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At least 19 recordsLinked to original sources
PubMed1978

A new approach in the kinetics of biological transport. The potential of reversible inhibition studies.

Kinetic equations are derived for reversible inhibition of both active and facilitated transport systems for seven common experimental arrangements. It is shown that the unique features of transport kinetics may be exploited to give new kinds of information. It is also shown that that the familiar rules of enzyme kinetics, though often applied to transport, can be seriously misleading. The analysis leads to the following general conclusions: (1) A competitive mechanism frequently gives rise to non-competitive kinetics, depending on the experimental design, but a non-competitive mechanism never produces competitive kinetics. (2) Inhibition studies on exchange diffusion at equilibrium in non-active systems or in the final steady state in active systems are the only unambiguous kinetic tests to distinguish competitive from non-competitive mechanisms. (3) Substrate analogs that are bound to the carrier and transported are readily distinguished by inhibition kinetics from those not transported, even though both may rapidly enter the cell by another route. (4) Even in non-active systems competitive inhibitors commonly have far different affinities for the substrate sites on the two membranes faces: where sufficient non-polarity allows their penetration into the cell, inhibition kinetics readily establish such sideness in their action. (5) Inhibition kinetics of the mixed competitive and non-competitive type result from moderately asymmetrical binding of inhibitor at the substrate site. (6) Asymmetry is a necessary feature of active transport: hence studies of inhibition kinetics should provide important insights into its mechanism.

Binding, Competitive↗
PubMed1978

Cytochalasin B and the kinetics of inhibition of biological transport: a case of asymmetric binding to the glucose carrier.

Cytochalasin B inhibits glucose transport in human erythrocytes by competing with glucose for the carrier on the inner surface of the cell membrane, but there is no cytochalasin site associated with the outware-facing form of the carrier. Such asymmetry may be demonstrated by zero trans exit and entry experiments, whereas Sen-Widdas exit experiments are not easily interpretable. The orientation of the transport system appears to be reversed in certain other cell types: chich embryo fibroblasts, Novikoff hepatoma cells and HeLa cells. Here the cytochalasin site is present in the external but not internal carrier form.

Binding Sites↗
PubMed1975

A model for steroid transport across biological membranes.

The absorption of a range of steroids (water soluble to lipid soluble) in the mouth was investigated under standard conditions and using gas chromatography. A two-compartment open model was used to describe the absorption of these steroids into, and through, the membrane. Rate constants were calculated on this basis using a feathering technique, and were used in an analogue computer program to predict steroid concentration which agree favourably with experimental data. Correlations between absorption and partition data were made in an attempt to relate the proposed model to anatomical features of the absorptive membrane and to make comparison with models proposed to describe drug absorption across the intestinal mucosa.

Absorption↗
PubMed1978

Kinetic properties of electrostatic pores with orientable dipoles, for Na+ and K+ transport through biological membranes.

The model, used previously to account for the transport of K+ ions through squid axon membranes under steady-state conditions, is extended to the description of the kinetic behavior of Na+ and K+ currents, for sudden variations of the applied potential. Theoretical curves are obtained by numerical integration of the electrodiffusion equation for ions within pores, with variable boundary conditions resulting from a progressive reorientation of dipoles at the pore surface. The pores are supposed to be selective and the dipole parameters are allowed to be different for Na+ and K+ pores. The K+ current varies with time, in agreement with the K+ dipole parameters deduced from the steady-state results of Gilbert and Ehrenstein (1969). The dipole parameters for Na+ current are deduced from the steady-state results of Armstrong, Bezanilla & Rojas (1973), where the inactivation phase of the Na+ current is suppressed by introducing pronase in the inside solution. The dipole reorientation is relavent to explain the sigmoid shape of the activation phase of the Na+ current, while the inactivation phase seems to resort to another physical mechanism. The predictions based on this model agree with the experimental results for the steady-state negative resistance and the gating current, associated both with a reorientation of surface dipoles, as well as the activation phase of the Na+ current using a consistent set of parameters for all these comparisons.

Biological Transport↗
PubMed1976

[Chemiosmotic mechanism of transport of biological macromolecules through bacterial membranes].

A general mechanism of the nucleic acids transport through bacterial membranes during genetic transformation, transfection, viral infection and bacterial conjugation, has been developed. The uptake of nucleic acid occurs due to the symport with H+ ions down to an electrochemical potential gradient ("minus" inside) generated by respiration or ATP hydrolysis within recipient cells. The nucleic acid anions of non--lethal viruses are extruded from the negatively charged host cell cytoplasm by electrostatic repulsion. The difference of electrochemical potentials between the conjugating cells cytoplasms is considered as a driving force for the transport of DNA from the donor to the recipient cell.

Adenosine Triphosphatases↗
PubMed1975

Models of ionic transport in biological membranes. Raman spectroscopy as a probe of valinomycin, gramicidin A', and rhodopsin conformations.

There is evidence that membrane proteins can serve as the functional units of ionic transport in biological membranes. Laser Raman spectroscopy has been used to probe specific molecular interactions inside two models of transport membrane proteins, valinomycin and gramicidin A. Conformational changes of these molecules, as well as specific interactions with ions, can be detected and may help elucidate how membrane transport proteins such as Na+ minus K+ ATPase and rhodopsin function. Resonance Raman spectroscopy has also been used to study conformational changes and protein-chromophore interactions in rhodopsin, the membrane protein that acts as the primary unit of visual excitation in the eye.

Adenosine Triphosphatases↗
PubMed1978

Physiology of transport regulation.

The regulation of biological transport is discussed on the basis of studies on sodium transport through amphibian skin. The following types of regulation are briefly considered: 1) Hormonal regulation 2) Regulation of Na entry by apparent or real saturation of entry path by outside Na 3) Regulation of Na transport by changes in resistance to the counter ion (mostly chloride) 4) Role of cellular Na concentration which may act both by controlling the passive entry of Na and by influencing the pumping rate. 5) Dependence of Na entry upon cell volume. It is shown that a moderate osmotic swelling of ouabain-poisoned skins leads to excessive swelling of the whole epithelium when NaCl is present on the outside. This indicates that cell swelling leads to opening of the Na channels, but it also indicates coupling between the different layers of the epithelium.

Animals↗
PubMed1979

Biological amine transport in chromaffin ghosts. Coupling to the transmembrane proton and potential gradients.

The effect of the transmembrane proton gradient (delta pH) and potential gradient (delta psi) upon the rate and extent of amine accumulation was investigated in chromaffin ghosts. The chromaffin ghosts were formed by hypo-osmotic lysis of isolated bovine chromaffin granules and extensive dialysis in order to remove intragranular binding components and dissipate the endogenous electrochemical gradients. Upon ATP addition to suspensions of chromaffin ghosts, a transmembrane proton gradient alone, a transmembrane gradient alone, or both, could be established, depending upon the compositions of the media in which the ghosts were formed and resuspended. When chloride was present in the medium, addition of ATP resulted in the generation of a transmembrane proton gradient, acidic inside of 1 pH unit (measured by [14C]methylamine distribution), and no transmembrane potential (measured by [14C]-thiocyanate distribution). When ATP was added to chromaffin ghosts suspended in a medium in which chloride was substituted by isethionate, a transmembrane potential, inside positive, of 45 mV and no transmembrane proton gradient, was measured. In each medium, the addition of agents known to affect proton or potential gradients, respectively, exerted a predictable mechanism of action. Accumulation of [14C]epinephrine or [14C]5-hydroxytryptamine was over 1 order of magnitude greater in the presence of the transmembrane proton gradient or the transmembrane potential than in the absence of any gradient and, moreover, was related to the magnitude of the proton or potential gradient in a dose-dependent manner. When ghosts were added to a medium containing chloride and isethionate, both a delta pH and delta psi could be generated upon addition of ATP. In this preparation, the maximal rate of amine accumulation was observed. The results indicate that amine accumulation into chromaffin ghosts can occur in the presence of either a transmembrane proton gradient, or a transmembrane potential gradient, and that the maximal rate of accumulation may exist when both components of the protonmotive force are present.

Adenosine Triphosphate↗
PubMed1978

Testing transport models with substrates and reversible inhibitors.

The kinetic behavior of five models for biological transport, only one of which is based on the classical carrier mechanism, is investigated. All give hyperbolic substrate saturation curves in accord with experimental observations on many systems. Several simple kinetic tests with substrates and competitive inhibitors serve to exclude or confirm proposed models. The tests involve measuring rates of efflux of radioactive substrate in the presence of (i) a competitive inhibitor outside the cell; (ii) inhibitor inside and outside; and (iii) unlabeled substrate outside. Rules for testing hypothetical mechanisms are presented in tables which may be consulted directly, disregarding the mathematical derivation.

Biological Transport↗