John S. Britten

Thallium activation of the (Na+-K+)-activated ATPase of rabbit kidney

Abstract Tl + has been found to replace K + in activation of the (Na + -K + )-sensitive ATPase of rabbit kidney. In comparison to the other known substitutes for K + in activating this ATPase, Tl + is unique in that it has an affinity approx. 10 times greater than K + for the K + activating site.

Transport properties of condensed monolayers

In an earlier paper, monolayer permeability was derived on the basis of the fluctuations that occur in monolayer density at equilibrium. In this paper, the same model is used to derive the monolayer transport properties. First, expressions are developed for the probability and for the frequency of local expansions. These expressions are then used to derive the self-diffusion coefficient (D), the viscosity (η), and the thermal conductivity (κ) of a monolayer in terms of properties that are related to the surface isotherm. Whereas D has a reasonable magnitude, η is extremely small and many orders of magnitude lower than measured surface viscosities. The differences are discussed in terms of the monolayer model and the effect of the subphase. The utility of the model for processes involving natural membranes is also considered.

Adsorption of Albumin on Rabbit Sperm Membranes

SummaryWhen mammalian sperm cells are exposed to solutions of albumin there are changes in the membranes of some species that resemble those that normally occur in the uterus prior to fertilization. We have shown that albumin molecules adsorb on to the membranes of ejaculated rabbit sperm cells, and that the equilibrium binding constant,K, (1) varies inversely with the albumin concentration, (2) is independent of the sperm cell concentration in the range 106–107 per ml, (3) is independent of the time of exposure of the sperm cells to the albumin solution, and (4) decreases in the presence of Ca++ and Mg++ ions. An unusual aspect of the adsorption is that if the albumin concentration is given the symbol [A],K[A] is a constant in our measurements. This means that for virtually the entire range of [A] studied, the sperm cells bind albumin so that half of the available surface is coated and half remains uncoated. This situation is rather remarkable and it suggests a role that adsorption could play in the physical processes preceding fertilization. In purely physical systems, the optimum for the bridging and flocculation of particles that are coated with adsorbed macromolecular films occurs when half of the available surface is covered. The sperm cell appears to provide the optimal situation for interacting with itself or with another surface.

Adsorption kinetics of ovalbumin monolayers

Abstract The adsorption kinetics of ovalbumin monolayers has been studied using simultaneous measurements of surface potential and surface tension. Both measures show that the adsorption rate is greatest at the IEP, and decreases when the molecule is charged. The rate also increases with the concentration of dissolved ovalbumin and with the ionic strength. These results are consistent with an adsorption rate that varies inversely with the surface pressure and is controlled by an asymmetric electrical barrier. The kinetic data also point up the important changes that occur in the properties of ovalbumin films at a surface pressure of about 5 dyn/cm.

225 - The surface compartment model of the steady state excitable membrane☆☆☆

Abstract Membrane bound ions can influence the kinetics of physiological transport processes, and may be the source of unusual transient ionic fluxes. To study these effects we have used the Surface Compartment Model (SCM) approximation: which divides a membrane system into discrete regions: two bulk reservoirs (the inner and outer solutions), two surface layers (on the inner and outer faces of the membrane) and the ultra-thin membrane. Ion flux and conservation equations can be written in terms of the various compartments, assuming that the transport processes are discontinuous and that the fluxes are proportional to the differences in the electro-chemical potential. Using the SCM it can be shown that the concentrations of ions in the surface regions of excitable membranes and the calculated permeabilities of the membranes to these ions are quite different from those that are generally accepted on the basis of bulk concentrations. Furthermore, it appears that small changes in the surface charge can alter the ionic concentrations at the membrane surfaces ad therefore, the ionic fluxes. This is especially true if appreciable numbers of ions are bound to the membrane surfaces and ion exchange processes occur as a result of sudden changes in ionic concentrations or current flow. The physiological processes ( e.g. , excitation and active transport) tht involve unusual ion fluxes in terms of bulk concentrations and potentials, may actually be occurring in the expected directions during transient states brought about by perfusion, electrical currents or chemical reactions.

Effects of cations on biologically active surfaces—specific binding sites in the NaKATPase

Abstract The interactions of a number of heavy ions with a membrane-bound enzyme, the Na+K+ sensitive adenosine triphosphatase, have been studied. The cations were found to inhibit enzymatic activity in either acetate or tris buffer in the order Hg2+ > Ag+ > Cu2+ > Cd2+ > Zn2+ > Pb2+ > Th4+ > La3+ > UO22+ > Ca2+ > Ni2+ > Mn2+ > Co2+ > Tl+. The nature of the inhibitory effects in the cases of Co2+, Mn2+, Ni2+, Pb2+, UO22+, Zn2+, and Tl+ suggests the possibility of four different specific mechanisms of ionic interaction with the enzyme. However, the majority of the cations appear to inhibit by a nonspecific mechanism that is correlated with the oxidation potential of the ion and with the association constants found in the binding of heavy metals to ethylenediamine, histidine, imidazole, and ATP. These results suggest the presence of a histidine near the active center of the enzyme.

The effect of surface charge on interfacial ion transport

Abstract Recent experiments have shown that the rate of ion flow through monolayers and bilayers varies with the surface charge density of the layer and the ionic strength of the aqueous solutions. In this paper, the observed dependence of ion transport on the properties of the two systems is explained by assuming that electrical double-layer theory describes the distribution of ions near an interface during ion flow. The agreement between the two sets of observations and the derived equations suggests that electrical double-layer theory is useful for systems that are not far from equilibrium. In addition, the ability to account for ion transport through monolayers and bilayers in this way suggests that the explanation also applies to related processes in natural membranes.

The Action of Phloridzin and Sugars on (Na+-K+)-Activated ATPase

SummaryThe action of phloridzin and simple sugars on the (Na+−K+)-activated ATPase obtained from rabbit kidney has been studied. Phloridzin 10−4to 10−3m was found to inhibit the enzyme at Na+:K+ ratios less than optimal for enzyme activity, whereas stimulation was noted at Na+:K+ ratios greater than optimal for enzyme activity. Some sugars in concentrations of 0.1 to 0.5m were found to inhibit the (Na+−K+)-activated ATPase. The sugars and related compounds could be ranked according to decreasing inhibitory potency as: D-mannose>D-arabinose, D-xylose>L-xylose>D-glucose>fructose, L-arabinose>D-galactose, myo-inositol, mannitol=0. No stimulatory effect or interaction with K+ was found with these compounds. The action of these substances on the (Na+−K+)-activated ATPase suggests an interaction of actively transported sugars and sodium-potassium transport at the level of the sodium pump that may be important in the biological coupling of the two systems.

Determination of yield stress in surface films of lung extract

Abstract A simple apparatus based on the canal viscometer has been used to measure the yield stress in surface films. The apparatus is described and the two-dimensional flow is analyzed as a function of both the yield stress and the surface viscosity. The yield stress of surface films of lung extracts has been measured as a function of the age of extract solution, and the data show a decrease of both the yield stress and the surface viscosity with age. The experiments indicate that the surfactant present in normal lung has the ability to form a two-dimensional network quite rapidly and that this structure has a yield stress of about 3 dynes/cm.

Electron flow at the polarized mercury-water interface in the presence of membrane fragments rich in Na+−K+-Activated ATPase

SummaryMeasurements of interfacial electron flow indicate that membrane fragments rich in Na+−K+-ATPase are capable of absorbing and releasing electrons in the form of random currents at an electrode surface. The electron transporting system, which functions in the presence or absence of substrate and activating ions, may be part of or in contact with the enzyme system, but it is not related to the ATPase activity. The observed electron transport at an electrode surface resembles physiological electron transport processes in being reversible, in extending over the same range of potential, and in being affected by some of the chemicals that interfere with electron transport and oxidative phosphorylation in mitochondria. Our experiments do not provide sufficient evidence to identify the substances that are responsible for the random currents, but the results suggest that the electro-active substances are similar to those which are involved in the reactions at the second phosphorylation site in mitochondria. Experiments with this technique provide a new approach to the study of the mechanism of biological electron transport processes and their possible relation to ATP synthesis and hydrolysis.