Some properties of an SH group essential for choline transport in human erythrocytes

[The individual and combined antioxidant effects of citicoline and ethylmethylhydroxypyridini succinas]

AIM

To study the antioxidant status of patients with chronic cerebral ischemia (CCI) during the individual treatment with 2-ethyl-6-methyl-3-hydroxypyridine-succinate (neurox) and in the combination with citicoline (neipilept).

MATERIAL AND METHODS

A study included 40 patients, 18 men and 22 women, aged from 54 to 72 years, with CCI, stage 2, at the decompensation stage complicated with the hypertensive crisis and/or arrhythmia.

RESULTS AND CONCLUSION

A significant increase in the serum superoxide dismutase activity after the complex therapy with neurox and neipilept was demonstrated compared to patients treated with neurox. A study of reduced sulfur-hydroxy groups in patients treated with 2-ethyl-6-methyl-3-hydroxypyridine-succinate and patients treated with the combination of 2-ethyl-6-methyl-3-hydroxypyridine-succinate and citicoline, revealed a significant increase in the number of reduced SH- groups after the treatment with neurox compared to the combined use of neurox and neipilept.

The transport of choline

Choline has many physiological functions throughout the body that are dependent on its available local supply. However, since choline is a charged hydrophilic cation, transport mechanisms are required for it to cross biological membranes. Choline transport is required for cellular membrane construction and is the rate-limiting step for acetylcholine production. Transport mechanisms include: (1) sodium-dependent high-affinity uptake mechanism in synaptosomes, (2) sodium-independent low-affinity mechanism on cellular membranes, and (3) unique choline uptake mechanisms (e.g., blood-brain barrier choline transport). A comprehensive overview of choline transport studies is provided. This review article examines landmark and current choline transport studies, molecular mapping, and molecular identification of these carriers. Information regarding the choline-binding site is presented by reviewing choline structural analog (hemicholinium-3 and 15, and other nitrogen/methyl-hydroxyl compounds) inhibition studies. Choline transport in Alzheimer's disease, brain ischemic events, and aging is also discussed. Emphasis throughout the article is placed on targeting the choline transporter in disease and use of this carrier as a drug delivery vector.

Modulation of high-affinity choline carrier activity following incubation of rat hippocampal synaptosomes with hemicholinium-3

Membrane carriers display structural and functional asymmetry with a substrate binding site which can be oriented alternately, but not simultaneously, to the extracellular and intracellular environment. Hemicholinium-3 is an inhibitor of the high-affinity choline carrier in cholinergic nerve terminals which binds to the transporter at the outer membrane surface but is not taken up into the cell. In the present study, we investigated the decline in choline transport which occurs during the first few minutes cholinergic nerve terminals are incubated in physiological salt solutions. Following incubation of rat hippocampal synaptosomes with hemicholinium-3, samples were washed free of the inhibitor and high-affinity choline uptake was measured. Choline uptake into hemicholinium-treated nerve terminals was significantly greater than control (132 +/- 4%). This effect appeared not to be due to an increase in uptake of choline above initial values in the hemicholinium-treated synaptosomes, but to a decrease in choline carrier activity in control samples by more than 25% during the first few minutes of incubation. Addition of hemicholinium-3 to samples after the preincubation induced decrease in choline uptake, followed by a wash period to remove the inhibitor resulted in elevation of choline uptake levels to initial levels. The effect of hemicholinium-3 was concentration-dependent, requiring near saturating concentrations of the inhibitor to elicit the effect. Measurement of acetylcholine content of synaptosomes at different points during the incubation procedure revealed that there was a trend for transmitter levels to vary inversely compared to choline uptake activity, but the differences were not statistically significant during treatments when significant changes in transport activity were measured.

Increased permeability to choline in simian erythrocytes after Plasmodium knowlesi infection

The permeability of simian erythrocytes to choline was found to be considerably increased after infection by the malaria parasite, Plasmodium knowlesi. Choline entry occurs by a facilitated-diffusion system involving a carrier, which displays temperature-dependence, saturability with choline (Km = 8.5 +/- 0.7 microM) and specificity. This carrier can also be inhibited by a thiol reagent, N-ethylmaleimide, at an inactivation rate which is, in the absence of choline, the same as in normal erythrocytes. Inactivation by N-ethylmaleimide can be accelerated by external choline and prevented by decamethonium, which acts as an inhibitor of choline entry in infected cells (as with dodecyltrimethylammonium). Both ethanolamine and imidazole act as inhibitors or activators of choline entry in infected erythrocytes, depending on the relative concentrations of choline and of the competing compound (i.e. ethanolamine or imidazole). After infection, the maximum velocity reached 2.84 +/- 0.5 nmol/min per 10(10) infected cells, which is more than 10 times the Vmax. of normal erythrocytes. Impairing the biosynthesis of phosphatidylcholine de novo in Plasmodium-infected erythrocytes by various methods (glucose or ATP depletion, high ethanolamine concentrations) did not result in any alteration of choline transport (Km or Vmax.), indicating that the constant triggering and transformation of choline into phosphatidylcholine by the parasite is not directly responsible for the increase in the choline transport rate after infection. This high increase in choline transport activity is more likely related to modifications in choline carriers and/or in their environment after Plasmodium infection.

The choline carrier of erythrocytes: location of the NEM-reactive thiol group in the inner gated channel

Choline transport across the human erythrocyte membrane is irreversibly inhibited when N-ethylmaleimide (NEM) reacts with a carrier SH group which is located outside the substrate site, and which is exposed in the inward-facing form of the carrier but prevented from reacting in the outward-facing form. The location of the SH group with respect to the membrane has now been determined by studying the dependence of the NEM-alkylation rate on the intracellular and extracellular pH. The results show that the reactive SH group equilibrates with hydrogen ions in the cytoplasm, but is completely isolated from hydrogen ions in the external medium. With this added evidence it becomes possible to conclude that the SH group is located in the inner gated channel of the choline carrier.

Effects on transport of rapidly penetrating, competing substrates: activation and inhibition of the choline carrier in erythrocytes by imidazole

The properties of the choline transport system are fundamentally altered in saline solution containing 5 mM imidazole buffer instead of 5 mM phosphate: (i) The system no longer exhibits accelerated exchange. (ii) Choline in the external compartment fails to increase the rate of inactivation of the carrier by N-ethylmaleimide. (iii) Depending on the relative concentrations of choline and imidazole, transport may be activated or inhibited. The maximum rates are increased more than fivefold by imidazole, but at moderate substrate concentrations activation is observed with low concentrations of imidazole and inhibition with high concentrations. (iv) The imidazole effect is asymmetric, there being a greater tendency to activate exit than entry. All this behavior is predicted by the carrier model if imidazole is a substrate of the choline carrier having a high maximum transport rate but a relatively low affinity, and if imidazole rapidly enters the cell by simple diffusion, so that it can add to carrier sites on both sides of the membrane. Addition at the cis side inhibits, and at the trans side activates. According to the carrier model, asymmetry is a necessary consequence of the potassium ion gradient in red cells, potassium ion being another substrate of the choline system.

Characterisation in vivo of the reactive thiol groups of the lactose permease from Escherichia coli and a mutant; exposure, reactivity and the effects of substrate binding

The reactivity and accessibility of the reactive thiol groups of the native lactose permease and a mutant have been studied in a number of circumstances and with a number of reagents, in particular using the specific thiol-disulphide exchange reaction. Seven different reactive states of the thiol in the native protein have been characterised by their different second-order rate constants. Interconversion between these states is dependent on the magnitude of the protonmotive force, pH and substrate binding. In the absence of galactoside, reactivity is controlled by an ionisation with apparent pKa 9.3. This pKa is not affected by the protonmotive force, but it is lowered in the presence of external galactoside. The conformation adopted by the permease when in equilibrium with saturating galactoside appears to be different from that of the intermediate that accumulates during net turnover. In the former state, the reactivity of the thiol group is depressed, whereas in the latter state it is enhanced. The thiol group of the native protein is buried in a hydrophobic environment that has a dielectric constant considerably lower than that of water. The environment is not greatly perturbed by changes in the magnitude of the protonmotive force, but it is affected by the binding of galactoside. In a strain which carries the YUN mutation (Wilson, T.H. and Kusch, M. (1972) Biochim. Biophys. Acta 255, 786-797), two reactive thiols were characterised. The more reactive of the two is more exposed than the thiol group of the native molecule and is in an environment that has a dielectric constant close to that of water. The less reactive thiol appears to be more deeply buried than that of the native protein. Thus the mutation appears to produce a conformation change in the central portion of the polypeptide chain that results in greater exposure of the reactive thiol to the aqueous environment.

The carrier reorientation step in erythrocyte choline transport: pH effects and the involvement of a carrier ionizing group

Under zero-trans conditions, the facilitated transport of choline across the erythrocyte membrane is limited by the rate of reorientation of the free carrier; as a result the pH dependence of this step can be investigated, independent of other steps in transport. It is found that as the pH declines (between 8.0 and 6.0) the rate of inward movement of the free carrier rises and the rate of outward movements falls, so that the partition of the free carrier increasingly favors the inward-facing form. When the pH of the cell interior and of the medium are varied independently, the partition responds to the internal but not the external pH. The membrane potential, which varies somewhat as the pH is altered, has no effect on the carrier partition. The analysis of the results indicates that the carrier mobility is dependent on an ionizing group of pKa 6.8, which is exposed on the cytoplasmic surface of the membrane in the inward-facing carrier; in the outward-facing carrier the ionizing group appears to be masked, in that its pKa is shifted downward by more than one unit. The observations can be explained by assuming that an ionizing group is located in the wall of a gated channel connecting the substrate site with the cytoplasmic face of the cell membrane.

Characterization of choline transport at maternal and fetal interfaces of the perfused guinea-pig placenta

Unidirectional influx and efflux of choline into the syncytiotrophoblast were investigated from both maternal and fetal circulations of the perfused guinea-pig placenta by using a single-circulation paired-tracer (extracellular reference and test substrate) dilution technique. Cellular uptake of [3H]choline at 0.05 mM was (mean percentage +/- S.E. of mean, n = 14 placentae) 51 +/- 2 and 49 +/- 2, on maternal and fetal sides, respectively. Kinetics of unidirectional influx (0.05-4.0 mM-choline) indicated the existence of saturable and non-saturable components on both sides: on maternal and fetal interfaces the Km (mM) values were respectively, 0.12 and 0.13, the Vmax (mumol min-1 g-1) values, 0.08 and 0.07 and the apparent linear transfer constants (min-1 g-1) 0.11 and 0.12. Efflux of [3H]choline from the placenta back into the ipsilateral circulation (backflux) was generally fast (20-60% in 5-6 min) and asymmetric with the fetal: maternal ratio usually above unity. Transplacental specific choline transfer in the dually perfused placenta, when observed, was small (less than 10% of the injected dose) following tracer injections in either direction based on the 5-6 min collection of the contralateral circulation (at 0.05 mM-choline). Placental retention of [3H]choline at the end of the 5-6 min period was about 25% of the injected dose when the tracers were injected from either circulation. Analogues of choline such as hemicholinium-3, thiamine, ethanolamine and N,N-dimethylethanolamine inhibited choline unidirectional influx, whereas betaine and acetate had no effect. The absence of the normal sodium gradient (perfusate sodium was replaced by Tris or by lithium) did not inhibit choline transport. The metabolic inhibitors dinitrophenol (1.0 mM) and potassium cyanide (1.0 mM) were essentially ineffective (up to 40 min perfusion). The sulphydryl reagent N-ethylmaleimide did not appear to inhibit the influx, in comparison with its effect on [3H]choline backflux which was greatly accelerated, resulting in a dramatic reduction in placental net uptake of the label. Our findings show that choline transport into the placenta is a rapid carrier-mediated process occurring at both maternal and fetal sides of the trophoblast, at physiological blood concentrations. This cellular uptake is possibly related to the synthesis of acetylcholine, which is known to occur in human placental tissue. Specific transplacental transfer of choline was a very slow process under the conditions of our experiments and this contrasted with the observed fast and high uptake into the trophoblast.

The comparative specificity of the inner and outer substrate transfer sites in the choline carrier of human erythrocytes

The substrate specificities on the inner and outer surfaces of the cell membrane have been compared by determining the relative affinities, inside and outside, of a series of choline analogs. The results of two different methods were in agreement: (1) the carrier distribution was determined in the presence of a saturating concentration of an equilibrated analog, using N-ethylmaleimide as a probe for the inward-facing carrier; (2) the degree of competition was measured between an equilibrated analog and choline in the external solution. The carrier sites are found to have markedly different specificities: the outer site is more closely complementary to the structure of choline than is the inner, and even a slight enlargement of either the trimethylammonium or hydroxyethyl group gives rise to preferential binding inside. It is also found that a nonpolar binding region, which is adjacent to the outer site, is absent from the inner site. As the transport mechanism involves the exposure of only one site at a time, first on one surface and then the other, it follows that an extensive reorganization of the structure of the substrate site may occur during the carrier-reorientation step, or alternatively that two distinct sites may be present, only one of which is exposed at a time.

The kinetics of transport inhibition by noncompetitive inhibitors

A new analysis of the conventional carrier model shows that noncompetitive inhibitors can give rise to either competitive, noncompetitive or uncompetitive kinetics; the true mechanism and also the relative affinity of the inhibitor on each surface of the membrane can be decided from the patterns of inhibition observed in different transport experiments. The principles governing the kinetics of inhibition apply to both reversible and irreversible inhibitors, for in either case the substrate may increase or decrease inhibition or be without effect. Ambiguity arises if the noncompetitive inhibitor acts on only one side of the membrane and if the substrate, in the course of being transported, alters the steady-state distribution of the carrier between inner and outer forms. In facilitated transport systems only equilibrium exchange should give rise to noncompetitive kinetics, whatever the location of the inhibitor. In active systems even the interpretation of exchange in the final steadystate is complicated if the energy-coupling mechanism produces a large displacement in the distribution of the free carrier or the substrate complex: the inhibition could be competitive or uncompetitive, depending on the location of the inhibitor. The actual mechanism is revealed in the uncoupled system.

Reaction of internal forms of the choline carrier of erythrocytes with N-ethylmaleimide: evidence for a carrier conformational change on complex formation

The choline carrier of human erythrocyte membranes exists in distinguishable outward-facing and inward-facing conformations, and previous studies demonstrated that only the latter reacts with N-ethylmaleimide, producing an irreversible inhibition of transport. We now report experiments to determine the individual reaction rates for the two inward-facing forms: the free carrier and the complex. The pseudo-first-order rate constant for the complex with a substrate analog, di-n-butylaminoethanol, is found to be nearly double that for the free carrier, showing that the carrier conformation is altered following addition of a ligand (with 1 mM N-ethylmaleimide at pH 6.8, 37 degree C, the constants are 0.57 +/- 0.05 min-1 and 0.33 +/- 0.02 min-1, respectively). Hence three different conformational states have been distinguished by experiment: (1) the inward-facing free carrier; (2) the inward-facing complex; and (3) the outward-facing carrier.

Evidence for a two-state mobile carrier mechanism in erythrocyte choline transport: effects of substrate analogs on inactivation of the carrier by N-ethylmaleimide

Choline transport in erythrocytes is irreversibly inhibited by N-ethylmaleimide. The hypothesis that the carrier alternates between outward-facing and inward-facing forms and that only the latter reacts with the inhibitor (Martin, K. (1971) J. Physiol. (London) 213:647--667; Edwards, P.A. (1973) Biochim. Biophys. Acta 311:123--140) is here subjected to a quantitative test. In this test the effects of a series of substrate analogs upon rates of inactivation and rates of choline exit are compared. By hypothesis the effect of an analog in the external solution on the inactivation rate depends only on how it affects the proportion of the inward-facing carrier. Since 14C-choline efflux is necessarily proportional to the concentration of free carrier in the inward-facing form, the analogs should have related effects on the two rates. In every case the observed effects were identical, whether the analogs accelerated transport or inhibited it. Analysis of the results demonstrates that (1) the transport mechanism depends on the operation of a mobile element; (2) distinguishable inward-facing and outward-facing conformations of the free carrier, carrier-substrate complex, and carrier-inhibitor complex exist, and only the inward-facing forms react at a significant rate with N-ethylmaleimide; (3) carrier mechanisms involving a single form of free carrier or a single form of carrier-substrate complex are ruled out; and (4) dissociation of the carrier-substrate complex is a rapid step with all substrate analogs.

Testing the simple carrier using irreversible inhibitors

1. We analyse the kinetics of irreversible inhibition of the simple carrier. We consider how the rate of inactivation is influenced by the concentrations of permeant on the two sides of the membrane. 2. We consider various kinetic schemes for the simple carrier and show that these are all indistinguishable kinetically, using steady-state transport or inactivation data. We point out the advantages of using the simplest kinetic scheme and the possible pitfalls of using more complicated schemes, including the conventional carrier model. 3. We show that in the absence of information on the transport properties of the system, irreversible inhibition data are ambiguous. Taken together with transport data, however, inactivation data provide new tests for the applicability of the simple carrier. 4. The new tests show that for the simple carrier model to be applicable, the substrate dependencies of transport and of inactivation must be identical in comparable experimental situations. Further, the maximal rates of transport and of stimulation (or inhibition) of inactivation must obey a simple relationship, which we derive. We illustrate the use of these tests with published data on the glucose and choline transport systems of the human red blood cell.

An irreversible effect of lithium administration to patients

1 Lithium administration to patients leads to a pronounced inhibition (about 90%) of the choline transport system in erythrocytes. The transport system does not recover when ghosts are prepared from the erythrocytes, thereby removing intracellular as well as extracellular lithium. 2 When a patient is taken off lithium, the choline transport in erythrocytes recovers only very slowly over a period of three months, i.e. at about the same rate at which the erythrocytes that had been exposed to lithium are replaced by new cells. 3 It is concluded that therapeutic concentrations of lithium produce an irreversible inhibition of the choline transport system in human erythrocytes.

Methylmercury and the skeletal muscle receptor

Methylmercury at bath concentration of 2 X 10(-5) M was capable of inhibiting muscular contractions of the isolated rat phrenic-nerve hemidiaphragm preparation. At the height of inhibition, nerve action potential could still be recorded and the muscles continued to respond to direct stimulation. The inhibition was not reversible with L-cysteine or D-penicillamine but limited protection was possible by prior treatment with (+)-tubocurarine. Treatment of frog rectus muscles with methylmercury (0-2 mM for 15 min) resulted in a shift to the right of 1 log unit in the dose response curve to acetylcholine and a reduction in the maximum response of the tissue. The observed inhibitory action of methylmercury on neuromuscular transmission may be explained by an action on the disulphide bond believed to be present on a cholinergic receptor.

Inhibition of amino acid transport in rabbit intestine by p-chloromercuriphenyl sulfonic acid

Influx of phenylalanine across the brush border of rabbit intestine is markedly reduced by treatment with 5 mM p-chloromercuriphenyl sulfonate (PCMBS). The effect is rapidly and completely reversed by dithiothreitol. Phenylalanine influx into PCMBS-treated tissue can be competitively inhibited by other neutral amino acids and follows saturation kinetics. PCMBS causes an increase in the apparent Michaelis constant from the value observed in control tissue but does not alter the maximal influx significantly. Treatment of the tissue with PCMBS leads to a significant reduction in the Na-sensitivity of the transport, and a number of results indicate that the major effect of the reagent is to cause a marked reduction in the affinity of the transport system for Na. The transport system can be partially protected against reaction with PCMBS by phenylalanine and tryptophan but not by methionine or norleucine. The results suggest that PCMBS reacts with a sulfhydryl group in the region of the transport site and may alter conformational changes associated with the binding of substrates.

The efflux of magnesium from single crustacean muscle fibres

1. In the large single muscle fibres from the barnacle Balanus nubilus, the total fibre Mg concentration was estimated as 15.1 m-mole/kg wet wt., of which about 3-3.5 m-mole/kg wet wt. was extracellular. The diffusible Mg, measured by internal sampling, was 11.5 m-mole/kg wet wt., of which at least half may be complexed to larger diffusible molecules. The free ionized Mg level was estimated as < 5 m-mole/kg wet wt.2. The loss of [(28)Mg]MgCl(2) from both Maia and Balanus muscle fibres following axial micro-injection approximated to first-order kinetics. The maximum rate constant for the loss was 1.51 +/- 0.20 (S.E.) x 10(-5) sec(-1) for Balanus (sixty-seven fibres) and 1.06 +/- 0.46 (S.E.) x 10(-5) sec(-1) for Maia (seven fibres) at 20-25 degrees C.3. The calculated Mg efflux was in the range 6-12 p-mole/cm(2).sec based on this rate constant, assuming isotopic equilibration internally and that the surface area of the fibres approximated to that of a simple cylinder. If account was taken of the area of the cleft system the efflux was reduced by about fifteen times.4. The diffusion coefficient for injected (28)Mg was estimated as 2-3 x 10(-6) cm(2) sec(-1), about half the value in free solution. Injections of 2 M-MgCl(2) or 200 mM-EDTA subsequent to the injection of the isotope caused about a 30% reduction in the tracer efflux.5. External application of salines containing 100 mM-Ca or Mg caused a rapid but reversible inhibition of the magnesium efflux. Similar effects were observed with salines containing 32 mM-Co or Mn chlorides or 1-2 mM-La or Gd chlorides. Polyarginine (200 mug/ml.) had no effect.6. The Mg efflux had a Q(10) of 3-4 over the temperature range of about 5-20 degrees C. It was irreversibly inhibited by the sulphydryl reagent NEM (1 mM), but PCMBS (0.2-2 mM) had no effect. Contractile agents (5 mM caffeine or 200 mM-K salines) and a variety of inhibitors of ion movement or active transport had no appreciable effect on the Mg efflux. Lowering the pH of the saline from 7 to 5 produced a 70% reduction in the efflux which was reversible over short periods of application.7. Replacement of external Na, but not Ca or Mg, with Li, choline or sucrose caused a rapid and partially reversible reduction of the Mg efflux, but increasing the internal Na by micro-injection in zero Na salines had no consistent effect. It is suggested that the extrusion of Mg from these muscle cells is largely dependent upon the inward movement of Na.