Partial characterization of the glucose transport activity in the golgi-rich fraction of fat cells

The glucose transport activity solubilized from the basal and plus insulin forms of the Golgi-rich fraction of adipocytes was partially characterized, and the results were compared with those of the activity obtained from the plus insulin form of the plasma membrane-rich fraction. The transport activity was determined in a cell-free, reconstituted, system. Prior to reconstitution, the activities in the three preparations were all (a) stable at 0°C for at least 4 h, but not at 37°C or above; (b) most stable at pH 7–9, and (c) less stable in Tes than in Tris buffer. After reconstitution, the three activities were all (d) stable at 0°C, (e) most active at pH 5.5, (f) mildly stimulated by divalent cations, (g) unaffected by insulin or 1 mM of several SH-blocking agents, (h) inhibited by heavy metal ions, 10–100 mM of monovalent salts, organic solvents, several sugar isomers, and specific sugar-transport inhibitors. The rates of d-glucose uptake by the three liposome preparations were all inhibited more strongly by 2-deoxy-d-glucose or $\text{3}\text{-O-}\text{methyl-}\text{d}\text{-glucose}$ than by d-glucose. These data indicate that the general properties of the glucose transport activity in the Golgi-rich fraction are similar to those of the activity in the plasma membrane-rich fraction.     

Structure of adenovirus chromatin

The structure of adenovirus type 2 chromatin isolated from wild-type and ts1 virions was investigated by micrococcal nuclease digestion and electron microscopy. Partial digestion of wild-type and ts1 chromatin with micrococcal nuclease generated a multimeric DNA smear devoid of the 200 basepair nucleosome repeating pattern characteristic of cellular chromatin. However, 11 S monomer cores of 150 basepairs were detectable. The chromatin of ts1 (39°C) was more resistant to digestion by micrococcal nuclease. Two-dimensional electrophoresis of the monomer core showed that wild-type core contained protein VII while ts1 (39°C) core contained PVI and PVII. Protein V appears to be located on the variable-length intermonomer region. Crosslinking studies suggest that proteins PVII and VII exist in dimeric form within the monomer core. Electron microscopy revealed a 5.5-fold-condensed two-micron-long beaded structure with about 200 monomer particles spaced irregularly. Based on these observations, a model for adenovirus prochromatin and chromatin is proposed that differs in important aspects from the model proposed previously (Corden, J., Engelking, H.M. and Pearson, G.D. (1976) Proc. Natl. Acad. Sci. USA 73, 401–404).     

Cytochalasin B inhibition and temperature dependence of 3-O-methylglucose transport in fat cells

The transport of $\text{3-O-}\text{methylglucose}$ in white fat cells was measured under equilibrium exchange conditions at $\text{3-O-}\text{methylglucose}$ concentrations up to 50 mM with a previously described method (Vinten, J., Gliemann, J. and Østerlind, K. (1976) J. Biol. Chem. 251, 794–800). Under these conditions the main part of the transport was inhibitable by cytochalasin B. The inhibition was found to be of competitive type with an inhibition constant of about 2.5 · 10^?7 M, both in the absence and in the presence of insulin (1μM). The cytochalasin B-insensitive part of the $\text{3-O-}\text{methylglucose}$ permeability was about 2 · 10^?9 cm · s^?1, and was not affected by insulin. As calculated from the maximum transport capacity, the half saturation constant and the volume/ surface ratio, the maximum permeability of the fat cell membrane to $\text{3-O-}\text{methylglucose}$ at 37°C and in the presence of insulin was 4.3 · 10^?6 cm · s^?1. From the temperature dependence of the maximum transport capacity in the interval 18–37°C and in the presence of insulin, an Arrhenius activation energy of $\text{14.8 ± 0.44}\text{kcal/mol}$ was found. The corresponding value was $\text{13.9 ± 0.89}$ in the absence of insulin. The half saturating concentration of $\text{3-O-}\text{methylglucose}$ was about 6 mM in the temperature interval used, and it was not affected by insulin, although this hormone increased the maximum transport capacity about ten-fold to 1.7 mmol · s^?1 per 1 intracellular water at 37°C.     

Studies on glutathione transport utilizing inside-out vesicle prepared from human erythrocytes

Adenosine triphosphate-dependent glutathione transport was characterized using inside-out vesicles made from human erythrocytes. Kinetic analysis of the glutathione disulfide (GSSG) transport showed a biphasic Line-weaver-Burk plot as a function of GSSG concentration suggesting the operation of two different processes. One phase had a high affinity for GSSG and a low transport velocity. Most active at acidic pH and at 25°C, this transport activity was easily lost during the storage of vesicles at 4°C. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for Mg-ATP was 0.63 mM; guanosine triphosphate (GTP) substituted for ATP gave a 340% stimulation of transport activity. Neither dithiothreitol nor thiol reagents affected this transport process. The other phase had a low affinity for GSSG and a high transport velocity. Most active at pH 7.2 and 37°C, this transport activity was stable during storage of vesicles at 4°C for several days. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for Mg-ATP was 1.25 mM; GTP substituted with no change in activity. Dithiothreitol increased the V but did not alter the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$, and thiol reagents inhibited the transport. These findings suggest that there are two independent transfer processes for GSSG in human erythrocytes.     

Infinite cis influx of cyclic AMP into human erythrocyte ghosts

Infinite cis uptake of cyclic AMP into red blood cell ghosts has been measured. The $\text{K}{}_{\mathrm{<mtext>i</mtext><mtext>c</mtext>}}{}^{\mathrm{<mtext>oi</mtext>}}$ is calculated from two different integrated rate equations that are applicable when the substrate concentration is unsufficient to cause volume changes. Values of 0.69 mM and 0.66 mM are obtained for the infinite cis$\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ at 30°C using these procedures. These values are only slightly higher than that predicted from zero trans net flux experiments.Lowering the temperature reduces $\text{K}{}_{\mathrm{<mtext>i</mtext><mtext>c</mtext>}}{}^{\mathrm{<mtext>oi</mtext>}}$ from 0.69 mM at 30°C to 0.478 mM at 20°C, 0.108 mM at 10°C and 0.072 mM at 4°C ($\text{Q}{}_{10}\text{= 2.4}$). The $\text{Q}{}_{10}$ for activation of influx permeability of 10^?5 M cyclic AMP is 1.55.     

In vitro activation of leukocyte glycogen synthetase

The activity of leukocyte glycogen synthetase in a freshly prepared homogenate is almost completely in the $\text{b}$ form. Incubation of the homogenate at 30°C caused a time dependent increase in the activity measured in the absence of G-6-P ($\text{b}$ to $\text{a}$ conversion). The K_a for G-6-P decreased from 0.7 to 0.01 mM. Freezing of the homogenate resulted in a complete loss of the capacity for activation. These results demonstrate that glycogen synthetase from leukocytes of normal human subjects can be converted $\text{in vitro}$ to a form, which is almost independent of G-6-P for activity.     

The stereospecific d-glucose transport protein in cholate extracts of human erythrocyte membranes Molecular sieve chromatography and estimation of molecular weight

Cholate extracts of human erythrocyte membranes (Lundahl, P., Acevedo, F., Fröman, G. and Phutrakul, S. (1981) Biochim. Biophys. Acta 644, 101–107) were fractionated by molecular sieve chromatography on Sepharose 6B, and the size and molecular weight of the active d-glucose transporter were estimated. The eluent contained 10 or 12.5 mM cholate, since higher concentrations inactivated the glucose transporter, and lower concentrations resulted in aggregation. The chromatographic distribution of the transport activity was reproducible, but was broader than one would expect for a homogeneous component. In the presence of 20 mM EDTA and 5 mM dithioerythritol, a combination which affords a highly stable transport activity, a molecular weight of $\text{400 000 ± 20 000}$ (Stokes' radius 5.9 nm) was estimated for the smallest active component. This value represents an upper limit, since the molecular weight of a non-spherical component would have been overestimated, and since bound cholate was calculated to represent about 12% of the molecular weight. The activity was completely recovered upon rechromatography. In 10 mM EDTA and 10 mM 2-mercaptoethanol, the estimated molecular weight of the smallest active component was $\text{210 000 ± 15 000}$, and this component was not stable upon rechromatography in 10 mM EDTA and 10 mM 2-mercaptoethanol. In the absence of chelating and reducing agents, cholate extracts from membranes which had been kept for 5 days at 4°C showed three additional active components smaller than 200 000 in molecular weight. Most of the phospholipids eluted later than the active components of molecular weight 400 000 or 210 000, in all experiments. Electrophoretic analysis in dodecyl sulfate of the chromatographic eluents indicates that at least one of the band 3-polypeptides (nomenclature according to Steck, T.L. (1974) J. Cell Biol. 62, 1–19) is a component of the active transporter. This band 3-polypeptide, which we denote 3.3, has an apparent molecular weight of 88 000. The stable transporter of molecular weight 400 000 might be a tetramer of the 3.3-polypeptide. Alternatively, a dimer of the 3.3-polypeptide in complex with lipids might account for this molecular weight. If the 3.3-polypeptide is the transporter subunit and if it binds cytochalasin B with high affinity ($\text{1.8 · 10}{}^5$ sites/cell) the recovered activity per 3.3-polypeptide is around 40% A degradation product of the 3.3-component (possibly a 4.5-component) might account for the unstable active transporter of molecular weight 210 000.     

Progesterone-induced inactivation of nuclear estrogen receptor in the hamster uterus is mediated by acid phosphatase

Inactivation of hamster uterine estrogen receptor was assayed in nuclear KCl extracts (30 min, 37°C, pH 7.5) after progesterone treatment $\text{in}\text{vivo}$ for 2h. At very low concentrations ($\text{>}$0.05 mM), molybdate and vanadate blocked the progesterone-induced increase in receptor inactivation. In contrast, only high concentrations ($\text{>}$10 mM) of the inhibitors blocked receptor inactivation in extracts from untreated hamsters. Gel electrophoresis and inhibition curves for phosphatases in nuclear extract demonstrated that acid, rather than alkaline, phosphatase activity is most likely responsible for these effects. These data suggest that progesterone antagonizes estrogen action in the hamster uterus by promoting estrogen receptor dephosphorylation leading to inactivation.     

Hydrophobic interactions of the apo and holo forms of human cobalamin binding proteins

The interactions of transcobalamin II (TC II), intrinsic factor (IF) and R-type binding protein of cobalamin (Cb1, vitamin B₁₂) with the hydrophobic chromatography matrix Phenyl-Sepharose CL-4B were investigated. IF-Cb1 and R-Cb1 complexes were not adsorbed on Phenyl-Sepharose at room temperature or at 4°C with buffer containing 50 mM sodium phosphate, pH 7.4 containing 150 mM sodium chloride. The TC II-Cb1 complex adsorbed and could be eluted with buffer containing 50% $\text{v}\text{v}$ glycerol. IF without Cb1 adsorbed and was eluted with 50% glycerol at room temperature and 4°C. At room temperature, R binder without Cb1 eluted with buffer, but later than the R-Cb1 complex. At 4°C, R binder completely adsorbed to the matrix. TC II-without Cb1 bound to the matrix at 4°C and room temperature and could not be eluted with glycerol. These results suggest that Cb1 binding proteins can be separated and identified based on their hydrophobic properties. In addition, upon binding Cb1, TC II, IF and R-type binders undergo a conformational change such that the protein-Cb1 complex shows reduced hydrophobicity.     

Enzyme loading of electrically homogeneous human red blood cell ghosts prepared by dielectric breakdown

Human red blood cell ghosts were prepared by electrical haemolysis at 0 °C in isotonic solutions using a discharge chamber which was part of a high voltage circuit. The size distribution of the ghosts was normally distributed, the modal ($\text{=}\text{mean}$) volume was approx. 115 μm³, performing the electrical haemolysis in the following solution: 105 mM KCl, 20 mM NaCl, 4 mM MgCl₂, 7.6 mM Na₂H₂PO₄, 2.4 mM Na₂HPO₄, 10 mM glucose, pH 7.2.Resealing was carried out at 0 °C for 10 min (after the haemolytic step) and then for futher 20 min at 37 °C.The mean volume of the ghost preparation could be changed by variation of the phosphate concentration in the above solution replacing a part of NaCl by phosphate (5 mM phosphate: 94 μm³, 15 mM phosphate: 134 μm³).The breakdown voltage of the ghost cell membranes measured with a hydrodynamic focusing Coulter Counter depends on the mean volume ($\text{94 μ}\text{m}{}^3\text{= 1.04 V, 134 μ}\text{m}{}^3\text{= 1.36}\text{V}$). On the other hand, the breakdown voltage is constant throughout each size distribution pointing to an “electrically homogeneous” ghost preparation. The sensitivity of the Coulter Counter to detect electrical inhomogeneities in the membranes of a ghost population is demonstrated by dielectric breakdown measurements of an apparently normally distributed ghost preparation containing two different “electrically homogeneous” ghost populations i.e. with two different breakdown voltages. The ghost cells obtained by electrical haemolysis in the above solution containing 10 mM phosphate were fairly impermeable to sucrose and behave like an ideal osmometer.It is further demonstrated that ghost cells can be loaded with enzymes (e.g. urease) and drugs using this technique and that these loaded ghost cells can be used as bioactive capsules for clinical application.     

DNA complementary to rabbit globin mRNA made by E. coli polymerase I

Incubation of liver microsomes with cytochrome $\text{b}{}_5$, purified after solubilization with detergents, caused an effective incorporation of the cytochrome into the microsomal membranes. The incorporated cytochrome was reducible by NADH and could not be removed by repeated washing with 0.3 M KCl or 10 mM EDTA. The incorporation was much more efficient at 37°C than at 0°C. Trypsin-solubilized cytochrome $\text{b}{}_5$, which lacks the hydrophobic tail of the native protein, could not be inserted into the membranes. These findings confirm the view that the hydrophobic tail of the cytochrome molecule is responsible for its tight binding to the microsomal membranes.     

Characteristics of leucine transport by isolated hepatocytes of antarctic fish at low temperatures

Hepatocytes prepared by collagenase perfusion from Antarctic nototheniid fish of genus Trematomus are active in uptake of [¹⁴C]leucine at 0, 5, and 10°C. The system is saturable with apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ about 1.0 mM. Isoleucine and phenylalanine were major competitors, valine was about one-half as effective, while alanine, glycine and histidine had no effect. Temperature dependency of rates in the 0–10°C range yielded $\text{E}{}_{\mathrm{<mtext>a</mtext>}}\text{= 65}\text{kJ/mol}$ ($\text{Q}{}_{10}\text{= 2.7}$). The average first-order rate constant at 0°C was 0.1 min^?1, one-third the value of 0.3 min^?1 estimated for clearance of [¹⁴C]leucine by liver of these species in vivo. Affinity and specificity agreed well with in vivo data on liver clearance of leucine, both in Antarctic fish at 0°C and in temperate fish acclimated to 10°C and 20°C. The results indicate similar modifications of leucine transport associated with evolutionary cold adaptation and seasonal acclimation in fish.     

Seasonal and temperature-related changes in mitochondrial membranes associated with torpor in the mammalian hibernator Spermophilus richardsonii

Seasonal variations in the thermal response of liver mitochondrial membranes from Richardson's ground squirrels (Spermophilus richardsonii) were determined by measuring succinate-cytochrome $\text{c}$ reductase activity and spin label motion over a temperature range of 2 °C to 35 °C. For seven summer animals from the field the Arrhenius-type plots for enzyme activity and spin label motion were biphasic indicating a transition in structure and function at $\text{22 + 2.3°C}\text{and}\text{23 ± 1.9°C}$, respectively; typical of homeothermic mammals. For 12 winter animals maintained at 19°C, the transition in structure and function was lowered to $\text{12 ± 1.1°C}$ and $\text{13 ± 1.4°C}$, respectively. The transition for 5 of 11 winter animals which were kept at 4°C and maintained normal activity and body temperature was similar to animals maintained at 19°C, while for the other six the transition was further lowered to less than 4°C. The transition for seven winter animals which were in deep hibernation was less than 4°C. The results for liver mitochondria show that lowering of the transition in membrane structure and function occurs as a two-stage process of about 10 deg. C for each stage and that the lowering is a requisite for hibernation rather than a response to the low-body temperatures experienced during hibernation.     

Rate-limiting steps of 2-deoxyglucose uptake in rat adipocytes

2-Deoxy[1-¹⁴C]glucose uptake in rat adipocytes was measured as a function of time in the absence and presence of unlabelled glucose or 2-deoxyglucose. Uptake of tracer alone was linear from 2 s to 6 min. At 37°C the rate of uptake in insulin-stimulated cells decreased markedly after a few seconds in the presence of glucose (0.5–10 mM) and after 0.5–2 min in the presence of deoxyglucose (2–10 mM). Similar data were obtained at 22°C. With 10 mM glucose (37°C, 30 s) approx. 80% of the intracellular radioactivity was non-phosphorylated deoxyglucose and with 10 mM deoxyglucose approx. 40% was non-phosphorylated. The results show that deoxy[¹⁴C]glucose uptake after a few minutes is mainly limited by hexokinase in the presence of glucose and at least partially in the presence of deoxyglucose. The data suggest caution in using deoxyglucose uptake as a measure of transport, especially in complex kinetic studies.In addition, the initial velocity of tracer ¹$\text{3-O-}\text{methylglucose}$ was found to be approx. 2-fold higher than that of tracer deoxyglucose even though both sugars inhibited the initial velocity of labelled methylglucose half-maximally at a concentration of 5 mM. These data suggest a fundamental difference between deoxyglucose and methylglucose transport.     

Guanylate cyclase from Dictyosteliumdiscoideum

Guanylate cyclase from crude homogenates of vegetative $\text{Dictyostelium}$$\text{discoideum}$ has been characterized. It has a pH optimum of 8.0, temperature optimum of 25°C and requires 1 mM dithiothreitol for optimal activity. It strongly prefers Mn⁺⁺ to Mg⁺⁺ as divalent cation, requires Mn⁺⁺ in excess of GTP for detectable activity, and is inhibited by high Mn⁺⁺ concentrations. It has an apparent Km for GTP of approximately 517 μM at 1 mM excess Mn⁺⁺.The specific activity of guanylate cyclase in vegetative homogenates is 50–80 pmoles cGMP formed/min/mg protein. Most of the vegetative activity is found in the supernatant of a 100,000 x g spin (S100). The enzyme is relatively unstable. It loses 40% of its activity after 3 hours storage on ice. Enzyme activity was measured from cells that had been shaken in phosphate buffer for various times. It was found that the specific activity changed little for at least 8 hours. Cyclic AMP at 10^?4 M did not affect the guanylate cyclase activity from crude homogenates of vegetative or 6 hour phosphate-shaken cells.     

Modification of ribulose bisphosphate carboxylase by 2,3-butadione

D-ribulose-1,5-bisphosphate carboxylases purified from barley or formate-grown $\text{Pseudomonas oxalaticus}$ were inactivated by 2,3-butadione. Pseudo first-order inactivation depended on the presence of borate and was reduced by product 3-phosphoglycerate. The half-times at 30°C and pH 8.3 in the presence of 2 mM 2,3-butadione are 10 and 60 minutes for the enzymes from $\text{P. oxalaticus}$ and barley, respectively. Saturation kinetics and arginine modification were demonstrated for the enzyme from $\text{P. oxalaticus}$.     

Cellular pH and water permeability control in frog urinary bladder a possible action on the water pathway

Mucosal acidification (from pH 8.1 to 6.0) reversibly inhibited the hydroosmotic responses to oxytocin, cyclic AMP and 8-bromo-cyclic AMP in frog urinary bladder. These inhibitory effects were only observed in the presence of a permeant buffer in the apical medium and could also be elicited by CO₂ bubbling, even when the mucosal pH was clamped at 8.1. Acid pH reduced the oxytocin-induced net water flux faster than norepinephrine or oxytocin removal and the difference was especially important at low temperature. The time course of recovery from acid pH inhibition was, at 20°C, similar to that of the hormonal action, but when the medium temperature was reduced to 6–7°C, the recovery from acid pH inhibition paradoxically became faster while the oxytocin action was markedly slowed down ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of changes in net water fluxes (expressed in min): oxytocin addition at 20°C, $\text{6.2 ± 0.9}$; at 6°C, $\text{24 ± 3}$; oxytocin removal at 20°C, $\text{4.7 ± 0.8}$; at 6°C, $\text{22 ± 3}$; pH inhibition at 20°C, $\text{2.6 ± 0.2}$; at 6°C $\text{2.5 ± 0.2}$; recovery from pH 6 at 20°C, $\text{6.5 ± 0.9}$; at 6°C, $\text{2.7 ± 0.3}$). These results can be explained by accepting two main loci sensitive to medium acidification: (1) the cyclase system and (2) an intracellular, temperature-independent, post-cyclic AMP site. The fact that the intramembranous particle aggregates associated with the oxytocin-induced water permeability increase did not disappear after the flow inhibition by acid pH at low temperature suggests that the second effect could be located at the water channel itself.     

Differential scanning calorimetry and enzymic activity of rat liver microsomes in the presence and absence of Δ1-tetrahydrocannabinol

The thermal transitions of rat liver microsomes and isolated lipids were investigated by using differential scanning calorimetry. Endothermic transitions at ≈?5°C and between ≈18° and 40°C were detected in the membranes and at ≈?10°C and between ≈10° and 20°C in the extracted lipids.Interaction with $\text{Δ}{}^1\text{-}\text{tetrahydrocannabinol}$ of microsomal membranes and of extracted lipids influences the thermotrophic behaviour as revealed by differential scanning calorimetry and eliminates the break in the Arrhenius plot of the enzymic activity of $\text{O-}\text{demethylase}$.