 1,7 degrees C), the beta-hydroxybutyrate dehydrogenase, the succinate dehydrogenase and the ATP-ase (delta t = 3,0 degrees C). 2. Specific activities of the above enzymes are distinctly lowered in preparation from aged rats. 3. Thermotropic differences of the particular enzyme activities completely disappeared after solubilization of the lipid components by Triton X-100. 4. ESR studies exhibit a clear decrease in the fluidity of membrane lipids as a function of age. 5. Analysis of the membrane lipids by gas-liquid chromatography reveals a distinct age-dependent fall in the content of polyunsaturated fatty acids accompanied by a slow increase in the degree of fatty acid saturation. 6. It is concluded from the results that aging influences enzyme-protein-lipid interactions by radical-induced peroxidation of the surrounding membrane lipids, but does not affect the intrinsic properties of the membrane-bound enzymes themselves.

Adenosine Triphosphatases↗