PubMed Health⌕ Search

Biomedical subjects

M T Tosteson

Publications and source records attributed to M T Tosteson.

29 records · Page 2Linked to original sources
PubMed1984

Formation and properties of tetramers of band 3 protein from human erythrocyte membranes in planar lipid bilayers.

Lipid bilayer experiments were performed in the presence of solubilized band 3 protein from human red cell membranes. Band 3 protein increased the conductance of the lipid membranes by several orders of magnitude. Membrane conductance was found to be dependent on the fourth power of protein concentration. This shows that four band 3 subunits form an ion permeable pathway in the lipid bilayer membranes. It also shows that, in the membranes, the protein molecules undergo an association equilibrium which involves at least the monomer and the tetramer of the protein, relaxation towards equilibrium being rapid on the time scale of the experiment. The increase in bilayer conductance induced by the band 3 tetramer could be inhibited by pretreatment of the protein with several SH-reagents (pCMB, pCMBS, DTNB) which also inhibit water transport across the human red cell membrane. Other SH-reagents which do not influence water transport (iodoacetamide, N-ethylmaleimide) did not show any influence on the band 3 induced conductance increase. A band 3-mediated exchange of anions comparable to that in the erythrocyte membrane did not occur in the system studied by us. Our results suggest that, in the human erythrocyte membrane, a pore formed by the band 3 tetramer could be the pathway responsible for the protein-mediated part of water transport.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1981

Protein interactions with lipid bilayers: the channels of kidney plasma membrane proteolipids.

Proteolipids extracted from bovine kidney plasma membrane induce irreversible changes in the electrical properties of lipid bilayers formed from diphytanoyl phosphatidylcholine. The interaction with the proteolipid produces channels which are cation selective. At low protein concentrations (i.e., less than 0.6 microgram/ml), the single-channel conductance is approximately 10 pS in 100 mM KCl and 3 pS in 100 mM NaCl. In the presence of protein concentrations above 1 microgram/ml, another population of channels appears. These channels have a conductance of about 100 pS in 100 mM KCl and 30 pS in 100 mM NaCl. Further, these channels are voltage dependent in KCl, closing when the voltage is clamped at values greater than or equal to 30 mV. The steady-state membrane conductance, measured at low voltages, was found to increase proportional to a high power (2-3) of the proteolipid concentration present in one of the aqueous phases. In 100 mM NaCl, the conductance increases at protein concentrations above 5 microgram/ml, whereas in 100 mM KCl it increases at protein concentrations above 0.6 microgram/ml. These measurements indicate that the higher steady-state conductance observed in KCl at a given proteolipid concentration in a multi-channel membrane presumably results because more channel incorporate in the presence of KCl than in the presence of NaCl. The two major fractions which comprise the proteolipid complex were also tested on bilayers. It was found that both fractions are required to produce the effects described.

Animals↗
PubMed1981

The sting. Melittin forms channels in lipid bilayers.

Melittin, a toxin of bee venom, is a cationic polypeptide composed of 26 amino acids. The six residues of the C-terminal end are polar and 19 of the 20 residues of the N-terminal end are hydrophobic. Exposure of the lecithin bilayer to melittin results in the formation of channels that are more permeable to anions that to cations. Unilateral addition of melittin produces a voltage-dependent increase in membrane conductance when the side where the polypeptide is present in made positive but not when it is made negative. At a fixed voltage, the conductance increases with the fourth power of the melittin concentration in the aqueous phase. At a fixed peptide concentration, the conductance increases approximately e-fold per 6-mV increase in the electrical potential difference across the membrane. These results suggest that four melittin monomers are needed to form a channel and, furthermore, that a minimum of four equivalent electronic charges need to be displaced by the electrical field to explain the voltage dependence of the conductance.

Bee Venoms↗
PubMed1980

Cholera toxin interactions with lipid bilayers.

The purpose of the experiments described in this paper was to assess the binding of cholera toxin to bilayers containing its receptor, the monosialoganglioside, GMl. The assay was based on the fact that GMl confers on the bilayer a negative surface charge. The magnitude of this surface charge was estimated by measuring the electrical conductance (G) of the bilayers exposed to nonactin-K+ under conditions where G is directly proportional to the potassium concentration in the aqueous solutions immediatey adjacent to the membrane surface. When bilayers were formed from mixtures of GMl and glycerolmonooleate (GMO), it was found that the molar ratio of the lipids in the bilayer was the same as that in the membrane forming solution. It was further found that cholera toxin or the binding subunit of the toxin (choleragenoid) bind to GMO bilayers containing GMl (but not to GMO bilayers containing phosphatidyl serine or disialoganglioside GDla). The value of the apparent dissociation constant for the binding of choleragen to its receptor was found to be 10(-11) M, comparable to values found in intact cells.

Binding Sites↗
PubMed1979

Organotin-mediated exchange diffusion of anions in human red cells.

Organotin cations (R3Sn+) form electrically neutral ion pairs with monovalent anions. It is demonstrated that the tin derivatives induce exchange diffusion of chloride in red cells and resealed ghosts, without any detectable increase of membrane permeability to net movements of chloride ions. The obligatory anion exchange is believed to be due to the permeation of electroneural ion pairs, whereas the organic cation (R3Sn+) has an extremely low membrane permeability. Exchange fluxes of chloride increased with the lipophilicity of the substituting group (R3). At the same molar concentration of organotin, the relative potencies of the tin derivatives as anion carriers (with trimethyltin as a reference) were: methyl 1, ethyl 30, propyl = phenyl 1,00, and butyl 10,000. Tributyltin-mediated anion exchange was studied in detail. The organotin-induced anion transport increased through the sequence: F- less than Cl- less than Br- less than I- = SCN- less than OH-. Partitioning of tributyltin into red cell membranes was greater in iodide than in chloride media (partition coefficients 6.6 and 1.7 x 10(-3) cm, respectively). Bicarbonate, fluoride, nitrate, phosphate, and sulphate did not exchange with chloride in the presence of tributyltin. Chloride exchange fluxes increased linearly with tributylin concentrations up to 10(-5) M, and with chloride concentrations up to at least 0.9 M. The apparent turnover number for tributyltin-mediated chloride exchange increased from 15 to 1,350 s-1 between 0 and 38 degrees C. These figures are minimum turnover numbers, because it is not known what fraction of the organotin in the membrane exists as chloride ion pairs.

Adsorption↗
PubMed1979

Tributyltin-mediated exchange diffusion of halides in lipid bilayers.

This paper describes the effect of tributyltin (TBT) on the inorganic anion permeability of lipid bilayers. When this compound is added in micromolar concentrations to one or both sides of a phosphatidyl ethanolamine (PE) membrane formed in 0.1 M NaCl or KCl (pH 7), there is no change in the electrical conductance. Under these circumstances, the Cl self-exchange flux measured with 36Cl (MCl) increases from a value of approximately 10(-12) mol.cm-2.s-1, to approximately 10(-8) mol.cm-2.s-1. It was further found that the relation between chloride flux and [TBT] and [Cl] can be described as: MCl = B[TBT] [Cl]. When chloride was replaced by an equimolar concentration of different univalent anions in the trans compartment, the heteroexchange flux of chloride followed the sequence: I greater than Br greater than Cl greater than F greater than NO3. Under all experimental conditions tested, the chloride flux was always more than 10(3) times the maximum flux predicted from the value of the membrane conductance, and at least 100 times higher than the expected fluxes of ion pairs (TBT-Cl) diffusing across the unstirred layers. Thus, the mechanism by which tributyltin increases anion permeability in bilayers seems to be that of an obligatory exchange diffusion, with the reaction between tributyltin and the halides occurring at the membrane surface. Measurements of interfacial potentials indicate that tributyltin chloride lowers the positive intrinsic dipole potential of PE membranes by approximately 70 mV (at a TBT concentration of 30 microM) without substantial alteration of other parameters of the bilayer. The estimated adsorption coefficient of TBT-Cl was found to be 3 x 10(-4) cm.

Bromides↗
PubMed1978

Some effects of trinitrocresolate and valinomycin on Na and K transport across thin lipid bilayer membranes: a steady-state analysis with simultaneous tracer and electrical measurements.

This paper describes the effect of trinitrocresolate anions (TNC-) on the electrical conductance (Gm), and tracer-measured unidirectional Na and K fluxes (MNa and MK) across bilayers formed from sheep red cell lipids dissolved in decane. In the absence of TNC-, typical low conductances were observed, while the cation fluxes were too low to measure by our techniques (less than 10(-12) moles cm-2 sec-1). In the presence of TNC- (10(-2) M), Gm increased and TNC- was the main charge carrier in the system. The cationic fluxes were also much increased, but the membranes showed no significant selectivity between K and Na. Furthermore, the Na and K fluxes were at least two orders of magnitude larger than the ionic fluxes calculated from Gm. Thus, almost all of the K and Na transport across the membrane in the presence of TNC- is electrically silent and is probably carried out as KTNC and NaTNC ion pairs. In the presence of valinomycin (10(-6) M) and no TNC-, both the ion fluxes and Gm were 10(3) times larger in KCl than in NaCl, thus exhibiting the characteristic high selectivity of valinomycin for K over Na. In the presence of both valinomycin (10(-6) M) and TNC- (10(-2) M), this selectivity disappeared in that both Gm and MNa in the NaCl system were similar to the respective values in the KCl system. Even under these conditions, most of the Na is still transported by a process which does not carry charge. Both Gm and Mx increased alike and monotonically with increasing temperature over the range 7 to 30 degrees C. In the absence of TNC- the enthalpies of activation were invariably higher in KCl than in NaCl. Addition of TNC- produced equal enthalpies of activation for both Na and K containing systems suggesting a common, temperature-dependent, rate-determining step in charge transfer and the electrically silent cation fluxes.

Biological Transport↗
PubMed1974

Effect of peptide PV on the ionic permeability of lipid bilayer membranes.

THIS PAPER REPORTS THE EFFECTS OF PEPTIDE PV (PRIMARY STRUCTURE: cyclo-(D-val-L-pro-L-val-D-pro)(delta)) on the electrical properties of sheep red cell lipid bilayers. The membrane conductance (G(m)) induced by PV in either Na(+) or K(+) medium is proportional to the concentration of PV in the aqueous phase. The PV concentration required to produce a comparable increase in G(m) in K(+) medium is about 10(4) times greater than for its analogue, valinomycin (val). Although the selectivity sequence for PV and val is similar, K(+) greater, similar Rb(+) > Cs(+) > NH(4) (+) > TI(+) > Na(+) > Li(+); the ratio of GGm in K(+) to that in Na(+) is about 10 for PV compared to > 10(3) for val. When equal concentrations of PV are added to both sides of a bilayer, the membrane current approaches a maximum value independent of voltage when the membrane potential exceeds 100 mV. When PV is added to only one side of a bilayer separating identical salt solutions of either Na(+) or K(+) salts, rectification occurs such that the positive current flows more easily away rather than toward the side containing the carrier. Under these conditions, a large, stable, zero-current potential (VVm) is also observed, with the side containing PV being negative. The magnitude of this VVm is about 90 mV and relatively independent of PV concentration when the latter is larger than 2 Times; 10(-5) M. From a model which assumes that V(m) equals the equilibrium potential for the PV-cation complexes (MS(+)) and that the reaction between PV and cations is at equilibrium on the two membrane surfaces, we compute the permeability of the membrane to free PV to be about 10(-5) cm s(-1), which is about 10(-7) times the permeability of similar membranes to free val. This interpretation is supported by the fact that the observed values of V(m) are in agreement with the calculated equilibrium potential for MS(+) over a wide range of ratios of concentrations of total PV in the two bathing solutions, if the unstirred layers are taken into account in computing the MS(+) concentrations at the membrane surfaces.

Animals↗