Effects of ketohexosemia on the ketohexose transport in the small intestine of rats

It was observed previously (Csáky, T.Z. and Fischer, E. (1981) Diabetes 30, 568–574), that sustained hyperglycemia enhances the intestinal transport of aldohexoses; on the other hand, hyperfructosemia affects primarily the transport of fructose. The present study examines in detail the hyperketosemia-induced intestinal ketose transport. Intravenously infused $\text{3-O-}\text{methylfructose}$ produces marked $\text{3-O-}\text{methylfructosemia}$ without concomitant hyperglycemia; in such animals the intestinal transport of both fructose and $\text{3-O-}\text{methylfructose}$ increased. The hyperketosemia-induced increased ketose transport was inhibited by phloretin but only if placed on the serosal compartment. Phlorizin affects neither the basal nor the induced intestinal ketohexose transport. The enhancement of the intestinal ketohexose transport is not sodium-dependent and is not inhibited by ouabain.     

Lactose transport in Escherichia coli cells Dependence of kinetic parameters on the transmembrane electrical potential difference

We determine the kinetic parameters $\text{V}$ and $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ of lactose transport in Escherichia coli cells as a function of the electrical potential difference (Δψ) at pH 7.3 and $\text{Δ}\text{pH}\text{= 0}$. We report that transport occurs simultaneously via two components: a component which exhibits a high $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ (larger than 10 mM) and whose contribution is independent of Δψ, a component which exhibits a low $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ independent of Δψ (0.5 mM) but whose $\text{V}$ increases drastically with increasing Δψ. We associate these components of lactose transport with facilitated diffusion and active transport, respectively. We analyze the dependence upon Δψ of $\text{K}{}_{\mathrm{<mtext>T</mtext>}}$ and $\text{V}$ of the active transport component in terms of a mathematical kinetic model developed by Geck and Heinz (Geck, P. and Heinz, E. (1976) Biochim. Biophys. Acta 443, 49–63). We show that within the framework of this model, the analysis of our data indicates that active transport of lactose takes place with a H⁺/lactose stoichiometry greater than 1, and that the lac carrier in the absence of bound solutes (lactose and proton(s)) is electrically neutral. On the other hand, our data relative to facilitated diffusion tend to indicate that lactose transport via this mechanism is accompanied by a H⁺/lactose stoichiometry smaller than that of active transport. We discuss various implications which result from the existence of H⁺/lactose stoichiometry different for active transport and facilitated diffusion.     

Kinetic mechanism of chlorpromazine inhibition of erythrocyte 3-O-methylglucose transport

The kinetic mechanism of chlorpromazine inhibition of erythrocyte hexose transport was investigated using the non-metabolizable glucose analog $\text{3-O-}\text{methylglucose}$. It was found that chlorpromazine added to the external medium is a non-competitive inhibitor of both equilibrium exchange and net $\text{3-O-}\text{methylglucose}$ transport at pH 7.8, 15°C. The $\text{K}{}_{\mathrm{<mtext>i</mtext>}}$ for equilibrium exchange is $\text{76 ± 21 μM}$. When net efflux and equilibrium exchange were measured on the same population of cells the equilibrium exchange was 2.5-times the maximum net efflux. The percent reduction of $\text{3-O-}\text{methylglucose}$ flux by chlorpromazine is dependent upon chlorpromazine concentration and not $\text{3-O-}\text{methylglucose}$ concentration as expected for a non-competitive inhibitor. Equilibrium exchange and net efflux show the same extent of inhibition at each concentration of chlorpromazine evaluated. These results suggest that exchange and net efflux of $\text{3-O-}\text{methylglucose}$ in the human erythrocyte may share a common transport system.     

The kinetics of ionophore X-537A-mediated transport of manganese through dipalmitoylphosphatidylcholine vesicles A 1H-NMR study

We have studied the kinetics of ionophore X-537A-mediated transport of manganese ions into small unilamellar vesicles formed from dipalmitoylphosphatidylcholine. To follow the transport we used the paramagnetic effect of manganese on the ¹H-NMR signal from choline trimethylammonium groups on the inner phospholipid monolayer. The transport of only one manganese ion produces an intravesicular concentration which is high enough (approx. 1 mM) to substantially broaden this signal. The observed signal thus arises predominantly from those vesicles which contain no manganese. Therefore, as manganese is transported into the vesicles the observed signal decreases in intensity, but does not broaden. The initial time-dependence of the intensity of the signal, $\text{S(t)}$, can be approximated by the simple first-order rate law: $\text{S(t) = S}\text{(O)}\text{exp}\text{(?K′t)}$, where $\text{K′}$ is the probability per unit time for the transport of a manganese ion from the external medium to the intravesicular space. From the dependence of $\text{K′}$ on the ionophore X-537A concentration we conclude that manganese is transported into the vesicles via both 1 : 1 and 2 : 1 complexes with ionophore X-537A. At low ratios of ionophore X-537A to vesicles transport via the 1 : 1 complex predominates; at high ratios transport via the 2 : 1 complex predominates. From the dependence of $\text{K′}$ on manganese concentration we determined that under our conditions the equilibration of ionophore X-537A between vesicles is much faster than the transport of manganese through the vesicles. Lastly, from the dependence of $\text{K′}$ on temperature, we conclude that the ionophore X-537A-mediated transport of manganese into the dipalmitoylphosphatidylcholine vesicles is very sensitive to the gel-liquid crystalline phase transition.     

The loss of the phoS periplasmic protein leads to a change in the specificity of a constitutive inorganic phosphate transport system in Escherichia coli

The phoS periplasmic protein, implicated in alkaline phosphatase regulation, is shown to be involved in inorganic phosphate (Pi) transport in $\text{E. coli}$. Although $\text{pho}\text{S}{}^{\mathrm{?}}$ cells dependent upon the PST system for Pi transport can grow in minimal medium with 1 mM Pi as source of phosphorus, the affinity of these cells for Pi is greatly reduced; Km = 18 μM compared with Km = 0.4 μM for $\text{phoS}{}^{\mathrm{+}}$ cells. $\text{pho}\text{S}{}^{\mathrm{?}}$ cells dependent upon the PST Pi transport system acquire the ability to accumulate Asi from the medium in contrast to $\text{pho}\text{S}{}^{\mathrm{+}}$ cells which exclude this toxic anion. It would appear that the periplasmic phoS protein is not essential for Pi accumulation but is involved in maintaining the specificity of the PST Pi transport system.     

Ammonium (methylammonium) transport by Klebsiella pneumoniae

Klebsiella pneumoniae can accumulate methylammonium up to 80-fold by means of a transport system as indicated by the energy requirement, saturation kinetics and a narrow pH profile around pH 6.8. Methylammonium transport (apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{= 100 μ}\text{M}$, $\text{V = 40 μ}\text{mol/min}$ per g dry weight at 15°C) is competitively inhibited by ammonium (apparent $\text{K}{}_{\mathrm{<mtext>i</mtext>}}\text{= 7 μ}\text{M}$). The low $\text{K}{}_{\mathrm{<mtext>i</mtext>}}$ value and the finding that methylammonium cannot serve as a nitrogen source indicate that ammonium rather than methylammonium is the natural substrate. Uphill transport is driven by a component of the protonmotive force, probably the membrane potential. The transport system is under genetic control; it is partially repressed by amino acids and completely by ammonium. Analysis of mutants suggest that the synthesis of the ammonium transport system is subject to the same ‘nitrogen control’ as nitrogenase and glutamine synthetase.     

Resonance Raman spectroscopy of cytochrome c oxidase and electron transport particles with excitation near the Soret band

We report the resonance Raman spectra of cytochrome $\text{c}$ oxidase, both solubilized and in electron transport particles using laser excitation near the Soret band. As in the spectra of other hemoproteins, such as cytochrome $\text{c}$, the shape and intensity of a number of bands change when the oxidation state is varied. However, one of the hemes of solubilized cytochrome $\text{c}$ oxidase shows redox behavior which is anomalous. Spectra of electron transport particles are dominated by cytochrome $\text{c}$ oxidase. There are, however, definite differences between spectra of solubilized cytochrome $\text{c}$ oxidase and electron transport particles in the oxidized states.     

The in vivo equivalence of a cell-free system for RNA processing and transport

The equivalence of messenger RNA released (transported) from isolated rat liver nuclei to three selected media, with messenger RNA normally released to liver cytoplasm $\text{in vivo}$, has been evaluated by competitive DNA: RNA hybridization. Near normal nuclear restriction was exhibited by nuclei in media fortified with ATP, salts, spermidine and dialyzed cytosol. The RNA transport in the latter system was markedly inhibited by colchicine as was also the transport of RNA $\text{in vivo}$. Both nuclear restriction and sensitivity of the RNA transport to colchicine in media lacking spermidine and cytosol deviated significantly from the $\text{in vivo}$ norm. The results emphasize the importance of establishing the $\text{in vivo}$ equivalence in cell-free systems designed to study RNA synthesis, processing and transport.     

Complexation of Pr3+ by two different ionophores enhances markedly its transport rate

Transport of Pr³⁺ across phosphatidylcholine vesicles, monitored through ³¹P nmr, is first-order in monensin ($\text{1}$), second-order in etheromycin ($\text{2}$) or in lasalocid A ($\text{3}$). When $\text{1}$ and $\text{2}$ (or $\text{2}$ and $\text{3}$) are incorporated $\text{together}$ in 1:1 ratio into the lipidic phase, transport is faster than with each ionophore alone. For instance, assuming that the complexes $\text{2}$.Pr³⁺.$\text{2}$, $\text{3}$.Pr³⁺.$\text{3}$, and $\text{2}$.Pr³⁺$\text{3}$ are equiprobable, they effect transport at intrinsic relative rates of 1, 2, and 13.5, $\text{i.e.}$ a remarkable synergism is set up.     

Action of Solanum malacoxylon on calcium metabolism in the rat

The administration of an aqueous extract of the leaves from $\text{Solanum malacoxylon}$ to vitamin D-deficient rats fed a normal calcium, normal phosphorus diet markedly increased serum calcium concentration within 48 hours. The $\text{Solanum malacoxylon}$ extract also stimulated intestinal calcium transport in the vitamin D-deficient rat but was without effect on the mobilization of calcium from bone. The extract from 100 mg of dry $\text{Solanum malacoxylon}$ leaves was more effective than 25 units of vitamin D given daily to vitamin D-deficient rats in stimulating intestinal calcium transport but its effect was not additive to that of the vitamin D. The results demonstrate that the action of $\text{Solanum malacoxylon}$ is independent of vitamin D and, although it can substitute for vitamin D in the stimulation of intestinal calcium transport activity, it cannot substitute for vitamin D in the mobilization of calcium from bone.     

Double dependence of organic acid active transport in proximal tubules of surviving frog kidney on sodium ions I. Influence of sodium ions in bath medium on the uptake and run out of fluorescein and uranin

With the aid of direct microfluorimetric determination of marker organic anions (fluorescein and uranin) accumulated in the proximal tubules the influence of Na⁺ in the bath medium on the active transport of these anions was studied. Kinetic analysis of the rate dependence of organic acid active transport into tubules on their concentration in the bath medium with a constant Na⁺ concentration permitted to define values of apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and $\text{V}$ for uranin and fluorescein transport in the medium with different Na⁺ content. It was shown that a decrease of Na⁺ concentration in the medium increases $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and lowers the $\text{V/K}{}_{\mathrm{<mtext>m</mtext>}}$ ratio with uncharged $\text{V}$. By varying the Na⁺ concentration in the medium with a constant concentration of the marker anion the and values for fluorescein and uranin transport were determined. A value for fluorescein in twice as much that for uranin. The $\text{1/K}{}_{\mathrm{<mtext>m</mtext>}}$ value for uranin transport is a linear function of Na⁺ concentration, while for fluorescein transport it is a quadratic one. Therefore it is concluded that two Na⁺ from the medium participate in active transfer of one fluorescein anion whereas only one Na⁺ from the medium is required for active transfer of one uranin anion. The run out of fluorescein from tubules preloaded with this acid is sharply reinforced by the Na⁺ omission from the medium. Thus, active transport of organic acids in proximal tubules of frog kidney is Na⁺-dependent, and Na⁺ from the medium is likely to participate directly in formation of a transport complex. When Na⁺ is absent in the medium a carrier fulfils a facilitated diffusion only.     

The inhibitor effect of probenecid and structural analogues on organic anions and chloride permeabilities in ox erythrocytes

Probenecid inhibits anion movements (organic anions and chloride) in ox erythrocytes. The I₅₀ is 4 · 10^?5 M. Structural analogues such as carinamide, $\text{p-}\text{carboxybenzene}$ sulfonamide and $\text{p-}\text{carboxy}$$\text{N,N}$ diethyl benzene sulfonamide, which are drugs of the sulfonamide class, were also found to inhibit anion transport. These results reinforce the previously discussed view based on structural considerations, that sulfonamides act on the red cell membrane as competitors of anion transport. It is possible that probenecid and carinamide act in a similar way in the kidney.     

The detection of a bound ferredoxin in the photosynthetic lamellae of blue-green algae and other oxygen evolving photosynthetic organisms

The presence of an electron transport component with an EPR spectrum similar to that of a ferredoxin has been demonstrated in the blue-green alga $\text{Anabaena}$$\text{cylindrica}$, the green alga $\text{Euglena}$$\text{gracilis}$, and in chloroplasts from sorghum ($\text{Sorghum}$$\text{bicolour}$) and beans ($\text{Phaseolus}$$\text{vulgaris}$). The component is photoreduced at 77°K and is very similar to that previously reported in spinach. It seems likely that this component is a primary electron acceptor in photosynthesis in all of these organisms.     

An allosteric pore model for sugar transport in human erythrocytes

Glucose transport in human erythrocytes is characterized by a marked asymmetry in the $\text{V}$ and $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values for entry and for exit. In addition, they show a high $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and a high $\text{V}$ for equilibrium exchange but low $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ values for infinite cis and for infinite trans exit and entry. An allosteric pore model has been proposed to account for these characteristics. In this model, substrate-induced conformational changes destabilize the interfaces between protein subunits (the pore gates).Pores doubly occupied from inside destabilize the transport gates and result in high $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and high $\text{V}$ transport parameters. This effect is less marked when pores are doubly occupied from outside and therefore transport asymmetry results.     

On the transport of sugars and amino acids by newborn kidney: use of isolated proximal tubule

While the newborn Sprague-Dawley rat has evidence of hyperglycinuria without glycosuria transport studies employing renal cortical slices $\text{in}$$\text{vitro}$ have shown paradoxically no impairment of glycine uptake by tubule cells but an inability to actively transport sugars. In order to eliminate the architecture of the cortical slice as a complicating factor in measuring cellular uptake isolated proximal tubule fragments have been prepared from newborn rats and the cellular uptake of glycine and α-methyl-D-glucoside has been measured. The entry of glycine into the cells of newborn tubules is similar to that in adult tubules and confirms data obtained with slices. On the other hand, concentrative uptake of glucoside though impaired in newborn tubules is easily demonstrable. Isolated tubules of the newborn should provide a model $\text{in}$$\text{vitro}$ system to assess the changing characteristics of sugar transport during development.     

Intestinal calcium and phosphate transport in genetic hypophosphatemic mice

Serum phosphate, serum calcium, intestinal phosphate and intestinal calcium transport were measured in normal ($\text{C57BL}\text{6J}$ +/Y) and genetic (X-linked) hypophosphatemic mice ($\text{C57BL}\text{6J}\text{Hyp}\text{Y}$). The hypophosphatemic mice had low serum phosphorus levels and dramatically decreased intestinal phosphate transport compared with normal controls. On the other hand, normal and hypophosphatemic mice had equivalent levels of intestinal calcium transport. The hypophosphatemic mice did illustrate a slightly decreased serum calcium concentration, however. Administration of 1,25-dihydroxy-vitamin D₃, the principal active metabolite of vitamin D, stimulated intestinal calcium transport but not intestinal phosphate transport in the genetic hypophosphatemic mice. The results obtained are consistent with the hypothesis that the primary metabolic disturbance in familial hypophosphatemia involves a defect in phosphate transport mechanisms.     

Suppression of iron uptake deficiency in Escherichia coli K-12 by loss of two major outer membrane proteins.

A novel iron uptake system was observed in pseudorevertants of $\text{Escherichia}$,$\text{coli}$ strains defective in ferrienterochelin transport. The new system is unique in that it is an active transport system that does not utilize any known siderophore. Acquisition of the new uptake system occurs concomitantly with the loss of two major outer membrane proteins (b and c) believed to function as structural components of transmembrane pores.     

The thermodynamic degree of coupling between metabolism and sodium transport in frog skin

The tightness of coupling between two processes is advantageously evaluated by the thermodynamic degree of coupling $\text{q}$, varying in absolute value from zero for uncoupled processes to unity for processes which are related stoichiometrically. Two methods for the determination of $\text{q}$ in the active pathway in frog skin have been developed, employing amiloride to abolish active sodium transport. The values of $\text{q}$ in 6 frog skins varied, but were always less than unity (mean $\text{0.79 ± 0.06}$ S.E. according to one method, $\text{0.78 ± 0.06}$ S.E. according to the other). This indicates that metabolism and sodium transport are incompletely coupled in this tissue even when passive transepithelial leakage pathways are taken into account.