Search PubMed⌕ Search

Biomedical subjects

S R Caplan

Publications and source records attributed to S R Caplan.

At least 37 records · Page 2Linked to original sources
PubMed1987

Consequences of detailed balance in a model for sensory adaptation based on ligand-induced receptor modification.

A model for exact sensory adaptation has been published by Segel and co-workers in several papers [e.g., Knox, B. E., Devreotes, P. N., Goldbeter, A. & Segel, L. A. (1986) Proc. Natl. Acad. Sci. USA 83, 2345-2349]. The model comprises a pair of states whose relative probabilities are determined by the binding of a ligand. A second pair of states related by the same ligand binding is accessible as a consequence of either a conformational change or a "covalent modification." By taking proper account of detailed balance, we show that the notion of covalent modification in this context includes three cases, two of which involve dissipation of metabolic energy. The condition for exact adaptation is dependent on metabolite concentrations in all cases of covalent modification. The performance of the model is critically examined on thermodynamic and kinetic grounds.

Adaptation, Physiological↗
PubMed1986

Double-inhibitor and uncoupler-inhibitor titrations. 1. Analysis with a linear model of chemiosmotic energy coupling.

The results of double-inhibitor and uncoupler-inhibitor titrations have been simulated and analyzed with a linear model of delocalized protonic coupling using linear nonequilibrium thermodynamics. A detailed analysis of the changes of the intermediate delta muH induced by different combinations of inhibitors of the proton pumps has been performed. It is shown that with linear flow-force relationships the published experimental results of uncoupler-inhibitor titrations are not necessarily inconsistent with, and those of double-inhibitor titrations are inconsistent with, a delocalized chemiosmotic model of energy coupling in the presence of a negligible leak. Also shown and discussed are how the results are affected by a nonnegligible leak and to what extent the shape of the titration curves can be used to discriminate between localized and delocalized mechanisms of energy coupling.

Adenosine Triphosphatases↗
PubMed1986

Double-inhibitor and uncoupler-inhibitor titrations. 2. Analysis with a nonlinear model of chemiosmotic energy coupling.

The results of double-inhibitor and uncoupler-inhibitor titrations have been simulated and analyzed with a nonlinear model of delocalized protonic coupling obtained by linking two proton pump models of the kind studied by Pietrobon and Caplan [Pietrobon, D., & Caplan, S. R. (1985) Biochemistry 24, 5764-5776] through their common intermediate delta mu H. It is shown that the results predicted by a delocalized chemiosmotic model are highly dependent on the kind of relationships existing between rate of ATP synthesis, Jp, and delta mu H and rate of electron transfer, Je, and delta mu H. With nonlinear flow-force relationships all the results reported so far are not necessarily inconsistent with the delocalized chemiosmotic model provided that the relationships between rates and delta mu H satisfy the following requirements: Jp/delta mu H increases and/or Je/delta mu H decreases as (delta mu H) increases.

Adenosine Triphosphatases↗
PubMed1986

Intrinsic uncoupling of mitochondrial proton pumps. 2. Modeling studies.

The thermodynamic and kinetic properties associated with intrinsic uncoupling in a six-state model of a redox proton pump have been studied by computing the flow-force relations for different degrees of coupling. Analysis of these relations shows the regulatory influence of the thermodynamic forces on the extent and relative contributions of redox slip and proton slip. Inhibition has been introduced into the model in two different ways, corresponding to possible modes of action of experimental inhibitors. Experiments relating the rate of electron transfer to delta microH at static head upon progressive inhibition of the pumps have been simulated considering (1) the limiting case that the nonzero rate of electron transfer at static head is only due to intrinsic uncoupling (no leaks) and (2) the experimentally observed case that about 30% of the nonzero rate of electron transfer at static head is due to a constant proton leakage conductance in parallel with the pumps, the rest being due to intrinsic uncoupling. The same simulations have been performed for experiments in which the rate of electron transfer is varied by varying the substrate concentration rather than by using an inhibitor. The corresponding experimental results obtained by measuring delta microH and the rate of electron transfer at different succinate concentrations in rat liver mitochondria are presented. Comparison between simulated behavior and experimental results leads to the general conclusion that the typical relationship between rate of electron transfer and delta microH found in mitochondria at static head could certainly be a manifestation of some degree of intrinsic uncoupling in the redox proton pumps.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Further comments on the logic of the application of uncoupler- inhibitor titrations for the elucidation of the mechanisms of energy coupling.

Following the appearance of two papers in this journal commenting on the logic of the application of uncoupler-inhibitor titrations as a means of discriminating between 'delocalized' and 'localized' chemiosmotic mechanisms [(1984) FEBS Lett. 176,79-82; (1985) FEBS Lett. 186, 8-10], and in contrast with the arguments presented there and elsewhere, we show that in a linear model the increase in delta mu H which accompanies partial inhibition of the ATPases always leads to a relatively higher decrease of the rate of ATP synthesis by a given concentration of uncoupler in the presence of an ATPase inhibitor than in its absence. This is due to the fact that the same titre of uncoupler leads to a higher dissipative H+ flow in the presence of inhibitor, since the driving force delta mu H is higher.

Adenosine Triphosphatases↗
PubMed1985

Flow-force relationships for a six-state proton pump model: intrinsic uncoupling, kinetic equivalence of input and output forces, and domain of approximate linearity.

General flow-force relations have been determined, by the Hill diagram method, for a six-state proton pump model with and without intrinsic uncoupling (molecular slipping). A computer-aided analysis of the resulting sigmoidal flow-force curves has been performed by using a set of physically meaningful rate constants. It is shown that gating effects and apparent irreversibility can arise from sigmoidicity. The regions of approximate linearity in the vicinity of inflection points, which may be far from equilibrium, have been examined with a view to characterization in terms of linear phenomenological equations, with due regard to the problems of kinetic equivalence of the forces and symmetry. The determination of thermodynamic parameters such as the degree of coupling, the phenomenological stoichiometry, and the efficiency in these regions is discussed, and their meaning is analyzed in relation to the parameters characterizing the Onsager domain close to equilibrium. The application of the phenomenological equations of near-equilibrium nonequilibrium thermodynamics to such regions is at best a simplification to be treated with great caution. A knowledge of the distance from equilibrium of the flow-controlling ranges of the forces (i.e., the ranges of approximate linearity) turns out to be crucial for the interpretation of thermodynamic parameters determined by manipulating one of the forces while the other remains constant, as well as for the interpretation of measurements of force ratios at static head. The latter approaches can give good estimates of the magnitude of the mechanistic stoichiometry and of the constant force if the pumps are highly coupled and are operating not far from equilibrium. The force-flow relationships are shown to be modified by intrinsic uncoupling, reflecting the regulatory influence of the forces on the extent and nature of the slip. Thus reaction slip increases, for example, as the force against which the proton pump operates increases. The possible physiological significance of regulated intrinsic uncoupling is discussed.

Calorimetry↗
PubMed1984

Simulations of frequency-dependent photoacoustic magnitude signals and their implications for bacteriorhodopsin photocycle energetics.

The modulation frequency dependence of photoacoustic signals obtained from photoactive samples can provide information on the time-dependent enthalpy changes occurring during the light-induced process. The experimental requirements for this type of calorimetry, and the interpretation ot the results, are critically examined with reference to the light-driven proton pump bacteriorhodopsin. For a three-step unbranched model of the bacteriorhodopsin photocycle we derive an expression for the photoacoustic magnitude signal as a function of frequency. Simulations are performed for various values of the rate constants and energetic changes. It is shown that the net heat uptake during a low, final step postulated by some workers should be reflected in the photoacoustic magnitude frequency spectrum, giving rise to a characteristic maximum. However, this effect, which has been observed experimentally, may also be produced by a fast, strongly endothermic step occurring earlier. The precise chronology of an endothermic transition cannot be resolved unambiguously by magnitude measurements alone, although they are free from assumptions regarding difficult-to-measure phase relationships. Hence, the published photoacoustic observations showing the effect are consistent with a cyclic sequence of events in which the bacteriorhodopsin system first undergoes an increase of entropy, followed by a decrease on returning to the initial state, as well as the reverse. It is argued that the molecular disorder-order sequence is more probable.

Journal Article↗
PubMed1984

Determination of the distribution of catalyst activity across a permeable membrane containing an immobilized enzyme. Indeterminacy of a functional approach to a structural problem.

Porous membranes were fabricated from collodion and impregnated with papain, inhomogeneously through the thickness of the membrane. These membranes were placed between reservoirs containing N-alpha-benzoyl arginineamide, a substrate for the enzyme papain. The progress of the reaction was monitored by sampling the reservoirs on each side for ammonia, a reaction product. From these data the diffusion coefficient, enzyme activity, and distribution of enzyme activity of the membrane were estimated. The limitations of this approach are discussed in the context of the analysis of biological transport systems.

Collodion↗
PubMed1983

Efficiency of energy conversion in model biological pumps. Optimization by linear nonequilibrium thermodynamic relations.

Experimental investigations showed linear relations between flows and forces in some biological energy converters operating far from equilibrium. This observation cannot be understood on the basis of conventional nonequilibrium thermodynamics. Therefore, the efficiencies of a linear and a nonlinear mode of operation of an energy converter (a hypothetical redox-driven H+ pump) were compared. This comparison revealed that at physiological values of the forces and degrees of coupling (1) the force ratio permitting optimal efficiency was much higher in the linear than in the nonlinear mode and (2) the linear mode of operation was at least 10(6)-times more efficient that the nonlinear one. These observations suggest that the experimentally observed linear relations between flows and forces, particularly in the case of oxidative phosphorylation, may be due to a feedback regulation maintaining linear thermodynamic relations far from equilibrium. This regulation may have come about as the consequence of an evolutionary drive towards higher efficiency.

Energy Metabolism↗
PubMed1982

Compartmental analysis of light-induced proton movement in reconstituted bacteriorhodopsin vesicles.

Purified bacteriorhodopsin from purple membrane sheets isolated from Halobacter halobium was solubilized with a bile salt detergent, 3-[(3-cholamidopropyl) dimethyl-ammonium]-1-propanesulfonate (CHAPS). The detergent-solubilized protein was then incorporated into lecithin vesicles at either high (450:1) or low (65:1) lipid to protein ratios. Circular dichroism studies showed that the bacteriorhodopsin incorporated was in a monomeric form in the 450:1 vesicles. The 65:1 vesicles exhibited an exciton splitting characteristic of the aggregated state of bacteriorhodopsin. We then examined the light-induced movement of protons for these two preparations. Compartmental analysis was used to derive a kinetic model for the observed proton movement. The pumping was qualitatively the same for monomeric and aggregated protein. A three-compartment model provided an excellent description of proton movement in both sets of vesicles and at four different light intensities. This model demands two independent processes to account for the proton movement. The rate coefficients for both are linearly related to light intensity. However, the total flux of protons via one of these processes diminishes as a function of the hydrogen ion accumulation within the vesicles.

Bacteriorhodopsins↗
PubMed1982

Photoacoustic photocalorimetry and spectroscopy of Halobacterium halobium purple membranes.

Enthalpy changes associated with intermediates of the photocycle of bacteriorhodopsin (bR) in light-adapted Halobacterium halobium purple membranes, and decay times of these intermediates, are obtained from photoacoustic measurements on purple membrane fragments. Our results, mainly derived from modulation frequency spectra, show changes in the amount of energy stored in the intermediates and in their decay times as a function of pH and/or salt concentration. Especially affected are the slowest step (endothermic) and a spectroscopically unidentified intermediate (both at pH 7). This effect is interpreted in terms of cation binding to the protein, conformational changes of which are thought to be connected with the endothermic process. Wavelength spectra are used to obtain heat dissipation spectra, which allow identification of wavelength regions with varying photoactivity, and estimation of the amounts of enthalpy stored in the photointermediates. Because of bleaching and accumulation of intermediates, however, and because of the small fraction of light energy stored during photocycle, quantitative information cannot be obtained. From photoacoustic wavelength spectra of purple membrane fragments equilibrated at 63% relative humidity, rise and decay times of the bR570 and M412 intermediates are calculated.

Acoustic Stimulation↗
PubMed1982

Effect of furosemide on the electrical parameters of frog skin.

When added to the mucosal solution bathing isolated frog skin at concentrations ranging from 5 X 10(-4) to 3 X 10(-3) M, the diuretic furosemide increased both the active transport of sodium and the electrical potential difference across the tissue in a dose-dependent way. The same effect was observed in chloride-free solutions. Mucosal furosemide also decreased the passive unidirectional fluxes of chloride. We believe that as far as electrical parameters are concerned mucosal furosemide has a double effect in frog skin: it increases the active conductance to sodium across the mucosal membrane, thus increasing active transport, and decreases the passive permeability to chloride, thus altering the passive conductance of the skin. The relative increase in short-circuit current was, however, invariably greater than the increase of the active conductance, suggesting the influence of yet a third effect. The effect of mucosal furosemide on active sodium transport was blocked by amiloride (5 X 1-(-5) M) and was independent of vasopressin. Qualitatively the effect was similar to the effect produced by triphenylmethylphosphonium ion.

Amiloride↗
PubMed1982

Effect of furosemide on the thermodynamic parameters of frog skin.

The formalism of nonequilibrium thermodynamics (NET) was used to analyze the effect of diuretic furosemide on the transport mechanism of frog skin. Mucosal furosemide increased the active conductance of Na+ across the mucosal membrane of the cells, producing an increased transport of Na+ (short-circuit current). Furosemide also increased both the thermodynamic affinity of the metabolic reaction supplying energy to the sodium pump and the degree of coupling between transport and metabolism. It changed the phenomenological cross-coefficient of the NET description as well as the stoichiometric ratio, indicating that its effect cannot be explained simply on the basis of a change in Na+ conductance. The effect on the NET parameters was independent of the presence of either chloride or sulfate as the principal anion in the solutions, and was qualitatively similar to the effect produced by mucosal application of triphenylmethylphosphonium ion.

Animals↗
PubMed1981

Active transport: conditions for linearity and symmetry far from equilibrium.

The impressive linearity of force-flow relationships in epithelial active-transport systems suggests the utility of a linear, nonequilibrium-thermodynamic analysis. We here present a plausibility argument for the appropriateness of such a treatment. Conventional phenomenological equations of nonequilibrium thermodynamics constitute an incomplete description of the processes under study, because a given thermodynamic force may be induced in an infinite number of ways. In general, therefore, flows are nonlinear functions of the forces, and the Onsager reciprocal relations are obeyed only very near equilibrium. If, however, the forces of two coupled processes can be constrained to "proper" pathways, each flow is a linear function of each force, and the phenomenological cross-coefficients are equal far from equilibrium. The nature of such proper pathways is investigated in terms of a simple model of a sodium-active transport system. Where the treatment is appropriate (i.e., for sufficiently small perturbations about a steady state far from equilibrium) it permits a complete thermodynamic characterization of a system, even when only one of the two forces can be controlled experimentally while the other remains constant.

Biological Transport, Active↗
PubMed1981

Reciprocity or near-reciprocity of highly coupled enzymatic processes at the multidimensional inflection point.

It has recently been demonstrated that coupled enzymatic processes may possess, for a particular choice of the state variables, multidimensional inflection points in thermodynamic force-flow space. The conditions for reciprocity in the linear region near such a reference state, which may be far from equilibrium, are of considerable interest. It is shown by examining the associated Hill diagrams that all cycles in which a given pair of forces act contribute a corresponding pair of symmetrical terms to the Jacobian matrix characterizing perturbations about this stationary state. To the extent that these cycles dominate--i.e., to the extent that the system is highly coupled--reciprocity or near-reciprocity will be obeyed. This would be expected to be the case in most biological systems.

Adenosine Triphosphate↗
PubMed1980

Proton and carbon-13 nuclear magnetic resonance studies of the polar lipids of Halobacterium halobium.

The thermotropic behavior and the dynamic properties of the polar lipids of Halobacterium halobium were studied by 1H and 13C NMR. The studies were performed on three different preparations: micellar solutions in deuterated chloroform, multilamellar dispersions in 2H2O, and unilamellar vesicles obtained from the latter by ultrasonication. Due to the methyl side groups in the alkyl chains of these lipids, the 13C spectra are highly resolved in particular in the micellar solution, allowing peak assignment of the alkyl chain carbons. Proton and 13C T1 relaxation and line width measurements were made for all three preparations. In both lamellar dispersions, sharp discontinuities in these parameters were observed around 35 degrees C. This behavior is attributed to an abrupt change in the dynamics of the lipids within the membrane. The variation of the specific relaxation rate 1/NT1 of the 13C nuclei along the hydrocarbon chains of the H. halobium lipids exhibits maxima at the tertiary carbons to which the methyl side groups are attached. These maxima are particularly pronounced in the lamellar phases and suggest that the segmental motion ("kink" formation) at the tertiary carbons is hindered by the presence of the methyl groups.

Halobacterium↗