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G F Fuhrmann

Publications and source records attributed to G F Fuhrmann.

At least 37 records · Page 2Linked to original sources
PubMed1984

The mechanism of vanadium action on selective K+-permeability in human erythrocytes.

Low concentrations of chelating agents such as EDTA prevent the air oxidation of vanadyl (VO2+, +4 oxidation state) to vanadate (VO3-, +5 oxidation state). Under these conditions, the ionophore A23187 mediates the rapid entry of vanadyl into human erythrocytes. In the presence of A23187, vanadyl at concentrations in excess of EDTA gives rise to a dramatic increase in K+ permeability, which is very similar to the Gardos Ca2+-induced K+ permeability increase with respect to ion selectivity, response to inhibitors, effects of pH, and stimulation by external K+. In ultrapure media with very low Ca2+, however, vanadyl has no effect on K+ permeability. These experiments suggest that Ca2+ is displaced from EDTA by vanadyl and then enters the cell via A23187 where it triggers the increase in K+ permeability. This hypothesis is confirmed by experiments demonstrating that vanadyl does displace Ca2+ from EDTA. Vanadate, an inhibitor of Ca2+-ATPase, causes a selective increase in K+ permeability in metabolically depleted cells, but the increase is abolished by low concentrations of EDTA, indicating that this effect is also due to entry of extracellular Ca2+. Earlier observations of effects of vanadyl and vanadate on erythrocyte K+ permeability can thus be explained on the basis of inhibition of the Ca2+ pump by vanadium, leading to an increase in intracellular Ca2+ concentration.

Biological Transport↗
PubMed1984

Effects of vanadate on intracellular reduction equivalents in mouse liver and the fate of vanadium in plasma, erythrocytes and liver.

Intraperitoneal injection of 180 mumol sodium vanadate/kg body wt in mice inhibits the in vivo metabolism of drugs by the hepatic monooxygenase, as measured by exhalation of 14CO2 after treatment with appropriately labeled [14C]methacetin. Determinations of hepatic GSH, NADPH and NADH after vanadate injection show an initial and transient decrease of GSH (10 min, 20%), followed by a transient decrease of NADPH (30 min, 23%), followed by a decrease of NADH (40 min, 23% and 60 min, 26%). Rat liver organ spectrophotometry in the dual-wavelength mode shows an immediate response of the NAD(P)H level, which decreases transiently after addition of vanadate. Furthermore, EPR and AAS measurements indicate that vanadium occurs in the plasma in the oxidation states +IV and +V, whereas in intracellular compartments in liver and erythrocytes vanadium exists practically only in the +IV form (vanadyl). The duration of the inhibition for about 2 h coincides well with the transient concentration of vanadate in plasma, which decreases more rapidly than vanadyl. Maximal drug inhibition is associated with the phase of rapid formation of vanadyl in the liver. The experiments are in accordance with the hypothesis that vanadate inhibits the cytochrome P-450 dependent oxidative drug metabolism by diversion of reducing equivalents away from cytochrome P-450. Further evidence for such a hypothesis is provided by in vitro experiments in microsomes. Vanadate causes a dose dependent decrease of ethoxyresorufin deethylation which is reversible by the addition of NADPH.

Acetamides↗
PubMed1983

Vanadium inhibits oxidative drug demethylation in vivo in mice.

Sodium vanadate inhibits the oxidative demethylation of substrates of the cytochrome P-450-dependent monooxygenase system in vivo in mice. [14C]Methacetin and 7-[methoxy-14C] coumarin were used as substrates, and the exhaled 14CO2 was monitored using the technique of the breath test. The inhibition is of short duration and begins to subside after about 10 min. The inhibition is dose-dependent; half-maximal effect is achieved at a dose of approximately 60 mumol/kg. The inhibition pattern is identical for both substrates, although 62% of the label of [14C] methacetin and only 10% of 7-[methoxy-14C] coumarin are enhaled within 1 h. Pretreatment with ascorbic acid (50 mg/kg p.o.) drastically diminishes the observed inhibitory effect of vanadate. Similarly, application of an equimolar dose of vanadyl sulphate produces a comparatively weak retardation of 14CO2 exhalation. The effect of vanadate is thought to occur by its competition for electrons normally transferred to cytochrome P-450.

Acetamides↗
PubMed1982

Purification and properties of a phospholipid acyl hydrolase from plasma membranes of Saccharomyces cerevisiae.

The properties of a phospholipid acyl hydrolase bound to yeast plasma membranes are described in detail. The enzyme is capable of splitting all phospholipids which can be extracted from yeast cells. The specific activity with lysophosphatidylcholine as substrate was much higher than with phosphatidylcholine. With dipalmitoyl phosphatidylcholine as substrate a broad pH optimum was measured between pH 3.0 and 4.5. The membrane-bound enzyme was activated strongly by the anionic detergents SDS, deoxycholate and, to a lesser extent, by cholate. The uncharged detergent Triton X-100 and the zwitterionic detergent SB12 exerted an only slightly activating effect. KCl, NaCl, MgCl2, and CaCl2 were inhibitory in the presence of glycine/acetic acid buffer at pH 4.0. THe enzyme was solubilized by cholate or by SB12 in an active form from the plasma membrane and purified by acetone and (NH4)2SO4 precipitation or gel filtration with Sephadex G-200. THe phospholipid acyl hydrolase was identified as a glycoprotein with an apparent molecular weight of 145,000 by SDS slab gel electrophoresis.

Cell Membrane↗
PubMed1979

Plasma membrane from Candida tropicalis grown on glucose or hexadecane. II. Biochemical properties and substrate-induced alterations.

Isolated plasma membranes from the yeast Candida tropicalis grown on two different carbon sources (glucose or hexadecane), had similar contents of protein (60% of total dry weight), lipid (21-24%) and carbohydrates (16-21%). Sodium dodecyl sulphate gel electrophoresis of the membrane proteins revealed 17 and 19 protein bands, respectively, for glucose and hexadecane grown cells. There were marked differences in RF values and relative peak heights between the two gels. Sterols and free fatty acids were the major components of the plasma membrane lipids. Phospholipid content was less than 2% of total plasma membrane lipids. Membrane microviscosity, as determined by fluorescence polarization, was very high (16.6 P). Fatty acid determination of membrane lipids by gas chromatography showed a significant increase of C16 fatty acids in plasma membranes of cells grown on hexadecane. Reduced-oxidized difference spectra demonstrated the presence of a b-type cytochrome in both Saccharomyces cerevisiae and C. tropicalis plasma membranes. Its concentration in C. tropicalis plasma membranes was three-fold greater in cells grown on hexadecane than in glucose grown cells.

Alkanes↗
PubMed1978

Plasma membranes from Candida tropicalis grown on glucose or hexadecane. I. Isolation, identification and purification.

Plasma membranes from Candida tropicalis grown on glucose or hexadecane were isolated using a method based on the difference in surface charge of mitochondria and plasma membranes. After mechanical disruption of the cells, a fraction consisting of mitochondrial and plasma membrane vesicles was obtained by differential centrifugation. Subsequently the mitochondria were separated from the plasma membrane vesicles by aggregation of the mitochondria at a pH corresponding to their isoelectric point. Additional purification of the isolated plasma membrane vesicles was achieved by osmolysis. Surface charge densities of mitochondria and plasma membranes were determined and showed substrate-dependent differences. The isolated plasma membranes were morphologically characterized by electron microscopy and, as a marker enzyme, the activity of Mg2+-dependent ATPase was determine. By checking for three mitochondrial marker enzymes the plasma membrane fractions were estimated to be 94% pure with regard to mitochondrial contamination.

Adenosine Triphosphatases↗
PubMed1977

The relationship between anion exchange and net anion flow across the human red blood cell membrane.

The conductive (net) anion permeability of human red blood cells was determined from net KCl or K2SO4 effluxes into low K+ media at high valinomycin concentrations, conditions under which the salt efflux is limited primarily by the net anion permeability. Disulfonic stilbenes, inhibitors of anion exchange, also inhibited KCl or K2SO4 efflux under these conditions, but were less effective at lower valinomycin concentrations where K+ permeability is the primary limiting factor. Various concentrations of 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) had similar inhibitory effects on net and exchange sulfate fluxes, both of which were almost completely DIDS sensitive. In the case of Cl-, a high correlation was also found between inhibition of net and exchange fluxes, but in this case about 35% of the net flux was insensitive to DIDS. The net and exchange transport processes differed strikingly in their anion selectivity. Net chloride permeability was only four times as high as net sulfate permeability, whereas chloride exchange is over 10,000 times faster than sulfate exchange. Net OH-permeability, determined by an analogous method, was over four orders of magnitude larger than that of Cl-, but was also sensitive to DIDS. These data and others are discussed in terms of the possibility that a common element may be involved in both net and exchange anion transport.

Bicarbonates↗
PubMed1976

Sugar transport and potassium permeability in yeast plasma membrane vesicles.

Plasma membrane vesicles were isolated from homogenised yeast cells by filtration, differential centrifugation and aggregation of the mitochondrial vesicles at pH 4. As judged by biochemical, cell electrophoretic and electron microscopic criteria a pure plasma membrane vesicle preparation was obtained. The surface charge density of the plasma membrane vesicles is similar to that of intact yeast cells with an isoelectric point below pH 3. The mitochondrial vesicles have a higher negative surface charge density in the alkaline pH range. Their isoelectric point is near pH 4.5, where aggregation is maximal. The yield of vesicles sealed to K+ was maximal at pH 4 and accounted for about one third of the total vesicle volume. The plasma membrane vesicles demonstrate osmotic behaviour, they shrink in NaCl solutions when loosing K+. As in intact yeast cells the entry and exit of sugars like glucose or galactose in plasma membrane vesicles is inhibited by UO22+. Counter transport in plasma membrane vesicles with glucose and mannose and iso-counter transport with glucose suggests that a mobile carrier for sugar transport exists in the plasma membrane. After galactose pathway induction in the yeast cells and subsequent preparation of plasma membrane vesicles the uptake of galactose into the vesicles increased by almost 100% over the control value without galactose induction. This increase is explained by the formation of a specific galactose carrier in the plasma membrane.

Biological Transport, Active↗