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R Bohnensack

Publications and source records attributed to R Bohnensack.

At least 37 records · Page 2Linked to original sources
PubMed1991

Stimulation of alanine metabolism by ammonia in the perfused rat liver. Quantitative analysis by means of a mathematical model.

The effect of ammonia on the catabolism of alanine was studied in the perfused rat liver. Addition of 0.5 mM NH4Cl to the perfusion medium containing 5 mM alanine plus 0.1 mM octanoate produced drastic changes in the metabolite concentrations in the efflux medium. Not only the rate of ureogenesis was activated, but also the formation of glucose, lactate and pyruvate. Additionally, respiration was stimulated, the output of ketone bodies decreased, and the redox ratios lactate/pyruvate as well as 3-hydroxybutyrate/acetoacetate became more oxidized. To interpret the causes of these metabolic changes, a mathematical model was developed. It contains kinetic equations by which fluxes through essential pathways of alanine catabolism, gluconeogenesis and energy metabolism were related to the intracellular concentrations of pyruvate, oxaloacetate and ammonia, as well as to the redox ratios lactate/pyruvate and 3-hydroxybutyrate/acetoacetate. Using a nonlinear regression procedure, the model was suitable to be fitted to the data found in the experiments. The consistency of the model and experiment allowed the changes caused by ammonia to be explained. Primarily, ammonia stimulated ureogenesis hence accelerating the deamination of alanine which led to the increased formation of pyruvate, lactate and glucose. The enhanced energetic load resulting from ureogenesis and gluconeogenesis shifted the mitochondrial and cytosolic NAD systems towards more oxidized states which additionally modified the flux rates. The results demonstrate that there is a high degree of cooperativity between the metabolic pathways.

3-Hydroxybutyric Acid↗
PubMed1991

Intramitochondrial fatty acid activation enhances control strength of adenine nucleotide translocase.

In incubations with isolated rat liver mitochondria we studied the fuel properties of octanoate as medium-chain fatty acid and respiratory substrate and the extent of control exerted by adenine nucleotide translocase on mitochondrial respiration. While, compared with pyruvate, octanoate improved the hydrogen supply in the active state to be seen from a high reduction of the mitochondrial NAD(P) system and an increased delta psi, it also decreased the efficiency of energy transduction indicated by a low ADP/O ratio. Based on measurements of the dependence of respiration on the extramitochondrial ATP/ADP ratio, we conclude that a switch-over from pyruvate to fatty acid oxidation does not change the kinetic parameters which make respiration respond to the ATP/ADP ratio. It is shown that the decrease of the exchangeable intramitochondrial adenine nucleotide pool due to the activation of octanoate results in a decrease of the activity of the adenine nucleotide translocase and an increase of its flux control coefficient.

Adenosine Diphosphate↗
PubMed1990

Estimation of flux control coefficients from inhibitor titrations by non-linear regression.

A mathematical model was developed to estimate flux control coefficients (Co) from titration studies with specific non-competitive inhibitors. In contrast to the normally used graphical determination the model pays regard to the dissociation equilibrium (KD) that exists between inhibitor and its binding sites (Eo) as well as to an objective estimation of the initial slope. The model was used for the analysis of titration experiments where the respiration of rat liver mitochondria was inhibited with carboxyatractyloside and antimycin A. It is shown that the graphical estimation of Eo and Co lead to significant overestimation if the ratio KD/Eo is larger than 10(-4) which can be avoided by using our model.

Animals↗
PubMed1990

Quantification of aerobic energy turnover in epididymal bull spermatozoa.

Turnover rates of oxidative energy metabolism were measured as oxygen consumption in untreated and caffeine-stimulated epididymal bull spermatozoa respiring with lactate. Incubation of spermatozoa with 1 mM caffeine led to an increase in respiration of approx. 60%. The rate of uncoupled respiration and the vanadate-insensitive part of oxygen consumption were not affected by caffeine. The small effect of ouabain on respiration (-10%) indicated a minor contribution of Na+/K+-ATPase to the ATP consumption. The major part of ATP turnover was caused by motility shown by the strong linear correlation between respiration and motility in untreated and caffeine-treated spermatozoa. In comparison with ejaculated spermatozoa investigated in a previous study, epididymal cells exhibited the same rates of uncoupled and ouabain-sensitive respiration. The efficiency of transforming mitochondrially-produced ATP into cell motion was the same in epididymal and ejaculated spermatozoa. The ATP-producing capacity of sperm mitochondria was utilized in untreated epididymal, in caffeine-stimulated epididymal and in ejaculated spermatozoa, by 20-25%, 40-45% and 45-50%, respectively. The results showed that the capacity of mitochondrial. ATP formation remains unchanged after ejaculation and is utilized to a higher extent by stimulated motility. Treatment with caffeine affected epididymal spermatozoa in a similar manner.

Aerobiosis↗
PubMed1990

Lactate and ornithine improve substrate hydrogen supply in isolated hepatocytes.

The influence of lactate and ornithine on the mitochondrial redox state as indicated by the 3-hydroxybutyrate/acetoacetate ratio was studied in isolated hepatocytes. The load on the hepatic energy metabolism when lactate and/or ornithine are additionally present can be higher than in the sole presence of octanoic acid. This was revealed by enhanced oxidative turnover rates and higher 3-hydroxybutyrate/acetoacetate ratios at identical rates of respiration. During gluconeogenesis from lactate, such an improved energy supply takes place through more intense oxidation of octanoic acid in the citric-acid cycle. In addition to octanoate oxidation, however, ornithine supplies substrate hydrogen through its catabolism to glutamate as was demonstrated by comparative studies in isolated rat liver mitochondria. The ambivalent character of lactate and ornithine with regard to loading and supplying the energy metabolism seems to substantially account for energetic homoeostasis of the liver.

3-Hydroxybutyric Acid↗
PubMed1989

Characterization of mitochondrial functions in digitonin-permeabilized rat liver cells.

Mitochondrial functions were investigated in permeabilized rat liver cells. For permeabilization isolated hepatocytes were treated with digitonin using a perifusion technique. After permeabilization the cell count was almost unchanged, and the mitochondrial marker enzyme, glutamate dehydrogenase, was decreased to as little as 83%. The mitochondria in permeabilized cells were functionally intact, a finding evident from a marked stimulation of respiration by ADP, inhibition by carboxyatractyloside, and stimulation by uncoupler. The ADP-stimulated and uncoupled respiration rates with succinate as substrate were comparable to those reported for isolated mitochondria, whereas the rates with NAD(+)-dependent substrates were somewhat higher. The ratios between ADP-stimulated and carboxyatractyloside-inhibited respiration rates were in the range noted for isolated mitochondria with identical substrates. Almost unchanged ADP-stimulated and carboxyatractyloside-inhibited respiration rates were found for at least 180 min after digitonin treatment. The preparation exhibited only a low extramitochondrial ATPase activity which was partially inhibited by vanadate.

Adenosine Diphosphate↗
PubMed1988

Stimulation of alanine metabolism in rat liver by ammonia.

The effect of ammonia on the alanine metabolism was investigated in perfused rat liver. Gluconeogenesis was found to be stimulated by physiological concentrations of ammonia, while being inhibited at higher concentrations (5-10 mM). The stimulating effect of 0.5 mM ammonia was studied in greater detail. In addition to glucose formation seen enhanced five times, increased rates were observed for ureogenesis as well as the formation of lactate and pyruvate, demonstrating also activation of the total alanine turnover. Furthermore, the mitochondrial and cytosolic NAD systems were increasingly oxidized as reflected by the beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios. The shift of the beta-hydroxybutyrate/acetoacetate ratio was correlated to the ATP demand by gluconeogenesis and ureogenesis. The elevated concentration of pyruvate was found to have caused stimulation of gluconeogenesis since there existed a Michaelis-Menten type relation between pyruvate concentration and glucose formation irrespective of the presence or absence of ammonia. The flux through glutamate dehydrogenase was calculated from the total alanine turnover and urea formation, and noted to be diminished in the presence of ammonia despite the increased alanine turnover. It is concluded that glutamate dehydrogenase, at least in part, controls the total alanine turnover in the absence of ammonia.

Alanine↗
PubMed1987

Regulation of oxidative phosphorylation in mitochondria of epididymal bull spermatozoa.

The regulation of oxidative phosphorylation was studied with digitonin-treated epididymal bull spermatozoa in which mitochondria are directly accessible to low molecular compounds in the extracellular medium. Due to the high extramitochondrial ATPase activity in this cell preparation, it was possible to stimulate respiration to a small extent only by added hexokinase in the presence of glucose and adenine nucleotides. Added pyruvate kinase plus phosphoenol pyruvate, however, strongly suppressed the respiration. Under these conditions, the respiration was found to depend on the extramitochondrial [ATP]/[ADP] ratio in the range of 1-100. The contribution of the adenine nucleotide translocator to this dependence was determined by titration with the irreversible inhibitor carboxyatractyloside in the presence of ADP. Using lactate plus malate as substrate, the active state respiration was controlled to about 30% by the translocator, whereas 12 and 4% were determined in the presence of L-glycerol-3-phosphate and malate alone, respectively. In order to compare the results with those for intact cells, the adenine nucleotide patterns were determined in intact and digitonin-treated spermatozoa under conditions of controlled respiration in the presence of vanadate and carboxyatractyloside, respectively. About 21% of total cellular adenine nucleotides were found in digitonin-treated cells representing the mitochondrial compartment. While allowing for the intramitochondrial amount of adenine nucleotides, the cytosolic [ATP]/[ADP] ratio was estimated to be 6-times higher than the mitochondrial ratio in intact cells. It is concluded from the data presented that the principal mechanism by which oxidative phosphorylation in sperm mitochondria is regulated via the extramitochondrial [ATP]/[ADP] ratio is the same as that demonstrated for other isolated mitochondria.

Adenine Nucleotides↗
PubMed1987

Dynamic compartmentation of adenine nucleotides in the mitochondrial intermembrane space of rat-heart mitochondria.

To investigate whether or not the mitochondrial intermembrane space together with the extramitochondrial space form a homogeneous pool for adenine nucleotides, rat-heart mitochondria were studied in reconstituted systems with pyruvate kinase and ADP-producing enzymes with varied localization. In the hexokinase system, ADP is produced extramitochondrially by added yeast hexokinase, whereas in the creatine kinase system mitochondrial creatine kinase is responsible for ADP regeneration in the intermembrane space. The dependence of mitochondrial respiration on the extramitochondrial [ATP]/[ADP] ratio in both systems was investigated experimentally and by means of computer simulation. Near the resting state, higher [ATP]/[ADP] ratios were found in the creatine kinase system than in the hexokinase system at the same rate of respiration. This and the maintaining of a substantial creatine kinase-stimulated respiration in the presence of pyruvate kinase in excess is explained by a two-compartment model considering diffusion limitations of adenine nucleotides. A diffusion rate constant of (8.7 +/- 4.7) 10(4) microliters X mg-1 X min-1 for ADP and ATP was estimated, resulting in rate-dependent concentration differences up to 13.7 microM AdN between the extramitochondrial space and the AdN-translocator at the maximum rate of oxidative phosphorylation of rat-heart mitochondria. The results support the assumption that ADP diffusion towards the AdN-translocator is limited if its extramitochondrial concentration is low, resulting in a dynamic compartmentation of adenine nucleotides in the mitochondrial intermembrane space.

Adenine Nucleotides↗
PubMed1987

The electroneutral transport of anions into mitochondria causes apparent saturation characteristics.

A theoretical analysis of the apparent saturation characteristics of anion accumulation by mitochondria is presented. The decrease in the accumulation ratio observed with increasing extramitochondrial anion concentration can be caused solely by the cotransport of protons due to the electroneutral transport mechanism and might not indicate that mitochondria have a limited anionic capacity. On this basis a mathematical model of anion distribution was developed which sufficiently describes the published experimental data of anion accumulation by mitochondria.

Animals↗
PubMed1986

Control of respiration and of motility in ejaculated bull spermatozoa.

The relations between motility and respiration were studied in ejaculated bull spermatozoa respiring with lactate. Motility was quantitatively evaluated by a turbidimetric procedure as percentage of cells moving per minute from the bottom of the cuvette into the light path. For selective inhibition of ATP-consuming reactions including motility or of mitochondrial respiration, vanadate or cyanide, respectively, were used. Both inhibitors were found to produce proportional changes in motility and respiration. The simultaneous changes in motility and respiration were linked to shifts in the cellular ATP/ADP ratio. Partial uncoupling of respiration in vanadate-inhibited cells gave similar relations between respiration and ATP/ADP ratios as stepwise inhibition of ATP-utilizing reactions by vanadate. Presuming saturation kinetics with respect to the ATP/ADP ratio, half maximum constants of 1.7 and 4.7 for the ATP/ADP ratio and maximum values of about 130% and 300% (in comparison to untreated cells) were estimated for motility and respiration, respectively. Respiration showed a much steeper dependence on the ATP/ADP ratio than motility resulting in an apparent cooperativity coefficient of 2.9. From these dependences on the ATP/ADP ratio, the shares in the control of ATP turnover in untreated cells were estimated. At sufficient supply with substrate, more than 80% of control were excreted by motility and other ATP-utilizing reactions, the rest by mitochondrial ATP production, i.e., the reactions of oxidative phosphorylation.

Adenosine Diphosphate↗
PubMed1986

Quantification of sperm motility by a turbidimetric assay. Correlation to cellular respiration.

The turbidimetric method according to SOKOLOSKI et al. (Fertil. Steril. 28, 1337-1341, 1977) was used for quantitative measurement of the motility of washed bovine spermatozoa. The method is based on the registration of the increase in absorbance (A538) which is produced by spermatozoa entering the light path after run from the bottom of a photometer cuvette. This increase in absorbance is dependent on the amount of motile cells and their average velocity. The increase in absorbance related to the total cell concentration was found to be a highly reproducible quantity (motility index M). Inhibitors of dynein ATPase (vanadate) and of the mitochondrial ATP-production (KCN, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), oligomycin) produced a diminution of the motility index M in a concentration-dependent manner. Under conditions of oxidative energy metabolism (lactate as substrate) a linear relation between respiration and M existed. Such a relationship was found under treatment of spermatozoa with KCN or vanadate as well as for untreated sperm suspensions. The linear correlation to the respiratory rate justifies to use the proposed motility index as a relative measure of the average motility of a sperm sample.

Adenosine Triphosphatases↗
PubMed1985

Interdependence of mitochondrial ATP production and extramitochondrial ATP utilization in intact spermatozoa.

The dependence of both respiration and total activity of ATP-consuming reactions on the cellular adenine nucleotide pattern was investigated in intact bovine spermatozoa. ATP consumption was manipulated by inhibition with vanadate and activation with caffeine, leading to a decrease or increase in the rate of respiration up to 70% or 20%, respectively. Oligomycin blocked the respiration to the same extent as did vanadate, suggesting that the total extramitochondrial ATP-consuming activity is vanadate-sensitive. The major part of ATP utilization must be linked to dynein ATPase, since inhibition of (Na+, K+) ATPase by ouabain showed only a small effect on respiration (-17%). Being a potent inhibitor of dynein ATPase, vanadate drastically reduced the amount of motile cells, whereas caffeine tended to increase the intensity of motion. The effects of vanadate or caffeine on respiration were paralleled by changes in cellular ATP, reflecting the response of mitochondrial respiration on the cellular ATP/ADP ratio. Respiration was found to depend on changes in the ATP/ADP ratio in the range from about 3 (+caffeine) to 9 (+vanadate). The range of response of ATP consumption to the ATP/ADP ratio was determined by varying the mitochondrial ATP production via the concentration of lactate which was used as substrate. The measured effects on both respiratory rate and ATP/ADP ratio suggested that ATP consumption was markedly dependent on ATP/ADP ratios below 5. It is concluded that lactate concentrations above 1 mM sufficiently supply bovine spermatozoa with substrate and the energy turnover is mainly limited by the activity of dynein ATPase rather than by the capacity of mitochondrial oxidative phosphorylation.

Adenosine Diphosphate↗
PubMed1985

Mathematical modeling of mitochondrial energy transduction.

A mathematical model of mitochondrial energy transduction is presented. The model contains rate equations for the main steps of oxidative phosphorylation. It was used to simulate the relations of respiration and ATP formation to extra- and intramitochondrial ATP/ADP ratios under various steady-state conditions. Furthermore, the model equations allowed to compute control coefficients, which quantify the control exerted by different steps on respiration. The distribution of control within mitochondria is demonstrated to depend on the metabolic state of mitochondria and also on the properties of extramitochondrial enzymes involved in ATP turnover. The simulated steady-state data as well as computed control coefficients were found in close agreement with experimental data.

Adenosine Diphosphate↗
PubMed1985

Effect of various substrates on mitochondrial and cellular energy state of intact spermatozoa.

Ejaculated bull spermatozoa separated from seminal plasma by repeated washing were used to study the effects of various substrates on respiration, mitochondrial energization, cellular ATP/ADP ratios, and motility. The respiratory rate in the presence of lactate, pyruvate, acetate or glycerol was about two to three times higher compared to cells oxidizing amino acids, fatty acids, or endogenous substrates. When well oxidizable substrates were present, the uncoupler FCCP produced a marked increase in respiration (two to three times at optimal FCCP concentrations). Mitochondrial energization was monitored in intact spermatozoa by spectrophotometry with the dye safranin. Lactate and pyruvate yielded more pronounced mitochondrial energization than did alanine or endogenous substrates. Cellular ATP/ADP ratios, with lactate, pyruvate, acetate or glycerol, were found to range from 6 to 10, whereas endogenous substrates and amino acids were able to establish ATP/ADP quotients as low as about 1 only. The capacity to oxidize various substrates was clearly reflected by the amount of motile cells. Motility in the presence of lactate, acetate or glycerol was seen in more than 60% of spermatozoa, while spermatozoa in the presence of alanine or butyrate showed a motility not above the endogenous level (approx. 30% motile cells). The experiments demonstrated that cellular performance is highly dependent on the energy state as made possible by the capacity of substrate utilization.

Acetates↗
PubMed1985

Theory of steady-state control in complex metabolic networks.

Based on the concept of control coefficients introduced by Kacser and Burns (Symp. Soc. Exp. Biol. 32 (1973) 65-104) and Heinrich and Rapoport (Eur. J. Biochem. 42 (1974) 83-95) a theoretical analysis of steady-state control in metabolic networks is presented. The control results from the kinetic properties of the reactions in the system, expressed by elasticity coefficients with respect to the intermediate signals, as well as from the stoichiometric structure of the system, expressed by pathway fractions of the pathway fluxes through the system. The intermediate signals and the pathway fluxes are defined as quantities which contain complete information about the patterns of the intermediates and fluxes in the steady-state without the constraints generated by the stoichiometric structure. Quantitative formulation leads to connectivity theorems relating the control coefficients with the kinetic properties of the reactions, and to pathway summation theorems relating the control coefficients with the stoichiometric structure. For larger systems a hierarchical substructure can be introduced; for the subsystems the term "functional compartment" is proposed. These compartments include several reactions but can be formally created as a single reaction. It is demonstrated that the control exerted by a reaction in the whole system is proportional to the control exerted in the isolated compartment if the compartment includes an unbranched flux structure, while this is not applicable in case of a branched structure.

Enzymes↗