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Biomedical subjects

K Van Dam

Publications and source records attributed to K Van Dam.

At least 37 records · Page 2Linked to original sources
PubMed1985

Differential effects of triphenyltin and 8-azido-ATP on the ATP synthesis, ATP-Pi exchange, and ATP hydrolysis in liposomes containing ATP synthase and bacteriorhodopsin.

The ATP hydrolysis activity of purified ATP synthase reconstituted in liposomes was inhibited by triphenyltin in a manner different from that of other thiol-specific reagents. In liposomes containing ATP synthase and bacteriorhodopsin, ATP hydrolysis and ATP-Pi exchange were inhibited by triphenyltin to a greater extent than the ATP synthesis, in contrast to what was found with an F1-specific inhibitor, 8-azido-ATP. The possibility is discussed that ATP hydrolysis and ATP synthesis are differently coupled to proton conduction through F0.

Adenosine Triphosphate↗
PubMed1983

Phloretin - an uncoupler and an inhibitor of mitochondrial oxidative phosphorylation.

The effect of phloretin on respiration by isolated mitochondria and submitochondrial particles was studied. In submitochondrial particles, both NADH- and succinate-dependent respiration was inhibited by phloretin. 50% maximum inhibition was reached at phloretin concentrations of 0.1 mM (NADH oxidation) and 0.7 mM (succinate oxidation). In isolated mitochondria, phloretin inhibited glutamate oxidation in both State 3 and State 4; 50% maximum inhibition occurred at about 30 microM. Succinate oxidation is inhibited in State 3 by phloretin, inhibition being half its maximum value at 0.5 mM, but in State 4 it is stimulated about 2-fold by phloretin at a concentration of 0.6 mM. Ascorbate oxidation is stimulated in both State 3 and State 4, maximum stimulation being equal to that obtained with an uncoupler of oxidative phosphorylation. Under all circumstances, phloretin lowered the transmembrane electrical potential difference in isolated mitochondria. These results are discussed in terms of mosaic non-equilibrium thermodynamics. We conclude that phloretin is both an uncoupler and an inhibitor of oxidative phosphorylation.

Animals↗
PubMed1983

Thermodynamic efficiency of microbial growth is low but optimal for maximal growth rate.

Thermodynamic efficiency of microbial growth on substrates that are more oxidized than biomass approaches 24%. This is the theoretical value for a linear energy converter optimized for maximal output flow at optimal efficiency. For growth on substrates more reduced than biomass, thermodynamic efficiencies correspond to those predicted for optimization to maximal growth rate (or yield) only.

Bacteria↗
PubMed1981

The hypothesis of localized chemiosmosis is unsatisfactory.

The hypothesis of 'localized' chemiosmosis have been put to an experimental test in the system of oxidative phosphorylation by rat liver mitochondria. We find that the variation of the ratio of phosphate potential to delta muH with delta muH does not depend on how delta muH is varied. This is in conflict with hypotheses of localized chemiosmosis. Of all coupling hypotheses, only the parallel-coupling hypothesis can explain the observations, unless variation of the H+/ATP stoichiometry of the ATPase proton pump is accepted. In the latter case. 'macroscopic' chemiosmosis can explain the observations equally well as hypotheses of localized chemiosmosis. It is concluded that either variation of H+/ATP stoichiometries must be accepted, or that the parallel-coupling hypothesis should be reformulated so that it becomes open to experimental tests.

Animals↗
PubMed1981

Mosaic nonequilibrium thermodynamics describes biological energy transduction.

A procedure, called "mosaic nonequilibrium thermodynamics," for describing ion movement and energy transduction in biological membranes is tested in a model system: bacteriorhodopsin liposomes. The important steps in the theoretical derivations are summarized; one of the experimental tests of the postulated fundamental flow-force relationships is shown. Furthermore, how the quantitative method, even if used only qualitatively, facilitates analysis and understanding of experimental results (in this case, the effect of medium composition on the development of pH gradient and membrane potential in the bacteriorhodopsin liposomes) is shown. The main advantage of this method lies in its quantitative description of the effect of variation of system parameters on the performance of, in this case, the reconstituted proton pump bacteriorhodopsin. As an example, the method is shown to explain quantitatively the dependence of the steady-state pH gradient on the light intensity. Even in more refined analyses of experiments, the quantitative theoretical description is in full accordance with the experimental results; this is illustrated by considering the effect of valinomycin on the dependence of the initial rate of proton uptake into bacteriorhodopsin liposomes on light intensity. It is concluded that mosaic nonequilibrium thermodynamics describes ion movement and energy transduction in the model system of bacteriorhodopsin liposomes and, therefore, may be applied to any other biological system performing such processes.

Bacteriorhodopsins↗
PubMed1981

31P-Nuclear magnetic resonance and freeze-fracture electron microscopic studies on reconstituted bacteriorhodopsin vesicles.

Bacteriorhodopsin has been reconstituted into egg-phosphatidylcholine vesicles by various methods. The resulting preparations have been analyzed on density gradients and by freeze-fracture electron microscopy. The homogeneity of the vesicle preparations and the light-induced intravesicular pH changes have been studied by 31P-NMR, using glucose 6-phosphate as pH probe. It is concluded that bacteriorhodopsin is incorporated in the inside-out mode in vesicles up to about 100 nm. Above this diameter, more or less random insertion takes place.

Bacteriorhodopsins↗
PubMed1980

Linear relation between rate and thermodynamic force in enzyme-catalyzed reactions.

Starting from enzyme kinetics, it is shown that generally a linear rather than a proportional relationship exists between rate and free energy changes in biochemical processes. In the derivation the boundary condition of constant substrate plus product is used, which is appropriate for many cellular systems. An example is the ADP plus ATP concentration is mitochondrial oxidative phosphorylation, as is illustrated experimentally.

Enzymes↗
PubMed1980

Amino acid transport in plasma-membrane vesicles from rat liver. Characterization of L-alanine transport.

Plasma-membrane vesicles, isolated from rat liver, catalyze active transport of L-alanine. The transient accumulation of L-alanine requires the presence of Na+ concentration gradient (outside greater than inside). The alanine-Na+ symport is an electrogenic process, since it is stimulated under conditions that allow compensatory ion movements: both co-transported anions as well as counter-transported cations influence the rate of alanine-Na+ symport. However, no uptake is observed in the presence of a membrane potential, when no Na+ concentration gradient is present. Sodium-gradient-stimulated alanine uptake is dependent on temperature and pH, stereospecific, and is affected by the presence of other amino acids. The activity of the L-alanine transport system is influenced both by the Na+ and by the L-alanine concentration. In the presence of 100 mM Na+, an apparent Km for alanine of 2 mM is observed; lowering the Na+ concentration results in an increase in the apparent Km, and a decrease in the apparent V. The apparent Km for Na+ is 34 mM in the presence of 0.2 mM L-alanine. Increasing the L-alanine concentration also results in a lower apparent Km for Na+, and a higher V.

Alanine↗
PubMed1975

On the stimulation of the light-induced proton uptake by uncoupling aminoacridine derivatives in spinach chloroplasts.

1. Light-induced proton uptake by spinach chloroplasts is enhanced several-fold by 9-(4-diethylamino-1-methylbutylamino)-6-chloro-2-methoxyacridine (atebrin). This stimulation does not depend on the chlorophyll concentration. The amount of extra protons taken up in the presence of atebrin is determined by the pKa values of atebrin and the pH of the incubation medium. 2. Both the stimulation of the proton uptake and the maximal binding capacity for atebrin is sensitive to uncouplers. However, the ratio of bound to free atebrin does not depend on the presence of uncoupler up to the saturating atebrin concentration. 3. From simultanious kinetic measurements of atebrin fluorescence and proton movement it seems that after binding of the completely protonated atebrin the dye and the protons can move separately. This can also be inferred from the spectral behaviour of atebrin in illuminated chloroplasts. 4. The stimulation of the proton uptake by atebrin does not depend on the presence of salts in the incubation medium. However, the 'saturating' atebrin concentration increases strongly with increasing salt concentration in the medium. 5. It is concluded that the interaction of atebrin and other acridines with energized chloroplasts most likely occurs at the level of the membrane proper. 6. It is proposed that uncoupling by atebrin is a consequence of the creation of a high proton activity at the periphery of the thylakoid membrane, which opposes a proton gradient across the membrane. The uncoupling by atebrin is not of the protonophoric type according to this mechanism.

Anilides↗