Thermodynamic Gradients Research Articles

ION CHANNELS IN LIPOSOMES

It may be reasonably argued that of all classes of known cellular proteins, the ion channels are the least understood biochemically. Of the large number of channel proteins known from cellular electrical behavior to exist in the mem­ branes of higher organisms, only three-the nicotinic acetylcholine receptor, the Na+ channel, and the mitochondrial porin channel-have been obtained in a relatively purified state (28). Even in these cases, serious questions remain about the physiologically correct functioning of these purified proteins. A single peculiarity of ion channel proteins explains this unhappy situation: the lack of a suitable functional assay for integral membrane channels. Seeking to purify an enzyme, for instance, a biochemist can straightforwardly develop a specific assay to pursue and eventually to apprehend the protein with a battery of purification methods. But the only criterion of functionality of an ion channel is its ability to make a membrane leaky to specific ions. To be sure, this leak may be turned on and off in very specific ways and may be modified pharmacologically, but the fact remains that ion channels, by their very definition, merely catalyze the passive flow of ions down their thermodynamic gradients. It is therefore necessary, in attacking purified channel proteins, to develop a system in which the flow of ions across a membrane may be detected. Histor­ ically, two such model membrane systems have been employed: planar bilayers and liposomes (10, 22, 26, 27, 32-36). The planar bilayer system has been largely favored for ion channel work because of its obvious advantages: the ability to voltage-clamp the model membrane and to measure single channel current fluctuations, and the easy accessibility to both aqueous phases. But there are two serious disadvantages in the use of planar bilayers in biochemical

Temperature gradient and electric field driven electrostatic instabilities

The stability of electrostatic waves to thermodynamic and electric potential gradients is investigated. A general instability parameter, ε = 2.6(εE+εp) +0.2εT, is identified which is related to thermodynamic gradients (εp and εT) and internal electric fields (εE). This parameter is evaluated at the instability threshold as a function of Te/Ti for both ion acoustic and electrostatic ion cyclotron waves. The major virtue of this analysis, other than its overall generality, is to show that thermodynamic gradients drive instabilities even when the internal electric field vanishes. This result does not emerge from previous analyses because skewing of the distribution function was not included in the dielectric.

Mechanism of nitrite oxidation and oxidoreductase systems inNitrobacter agilis

Chemiosmotic coupling mechanisms operate in the electron transfer reactions from: nitrite to O2, NO2 − to NAD+, ascorbate to O2, NADH to O2, and NADH to NO3 −. The enzyme systems catalyzing these reactions are named NO2 −:O2 oxidoreductase, ATP-dependent NO2 −:NAD+ oxidoreductase, ascorbate:O2 oxidoreductase, NADH:O2 oxidoreductase, and NADH:NO3 − oxidoreductase, respectively. All of the oxidoreduction reactions are exergonic with the exception of the ATP-dependent NO2 −:NAD+ oxidoreductase system, which involves reversed electron flow against the thermodynamic gradients. The mechanism for nitrite oxidation was found to be quite different from that of ascorbate oxidation; both systems were insensitive, however, to rotenone, amytal, antimycin A, and 2-n-heptyl 4-hydroxyquinolineN-oxide. These compounds, on the other hand, severely inhibited the electron transfer reactions catalyzed by NADH:O2 oxidoreductase, NADH:NO3 − oxidoreductase, and the ATP-dependent NO2 −:NAD+ oxidoreductase, indicating a common pathway of electron transport in these oxidoreductase systems. Cyanide inhibited all systems except the NADH:NO3 − oxidoredctase. The uncoupler carbonyl cyanide-m-chlorophenyl hydrazone strongly inhibited NO2 −:O2 oxidoreductase and ATP-dependent NO2 −:NAD+ oxidoreductase, which indicates the involvement of energy-linked reactions in both systems; the uncoupler caused a marked stimulation of the NADH:O2 oxidoreductase and NADH:NO3 − oxidoreductase without affecting the ascorbate:O2 oxidoreductase activities.

Theory of nonlinear transport processes in a dilute gaseous mixture

The modified moment method is applied for studying nonlinear transport processes in a dilute gaseous mixture described by the Boltzmann equation. The method supplies a variational functional which may be regarded as a nonlinear generalization of the Rayleigh–Onsager variational functional for linear irreversible processes. A variational principle is formulated with the variational functional, which yields a set of nonlinear equations for fluxes as the condition for the functional to be an extremum. By solving the set iteratively, we obtain nonlinear constitutive relations and transport coefficients which reduce to their linear counterpart as thermodynamic gradients decrease in magnitude. The Knudsen gas limits of the transport coefficients obtained are also briefly discussed, which show a slightly different behavior from that predicted by the collisionless Boltzmann equation.

The insulin transduction system: A biophysical model for mitogenesis

A model has been proposed to account for many of the intracellular actions of insulin. This model is called the insulin transduction system and consists of coordinated stimulation by insulin of the sodium pump and the Na:H exchange system in the plasma membrane. The primary end result of stimulation of the insulin transduction system is an elevation of intracellular pH, pHi. This model is a biophysical system which depends upon thermodynamic gradients across the plasma membrane and thus is a property only of the intact cell. Insulin is one of several agents that produce mitogenesis, that is, stimulate DNA synthesis and division in cultured cells. Arguments are presented that mitogenesis is triggered by activation of the insulin transduction system with a resulting elevation of PHi.

The phosphorylation potentials generated by respiring Ehrlich ascites tumor mitochondria

The extra- and intramitochondrial phosphorylation potentials (Δ G p(out) and Δ G p(in), respectively) generated by respiring Ehrlich ascites tumor mitochondria were determined, using succinate, pyruvate + malate, ascorbate + N,N,N′,N′-tetramethyl- p-phenylenediamine, and ascorbate + ferrocyanide as substrate systems. Values of Δ G p(out) exceeding 15 kcal mol −1 (62.8 kJ mol −1) in post-ADP state 4 respiration were found with succinate as substrate, in agreement with data on normal rat liver mitochondria. ΔG p(out) values exceeding 15 kcal mol −1 (62.8 kJ mol −1) were also observed with ascorbate + TMPD or ascorbate + ferrocyanide as substrates. Slightly lower values of Δ G p(out) were found with the NAD-linked substrates pyruvate + malate. The intramitochondrial Δ G p(in) developed by respiring Ehrlich ascites tumor mitochondria respiring on succinate approached 12 kcal mol −1 (50.2 kJ mol −1), in agreement with reported values on rat liver mitochondria. The prior accumulation of Ca 2+ and phosphate by the Ehrlich cell mitochondria did not lower the extramitochondrial Δ G p(out) developed after a subsequent addition of ADP. Although the rate of oxidative phosphorylation of Ehrlich ascites tumor cells is reduced by intramitochondrial Ca 2+ and phosphate ( Villalobo and Lehninger (1980) J. Biol. Chem., 255, 2457–2464 ) they are still capable of generating ATP in the suspending medium against a high thermodynamic gradient, as expressed by the [ATP]/[ADP][ P i]mass action ratio.

Mutual friction in the laminar flow of superfluid helium II through capillary tubes

The Hall-Vinen-Bekarevich-Khalatnikov theory is applied to the laminar flow of superfluid helium through capillary tubes. Velocity profiles obtained for the superfluid are interpreted in terms of the motion of vortex rings. The thermodynamic potential gradient as a function of the average superfluid and normal fluid velocities compares favourably with recent experimental results. It is concluded that the vortex rings originate at the wall and disappear at the tube axis.

Nonlinear transport in the Boltzmann limit

Formal expressions for the irreversible fluxes of a simple fluid are obtained as functionals of the thermodynamic forces and local equilibrium time correlation functions. The Boltzmann limit of the correlation functions is shown to yield expressions for the irreversible fluxes equivalent to those obtained from the nonlinear Boltzmann kinetic equation. Specifically, for states near equilibrium, the fluxes may be formally expanded in powers of the thermodynamic gradients and the associated transport coefficients identified as integrals of time correlation functions. It is proved explicitly through nonlinear Burnett order that the time correlation function expressions for these transport coefficients agree with those of the Chapman-Enskog expansion of the nonlinear Boltzmann equation. For states far from equilibrium the local equilibrium time correlation functions are determined in the Boltzmann limit and a similar equivalence to the Boltzmann equation solution is established. Other formal representations of the fluxes are indicated; in particular, a projection operator form and its Boltzmann limit are discussed. As an example, the nonequilibrium correlation functions for steady shear flow are calculated exactly in the Boltzmann limit for Maxwell molecules.

Chlorophyll and photosynthesis

Photosynthesis is the prerequisite of all life on earth. Chlorophyll fulfils the requirements for photosynthesis: the absorption of visible light, the photochemical capabilities, a rich supply of redox levels and chemical stability. The biosynthetic pathway of chlorophyll can be read as the evolutionary history of photosynthesis. Our exegesis is that the primary porphyrins served for early photosynthesis. The porphyrins readily photo-oxidize organic compounds under the reducing, aqueous conditions of this early era. The formation of oxidized substances in a reducing atmosphere supplied the thermodynamic gradient necessary for organized life processes. Conversely, the closed-shell metalloporphyrins, notable magnesium porphyrins, are powerful photoreducing agents. When coupled to the ultimate electron source, water, oxygen was produced and the modern era of photosynthesis was born. At the same time, the efficiency and usefulness of the photopigments was increased by incorporating them into the organized cellular system of membranes. The clear gradient of ionic to hydrophophic structures along the biosynthetic pathway from porphyrins to chlorophyll supports this view. Experimental evidence on the photochemistry of porphyrins pigments in solution and in lipid bilayers form the basis for these arguments. In this way we can relate the structure of chlorophyll to its function in photosynthesis.

Unimportance of counterflux in the energetics of "downhill" transport.

Adam Kepes suggested that the cellular transport and hydrolysis of orthonitrophenyl-beta-d-galactopyranoside is powered by the counterflux of the d-galactose resulting from beta-galactosidase action within the cell. His explanation would rationalize the unique insensitivity of this galactoside transport to energy poisons such as azide. But contrary to the predictions of this hypothesis, (i) there is no initial large inhibition that progressively lessens as galactose is produced. This was shown with a double wavelength stopped-flow spectrophotometer developed to eliminate interference from turbidity transients. (ii) The azide sensitivity does not increase with an external concentration of galactose sufficient to reverse the thermodynamic gradient. (iii) Mutation in galactose utilization or growth on highly catabolite-repressing regimens did not increase the azide sensitivity, and induction of galactose transport and metabolism did not decrease azide sensitivity. It was found that Kepes measurements must have contained two artifacts. One is that the control rate of hydrolysis decreases with time as the dense cell suspension becomes anaerobic. The other is that azide causes turbidity changes for some time after its introduction. If the former is avoided by magnetic stirring and the latter by double wavelength spectrophotometry or controls without substrate, the inhibition is constant from the earliest time that can be measured. It is therefore concluded that energy-unstarved cells, exposed to azide, still have adequate energy reserves to couple to the downhill transport, although their potential is not adequate to drive accumulation against a concentration gradient.

Kinetic equations for weak coupling brownian motion in a stationary temperature gradient

A non-isothermal kinetic equation for the distribution function of a sub-system weakly coupled to a bath is derived by modification of the analysis and assumptions of a previous paper [1]. The equation has the form of a generalized non-isothermal Fokker-Planck equation when it is linearized in thermodynamic gradients and only terms through second order in the coupling parameters are retained. Higher order terms in the coupling parameter do not diverge with time. The equation is compared with certain ‘exact’ model results of Ullersma and with the coherence time method. The equation is used to calculate a jump rate for diffusion of a harmonic particle weakly coupled to a lattice and it is found that the jump rate becomes independent of the mass of the particle for a heavy enough particle. The source of the discrepancy of this result with a similar calculation of Prigogine and Bak is indicated. The model of the jump rate is inappropriate for diffusion in a thermal gradient and more appropriate models of the ...

Measurement of the membrane potential and evidence for active transport of ions in Chlorella pyrenoidosa

1. 1. The internal concentrations of K +, Na + and Cl − have been determined for cells of Chlorella pyrenoidosa under fixed external conditions. The ratio of internal to external concentrations for light-treated cells gave equilibrium potentials of E Na + = −3 mV , E K + = −71 mV and E Cl − = +8 mV . 2. 2. In the dark the internal Na + and Cl − levels fell to about 30% of the light values but the K + content remained unchanged. 3. 3. A method was devised by which microelectrodes were inserted into individual Chlorella cells held at the tip of a suction pipette. 4. 4. A mean potential of −40 mV suggested that this organism accumulated K + and Cl − and extruded Na + against the thermodynamic gradient. 5. 5. Variation of the external ratio of K + to Na + resulted in changes of membrane potential. It was found that increasing the external K + concentration depolarised the potential and an estimate of the ratio of the permeability coefficients of Na + to K + gave values of about 0.1.