The metal permease ZupT from Escherichia coli is a transporter with a broad substrate spectrum

The Escherichia coli zupT (formerly ygiE) gene encodes a cytoplasmic membrane protein (ZupT) related to members of the eukaryotic ZIP family of divalent metal ion transporters. Previously, ZupT was shown to be responsible for uptake of zinc. In this study, we show that ZupT is a divalent metal cation transporter of broad substrate specificity. An E. coli strain with a disruption in all known iron uptake systems could grow in the presence of chelators only if zupT was expressed. Heterologous expression of Arabidopsis thaliana ZIP1 could also alleviate iron deficiency in this E. coli strain, as could expression of indigenous mntH or feoABC. Transport studies with intact cells showed that ZupT facilitates uptake of 55Fe2+ similarly to uptake of MntH or Feo. Other divalent cations were also taken up by ZupT, as shown using 57Co2+. Expression of zupT rendered E. coli cells hypersensitive to Co2+ and sensitive to Mn2+. ZupT did not appear to be metal regulated: expression of a Phi(zupT-lacZ) operon fusion indicated that zupT is expressed constitutively at a low level.     

Contributions of five secondary metal uptake systems to metal homeostasis of Cupriavidus metallidurans CH34

Cupriavidus metallidurans is adapted to high concentrations of transition metal cations and is a model system for studying metal homeostasis in difficult environments. The elemental composition of C. metallidurans cells cultivated under various conditions was determined, revealing the ability of the bacterium to shield homeostasis of one essential metal from the toxic action of another. The contribution of metal uptake systems to this ability was studied. C. metallidurans contains three CorA members of the metal inorganic transport (MIT) protein family of putative magnesium uptake systems, ZupT of the ZRT/IRT protein, or ZIP, family, and PitA, which imports metal phosphate complexes. Expression of the genes for all these transporters was regulated by zinc availability, as shown by reporter gene fusions. While expression of zupT was upregulated under conditions of zinc starvation, expression of the other genes was downregulated at high zinc concentrations. Only corA(1) expression was influenced by magnesium starvation. Deletion mutants were constructed to characterize the contribution of each system to transition metal import. This identified ZupT as the main zinc uptake system under conditions of low zinc availability, CorA(1) as the main secondary magnesium uptake system, and CorA(2) and CorA(3) as backup systems for metal cation import. PitA may function as a cation-phosphate uptake system, the main supplier of divalent metal cations and phosphate in phosphate-rich environments. Thus, metal homeostasis in C. metallidurans is achieved by highly redundant metal uptake systems, which have only minimal cation selectivity and are in combination with efflux systems that "worry later" about surplus cations.     

Effect of cutting on solute uptake by plasma membrane vesicles from sugar beet (Beta vulgaris L.) leaves.

The uptake of sucrose, 3-O-methylglucose (3-O-MeG), and valine were studied in discs and in purified plasma membrane vesicles (PMV) prepared from sugar beet (Beta vulgaris L.) exporting leaves. The uptake capacities of freshly excised leaf discs were compared with the uptake in discs that had been floated for 12 h on a simple medium (aging) and with discs excised from leaves that had been cut from the plant 12 h before the experiments (cutting). After cutting, sucrose uptake amounted to twice the uptake measured in fresh discs, whereas the uptake of 3-O-MeG and valine remained unaffected. In aged leaf discs, there was a general stimulation of uptake, which represented 400, 300, and 400% of the uptake measured in fresh discs for sucrose, 3-O-MeG, and valine, respectively. Sucrose uptake in fresh discs was sensitive to N-ethylmaleimide (NEM), to p-chloromercuribenzenesulfonic acid (PCMBS), and to mersalyl acid (MA). Although cutting induced the appearance of a sucrose uptake system that is poorly sensitive to NEM but sensitive to PCMBS and MA, aging induced the development of an uptake system that is sensitive to NEM but poorly sensitive to PCMBS and MA. Autoradiographs of discs fed with [14C]sucrose show that cutting resulted in an increase of vein labeling with little effect in the mesophyll, whereas aging induced an increase of labeling located mainly in the mesophyll. The data show that cutting is sufficient to induce dramatic and selective changes in the uptake properties of leaf tissues and that the effects of cutting and aging on the uptake of organic solutes are clearly different. Parallel experiments were run with purified PMV prepared from fresh and cut leaves. The uptake of sugars and amino acids was studied after imposition of an artificial proton motive force (pmf). Comparison of the uptake properties of PMV and of leaf tissues indicate that the recovery of the sucrose uptake system in PMV is better than the recovery of the hexose and of the valine uptake systems. As observed with the leaf discs, cutting induced a 2-fold increase of the initial rate of sucrose uptake in PMV but did not affect the uptake of valine and 3-O-MeG. Cutting induced an increase of both Vmax and Km of the sucrose transport system in PMV. Measurements of the pmf imposed on the vesicles indicated that the increase of sucrose uptake induced by cutting was not due to a better integrity of the vesicles. Hexoses did not compete with sucrose for uptake in PMV from fresh and cut leaves, and maltose was a stronger inhibitor of sucrose uptake in PMV from cut leaves than in PMV from fresh leaves. The sensitivity of sucrose uptake to NEM, PCMBS, and MA in PMV from fresh and cut leaves paralleled that described above for the corresponding leaf discs. These data show that (a) the changes induced by cutting on sucrose uptake by leaf discs are due to membrane phenomena and not to the metabolism of sucrose; (b) the study of sucrose uptake with PMB gives a good account of the physiological situation; and (c) the specific effects induced by cutting on the sucrose uptake system are not lost during the preparation of the PMV.     

Scavenger receptor class B, type I, mediates selective uptake of low density lipoprotein cholesteryl ester.

Scavenger receptor, class B, type I (SR-BI) is a cell-surface glycoprotein that mediates selective uptake of high density lipoprotein cholesteryl ester (CE) without the concomitant uptake and degradation of the particle. We have investigated the endocytic and selective uptake of low density lipoprotein (LDL)-CE by SR-BI using COS-7 cells transiently transfected with mouse SR-BI. Analysis of lipoprotein uptake data showed a concentration-dependent LDL-CE-selective uptake when doubly labeled LDL particles were incubated with SR-BI-expressing COS-7 cells. In contrast to vector-transfected cells, SR-BI-expressing COS-7 cells showed marked increases in LDL cell association and CE uptake by the selective uptake pathway, but only a modest increase in CE uptake by the endocytic pathway. SR-BI-mediated LDL-CE-selective uptake exceeded LDL endocytic uptake by 50-100-fold. SR-BI-mediated LDL-CE-selective uptake was not inhibited by the proteoglycan synthesis inhibitor, p-nitrophenyl-beta-D-xylopyranoside or by the sulfation inhibitor sodium chlorate, indicating that SR-BI-mediated LDL-CE uptake occurs independently of LDL interaction with cell-surface proteoglycan. Analyses with subclones of Y1 adrenocortical cells showed that LDL-CE-selective uptake was proportional to the level of SR-BI expression. Furthermore, antibody directed to the extracellular domain of SR-BI blocked LDL-CE-selective uptake in adrenocortical cells. Thus, in cells that normally express SR-BI and in transfected COS-7 cells SR-BI mediates the efficient uptake of LDL-CE via the selective uptake mechanism. These results suggest that SR-BI may influence the metabolism of apoB-containing lipoproteins in vivo by mediating LDL-CE uptake into SR-BI-expressing cells.     

Characterization of norepinephrine accumulation by a crude synaptosomal-mitochondrial fraction isolated from rat heart.

Norepinephrine (NE) uptake into a heart synaptosomal-mitochondrial fraction was assessed under conditions where neuronal uptake (type 1) was linear with respect to both time and protein concentration. The NE accumulation process was sensitive to incubation temperature, sodium ion concentration and medium osmolality. Furthermore, NE uptake was attenuated by the neuronal uptake inhibitor desmethylimipramine (DMI) in a concentration dependent manner; the IC50 value was approximately 10 nM and maximum inhibition was obtained at 100 nM. In contrast, the extraneuronal uptake inhibitor, metanephrine did not significantly attenuate NE uptake. Kinetic analysis demonstrated that the DMI sensitive NE accumulation is saturable with a KM of approximately 400 nM and that NE uptake occurs via a single uptake process. This demonstration of neuronal type NE uptake by a synaptosomal-mitochondrial fraction constitutes a successful demonstration of the preparation of a rat heart subcellular fraction containing functional synaptosomes.     

Evidence that nitroprusside stimulates glucose uptake in isolated rat cardiomyocytes via mitogen-activated protein kinase

Sodium nitroprusside (SNP), a nitric oxide (NO.) donor, stimulates glucose uptake in skeletal muscle. We investigated the stimulatory effect of SNP on glucose uptake in cardiomyocytes and the possible role of soluble guanylate cyclase, phosphatidylinositol-3-kinase (PI-3-kinase) and the mitogen-activated protein kinases (MAPKs). Cardiomyocytes were isolated from adult male Wistar rats by trypsin/collagenase perfusion and glucose uptake determined from the accumulation of 3H-2-deoxyglucose. SNP caused a dose-dependent increase in glucose uptake with 200-300% increase at 30 mM. Cytochalasin B completely prevented the SNP-induced increase in glucose uptake. 8-Br-cGMP (100 microM) and the NO. donor spermineNONOate (100 microM) were without effect on basal glucose uptake. SNP-stimulated glucose uptake was not inhibited by the guanylate cyclase inhibitor ODQ (10 microM). Sodium ferrocyanide (Na4Fe(CN)6), a compound structurally related to SNP, but without any NO. group, also stimulated glucose uptake in cardiomyocytes suggesting that the effect of SNP could be unrelated to liberation of NO. Wortmannin, an inhibitor of PI-3-kinase, inhibited insulin-stimulated glucose uptake completely but did not affect SNP-stimulated glucose uptake. SNP-stimulated glucose uptake was inhibited by 50 microM PD 098059 (inhibitor of the MAPK-kinases that activate external regulated kinase [ERK1/2]) and by 50 microM SB203580 (inhibitor of p38MAPK). In conclusion, high SNP concentrations dose-dependently stimulate glucose uptake in cardiomyocytes and our data suggest a role for MAPK signalling, but not PI-3-kinase and soluble guanylate cyclase, in stimulation of glucose uptake.     

Selective uptake of cholesteryl esters from low density lipoproteins in vitro and in vivo.

Evidence for the direct uptake ("selective uptake") of cholesteryl esters (CE) from low density lipoproteins (LDL) by perfused luteinized rat ovaries (Azhar, S., A. Cooper, L. Tsai, W. Maffe, and E. Reaven. 1988. J. Lipid Res. 29: 869-882) led to this examination of LDL selective uptake in cultured cells and in rats using LDL doubly labeled with intracellularly trapped tracers of the CE and apoB moieties. Studies in vitro demonstrated LDL selective uptake by human fibroblasts at a low rate relative to LDL particle uptake; the fractional rate of this selective uptake increased with decreasing LDL particle size. Mouse Y1-BS1 adrenal cortical tumor cells also selectively took up LDL CE; on ACTH treatment, LDL selective uptake increased in parallel with high density lipoproteins (HDL) selective uptake, and accounted for the majority of LDL CE uptake. Metabolism of doubly labeled LDL was examined in rats. Adrenal gland and liver selectively took up CE from rat LDL, as did lung and adipose tissue. Selective uptake from human LDL was at a lower fractional rate than from rat LDL, and could not be demonstrated in as many organs. Although selective uptake from LDL by ovaries of adult rats was not significant, ovaries of immature rats consistently exhibited LDL selective uptake; on treatment of these rats with hormones to produce superovulated, luteinized ovaries, LDL selective uptake increased in the ovaries and nowhere else. Selective uptake was also apparent in liver, where it accounted for 27% of total hepatic uptake of rat LDL CE. These studies indicate a significant contribution of selective uptake to LDL CE metabolism in rats, suggesting the possibility of a role in other animals as well.     

Anomalous kinetics of sucrose uptake in maize scutellum slices1

Abstract The kinetics of sucrose uptake into maize scutellum slices showed that the uptake mechanism had a saturable component with a Km of l.5mol m^?3 sucrose. Nevertheless, uptake rate was constant (zero order) over extended periods of time until the bathing solution was nearly depleted of sucrose. It is concluded that these anomalous uptake kinetics reflect sucrose influx across the plasmalemma because of the following results: (a) Efflux of sucrose into buffer was negligible compared with uptake rate, (b) When slices were incubated in fructose, sucrose was synthesized and there was a net release of sucrose to the bathing solution until a steady-state was reached when influx and efflux were equal in magnitude. After the steady-state was reached, efflux of sucrose from the slices was nearly the same in magnitude as the estimated rate of uptake that would have occurred from bathing solutions initially containing the steady-state sucrose concentration, (c) Exchange of sucrose between bathing solution and slices was negligible compared with uptake rate, (d) Pretreatment of slices with uranyl nitrate abolished sucrose uptake, but uptake rate was re-established in these slices after treatment with HCl (pH 2). Uptake rate was set by the initial sucrose concentration of the bathing solution, and was not influenced by the level of endogenous sucrose or by the rate at which the sucrose concentration of the bathing solution declined. Abrupt increases in sucrose concentration during the uptake period increased the rate of uptake only if the concentration was increased above that at the start of the uptake period. Following abrupt decreases in sucrose concentration, there was a lag of about 30 min before uptake rate decreased greatly. If slices were washed and replaced in a fresh sucrose solution during the uptake period, a new uptake rate was set to correspond to the new initial sucrose concentration. It is suggested that the sucrose carrier has a transport site with a relatively low Km (much below 1.5mol m^?3) and that the measured Km (1.5mol m^?3) is that of a site that binds sucrose and thereby controls the rate of uptake. The low Km suggested for the transport site would explain the zero order kinetics but a model of the uptake mechanism that includes the control site cannot, as yet, be constructed from the data.     

Characterization of glucosinolate uptake by leaf protoplasts of Brassica napus

The uptake of radiolabeled p-hydroxybenzylglucosinolate (p-OHBG) by protoplasts isolated from leaves of Brassica napus was detected using silicone oil filtration technique. The uptake was pH-dependent with higher uptake rates at acidic pH. Imposition of a pH gradient (internal alkaline) across the plasma membrane resulted in a rapid uptake of p-OHBG, which was inhibited in the presence of carbonyl cyanide m-chlorophenylhydrazone, indicating that the uptake is dependent on a proton motive force. Dissipation of the internal positive membrane potential generated a small influx as compared with that seen for pH gradient (DeltapH). Kinetic studies demonstrated the presence of two uptake systems, a saturable and a linear component. The saturable kinetics indicated carrier-mediated translocation with a K(m) of 1.0 mm and a V(max) of 28.7 nmol/microl/h. The linear component had very low substrate affinity. The carrier-mediated transport had a temperature coefficient (Q(10)) of 1.8 +/- 0.2 in the temperature range from 4-30 degrees C. The uptake was against a concentration gradient and was sensitive to protonophores, uncouplers, H(+)-ATPase inhibitors, and the sulfhydryl group modifier p-chloromercuriphenylsulfonic acid. The carrier-mediated uptake system had high specificity for glucosinolates because glucosinolate degradation products, amino acids, sugars, or glutathione conjugates did not compete for p-OHBG uptake. Glucosinolates with different side chains were equally good competitors of p-OHBG uptake, which indicates that the uptake system has low specificity for the glucosinolate side chains. Our data provide the first evidence of an active transport of glucosinolates by a proton-coupled symporter in the plasma membrane of rape leaves.     

Uptake, metabolism and distribution of organic and inorganic nitrogen sources by Pinus sylvestris

Although an increasing number of studies show that many plant species have the capacity to take up amino acids from exogenous sources, the importance of such uptake for plant nitrogen nutrition is largely unknown. Moreover, little is known regarding metabolism and distribution of amino acid-N following uptake or of the regulation of these processes in response to plant nitrogen status. Here results are presented from a study following uptake, metabolism, and distribution of nitrogen from NO(3)(-) NH(4)(+), Glu, or Ala in Scots pine (Pinus sylvestris L). In a parallel experiment, Ala uptake, processing, and shoot allocation were also monitored following a range of pretreatments intended to alter plant C- and N-status. Uptake data, metabolite profiles, N fluxes through metabolite pools and tissues, as well as alanine aminotransferase activity are presented. The results show that uptake of the organic N sources was equal to or larger than NH(4)(+) uptake, while NO(3)(-) uptake was comparatively low. Down-regulation of Ala uptake in response to pretreatments with NH(4)NO(3) or methionine sulphoximine (MSX) indicates similarities between amino acid and inorganic N uptake regulation. N derived from amino acid uptake exhibited a rapid flux through the amino acid pool following uptake. Relative shoot allocation of amino acid-N was equal to that of NH(4)(+) but smaller than for NO(3)(-) Increased N status as well as MSX treatment significantly increased relative shoot allocation of Ala-N suggesting that NH(4)(+) may have a role in the regulation of shoot allocation of amino acid-N.     

(99m)Tc-sestamibi uptake in rat skeletal muscle and heart: physiological determinants and correlations

The lipophilic cationic radiotracer (99m)Tc-sestamibi, known to be concentrated within mitochondria, is widely used for myocardial perfusion and to a lesser extent for muscle metabolism imaging. However, the exact distribution pattern in skeletal muscle has not been yet studied in detail. The present study aims to investigate the (99m)Tc-sestamibi uptake in rat skeletal muscle and myocardium in relation to their metabolic characteristics. (99m)Tc-sestamibi was i.v. administered in twenty adult male Wistar rats and uptake, as percent of injected dose per tissue gram (%ID/g), in the myocardium, soleus, extensor digitorum longus and gastrocnemius muscles was assessed 2 h after the injection. Muscle uptake was also correlated with myocardial uptake, muscle weight and body weight. Skeletal muscle (99m)Tc-sestamibi uptake was a small (9-16 %) fraction of that found in myocardium (1.71+/-0.63 %ID/g). Among the three hindlimb muscles considered, the slow-oxidative soleus muscle showed the highest uptake (0.28+/-0.16 %ID/g). Metabolically diverse parts of the gastrocnemius muscle showed different uptake. Skeletal muscle uptake was positively correlated with myocardial uptake and both were negatively correlated with tissue and body weight. Skeletal muscle and myocardium (99m)Tc-sestamibi uptake is related to their metabolic profile. Myocardium, with an exceptional rich mitochondrial concentration, shows much higher (99m)Tc-sestamibi uptake compared to skeletal muscles. Among muscles, uptake is dependent on their mitochondrial content. Evidence of matching exists between myocardial and muscle uptake, and both are size-dependent.     

Effects of salinity and light on organic carbon and nitrogen uptake in a hypersaline microbial mat

Utilization of dissolved organic matter (DOM) is thought to be the purview of heterotrophic microorganisms, but photoautotrophs can take up dissolved organic nitrogen (DON) and dissolved organic carbon (DOC). This study investigated DOC and DON uptake in a laminated cyanobacterial mat community from hypersaline Salt Pond (San Salvador, Bahamas). The total community uptake of (3)H-labeled substrates was measured in the light and in the dark and under conditions of high and low salinity. Salinity was the primary control of DOM uptake, with increased uptake occurring under low-salinity, 'freshened' conditions. DOC uptake was also enhanced in the light as compared with the dark and in samples incubated with the photosystem II inhibitor 3(3,4-dichlorophenyl)-1, 1-dimethylurea, suggesting a positive association between photosynthetic activity and DOC uptake. Microautoradiography revealed that some DOM uptake was attributed to cyanobacteria. Cyanobacteria DOM uptake was negatively correlated with that of smaller filamentous microorganisms, and DOM uptake by individual coccoid cells was negatively correlated with uptake by colonial coccoids. These patterns of activity suggest that Salt Pond microorganisms are engaged in resource partitioning, and DOM utilization may provide a metabolic boost to both heterotrophs and photoautrophs during periods of lowered salinity.     

Nitrogen uptake by size-fractionated plankton in permanently well-mixed temperate coastal waters

Nitrogen uptake by net- (15–200 µm), nano- (1–15µm) and picoplankton (<1 µm) was measured overseasonal cycles at two stations with different patterns of biologicaland chemical cycles in the Morlaix Bay (western English Channel).Though assimilable dissolved N nutrient pool at both stationswas nitrate-dominated, characteristics of biomass and N uptakeby netplankton differed from conventional patterns in two respects.In the first, biomass (26–30%) and N uptake (36–43%)were less important than those of nanoplankton. In the second,the netplankton did not show any marked preference for nitrateover ammonium (nitrate to ammonium uptake ratios of 0.98 and1.08). In contrast, nanoplankton had a preference for ammoniumover nitrate (ammonium to nitrate uptake ratios of 2 and 1.2).N uptake by picoplankton was only 8% of total N uptake at bothstations and was supported mainly by regenerated N (66% ammoniumand 17% urea), with nitrate uptake detectable in only one instanceand nitrite uptake in none. Substrate-dependent uptake of ammoniumin all fractions and a higher ammonium uptake in the nanoplanktonfraction in summer at both stations when ambient ammonium concentrationswere high indicated that while nitrate may satisfy a part ofN requirements, availability of ammonium and its flux throughnanoplankton determine the magnitude of total N uptake in thesewaters. Most of the N uptake in picoplankton appears to be autotrophic,suggesting that a substantial part of heterotrophic uptake,if any, could be localized in the fractions >1 µm,and mediated by free-living and particle-bound bacteria.     

Uptake of ammonium and nitrate by carob (Ceratonia siliqua) as affected by root temperature and inhibitors

Seedlings of carob ( Ceratonia siliqua L. cv. Mulata) were grown in nutrient solution culture for 5 weeks, with or without nitrogen at different root temperatures (10, 16, 22, 30, 35 or 40deg;C) and with the air temperature kept between 20 and 24°C. The nitrogen was given as either ammonium or nitrate. At all root temperatures studied, nitrogen-depleted plants developed higher net uptake rates for nitrogen than plants grown in the presence of nitrogen. Temperature affected the kinetic parameters of nitrate uptake more than those of ammonium uptake. With increasing root temperature, the K_m of ammonium uptake decreased, but to a lesser extent than the K_m for nitrate. The increase in V_max of ammonium uptake with temperature was also less noticeable than that for nitrate uptake. Ammonium and nitrate uptakes were inhibited in a similar way by respiratory or protein synthesis inhibitors. It may be noted that ammonium uptake in the presence of inhibitors at 40°C was higher than uptake at 10°C without inhibitors. Some similarities between the transport mechanisms for nitrate and ammonium are underlined in the present work. Components of both transport systems displayed saturation kinetics and depended on protein synthesis and energy. The following components of nitrate uptake were distinguished: (a) a passive net influx into the apparent free space; (b) a constitutive active uptake and (c) active uptake dependent on protein synthesis. We may similarly define three ammonium uptake systems: (a) a passive influx into the apparent free space; (b) passive diffusion uptake at high temperature and (c) active uptake dependent on protein synthesis. The possible role of the ratio between mechanism (c) and mechanism (b) as determinant of ammonium sensitivity is discussed.     

Scavenger receptor class B, type I is expressed in porcine brain capillary endothelial cells and contributes to selective uptake of HDL-associated vitamin E

It is clearly established that an efficient supply to the brain of alpha-tocopherol (alphaTocH), the most biologically active member of the vitamin E family, is of the utmost importance for proper neurological functioning. Although the mechanism of uptake of alphaTocH into cells constituting the blood-brain barrier (BBB) is obscure, we previously demonstrated that high-density lipoprotein (HDL) plays a major role in the supply of alphaTocH to porcine brain capillary endothelial cells (pBCECs). Here we studied whether a porcine analogue of human and rodent scavenger receptor class B, type I mediates selective (without concomitant lipoprotein particle internalization) uptake of HDL-associated alphaTocH in a similar manner to that described for HDL-associated cholesteryl esters (CEs). In agreement with this hypothesis we observed that a major proportion of alphaTocH uptake by pBCECs occurred by selective uptake, exceeding HDL3 holoparticle uptake by up to 13-fold. The observation that selective uptake of HDL-associated CE exceeded HDL3 holoparticle up to fourfold suggested that a porcine analogue of SR-BI (pSR-BI) may be involved in lipid uptake at the BBB. In line with the observation of selective lipid uptake, RT-PCR and northern and western blot analyses revealed the presence of pSR-BI in cells constituting the BBB. Adenovirus-mediated overexpression of the human analogue of SR-BI (hSR-BI) in pBCECs resulted in a fourfold increase in selective HDL-associated alphaTocH uptake. In accordance with the proposed function of SR-BI, selective HDL-CE uptake was increased sixfold in Chinese hamster ovary cells stably transfected with murine SR-BI (mSR-BI). Most importantly stable mSR-BI overexpression mediated a twofold increase in HDL-associated [14C]alphaTocH selective uptake in comparison with control cells. In line with tracer experiments, mass transfer studies with unlabelled lipoproteins revealed that mSR-BI overexpression resulted in a twofold increase in endogenous HDL3-associated alphaTocH uptake. The results of this study indicate that SR-BI promotes the uptake of HDL-associated alphaTocH into cells constituting the BBB and plays an important role during the supply of the CNS with this indispensable micronutrient.     

Gbu glycine betaine porter and carnitine uptake in osmotically stressed Listeria monocytogenes cells

The food-borne pathogen Listeria monocytogenes grows actively under high-salt conditions by accumulating compatible solutes such as glycine betaine and carnitine from the medium. We report here that the dominant transport system for glycine betaine uptake, the Gbu porter, may act as a secondary uptake system for carnitine, with a K(m) of 4 mM for carnitine uptake and measurable uptake at carnitine concentrations as low as 10 microM. This porter has a K(m) for glycine betaine uptake of about 6 micro M. The dedicated carnitine porter, OpuC, has a K(m) for carnitine uptake of 1 to 3 microM and a V(max) of approximately 15 nmol/min/mg of protein. Mutants lacking either opuC or gbu were used to study the effects of four carnitine analogs on growth and uptake of osmolytes. In strain DP-L1044, which had OpuC and the two glycine betaine porters Gbu and BetL, triethylglycine was most effective in inhibiting growth in the presence of glycine betaine, but trigonelline was best at inhibiting growth in the presence of carnitine. Carnitine uptake through OpuC was inhibited by gamma-butyrobetaine. Dimethylglycine inhibited both glycine betaine and carnitine uptake through the Gbu porter. Carnitine uptake through the Gbu porter was inhibited by triethylglycine. Glycine betaine uptake through the BetL porter was strongly inhibited by trigonelline and triethylglycine. These results suggest that it is possible to reduce the growth of L. monocytogenes under osmotically stressful conditions by inhibiting glycine betaine and carnitine uptake but that to do so, multiple uptake systems must be affected.     

Xylose uptake by the ruminal bacterium Selenomonas ruminantium.

Selenomonas ruminantium HD4 does not use the phosphoenolpyruvate phosphotransferase system to transport xylose (S. A. Martin and J. B. Russell, J. Gen. Microbiol. 134:819-827, 1988). Xylose uptake by whole cells of S. ruminantium HD4 was inducible. Uptake was unaffected by monensin or lasalocid, while oxygen, o-phenanthroline, and HgCl2 were potent inhibitors. Menadione, antimycin A, and KCN had little effect on uptake, and acriflavine inhibited uptake by 23%. Sodium fluoride decreased xylose uptake by 10%, while N,N'-dicyclohexylcarbodiimide decreased uptake by 31%. Sodium arsenate was a strong inhibitor (83%), and these results suggest the involvement of a high-energy phosphate compound and possibly a binding protein in xylose uptake. The protonophores carbonyl cyanide m-chlorophenylhydrazone, 2,4-dinitrophenol, and SF6847 inhibited xylose uptake by 88, 82, and 43%, respectively. The cations Na+ and K+ did not stimulate xylose uptake. The kinetics of xylose uptake were nonlinear, and it appeared that more than one uptake mechanism may be involved or that two proteins (i.e., a binding protein and permease protein) with different affinities for xylose were present. Excess (10 mM) glucose, sucrose, or maltose decreased xylose uptake less than 40%. Uptake was unaffected at extracellular pH values between 6.0 and 8.0, while pH values of 5.0 and 4.0 decreased uptake 28 and 24%, respectively. The phenolic monomers p-coumaric acid and vanillin inhibited growth on xylose and xylose uptake more than ferulic acid did. The predominant end products resulting from the fermentation of xylose were lactate (7.5 mM), acetate (4.4 mM), and propionate (5.1 nM), and the Yxylose was 24.1 g/mol.     

Nitrate uptake in the halotolerant cyanobacterium Aphanothece halophytica is energy-dependent driven by DeltapH

The energetics of nitrate uptake by intact cells of the halotolerant cyanobacterium Aphanothece halophytica were investigated. Nitrate uptake was inhibited by various protonophores suggesting the coupling of nitrate uptake to the proton motive force. An artificially-generated pH gradient across the membrane (DeltapH) caused an increase of nitrate uptake. In contrast, the suppression of DeltapH resulted in a decrease of nitrate uptake. The increase of external pH also resulted in an enhancement of nitrate uptake. The generation of the electrical potential across the membrane (Deltapsi) resulted in no elevation of the rate of nitrate uptake. On the other hand, the valinomycin-mediated dissipation of Deltapsi caused no depression of the rate of nitrate uptake. Thus, it is unlikely that Deltapsi participated in the energization of the uptake of nitrate. However, Na(+)-gradient across the membrane was suggested to play a role in nitrate uptake since monensin which collapses Na(+)-gradient strongly inhibited nitrate uptake. Exogenously added glucose and lactate stimulated nitrate uptake in the starved cells. N, N'-dicyclohexylcarbodiimide, an inhibitor of ATPase, could alsoinhibit nitrate uptake suggesting that ATP hydrolysis was required for nitrate uptake. All these results indicate that nitrate uptake in A. halophytica is ATP-dependent, driven by DeltapH and Na(+)-gradient.     

Evidence for the existence of distinct transporters for the polyamines putrescine and spermidine in B16 melanoma cells

The uptake of intracellular putrescine and spermidine was examined in B16 melanoma cells. It was found that difluoromethylornithine preferentially induced putrescine transport (28-fold) compared to that for spermidine (3.5-fold). Putrescine uptake was partially Na+ dependent, whereas spermidine uptake was not. Inhibition studies with the two polyamines showed that putrescine was a poor competitive inhibitor of spermidine uptake, exhibiting a Ki of 69-75 microM, whereas the estimated Km for putrescine uptake was only 5.36 microM. By contrast, spermidine inhibition of putrescine transport produced a non-linear Eadie-Scatchard plot suggesting that putrescine was taken up by a spermidine-sensitive and a spermidine-insensitive process. The estimated spermidine Ki for inhibition of the spermidine-sensitive process was 0.125 microM. Using a series of polypyridinium quaternary salts to inhibit transport, no correlation between inhibition of putrescine uptake and inhibition of spermidine uptake was seen. Finally, the photoaffinity label, 1,12-di(N5-azido-2-nitrobenzoyl)spermine selectively inactivated the putrescine transporter(s) without affecting spermidine uptake. From these observations, it was concluded that multiple polyamine transporters are present on B16 melanoma cells and that separate, distinct transporter(s) account for the uptake of putrescine and spermidine in this cell-line following induction with difluoromethylornithine. The present of different transporters for the two polyamines indicates that expression of uptake activity for putrescine and spermidine may be under separate cellular control.     

Inositol uptake in rat aorta.

The purpose of this study was to investigate the mechanism of inositol uptake into rat thoracic aorta. 3H-inositol uptake into deendothelialized aorta was linear for at least 2 h and was composed of both a saturable, Na(+)-dependent, and a nonsaturable, Na(+)-independent component. The Na(+)-dependent component of inositol uptake had a Km of 50 microM and a Vmax of 289 pmol/mg prot/h. Exposure to LiCl, ouabain, or Ca2(+)-free Krebs-Ringer bicarbonate solution inhibited uptake. Metabolic poisoning with dinitrophenol, as well as incubation with phloretin, an inhibitor of carrier-mediated hexose transport, also inhibited uptake. Exposure to norepinephrine decreased inositol uptake, while phorbol myristate acetate was without effect. Isobutylmethylxanthine significantly increased inositol uptake, while the increased uptake due to dibutyryl cyclic AMP and forskolin were not statistically significant. Sodium nitroprusside, an activator of guanylate cyclase, and 8-bromo cyclic GMP, were without effect on uptake, as was methylene blue, an inhibitor of guanylate cyclase. Inositol uptake into the aorta was increased when the endothelium was allowed to remain intact, although this effect was likely due to uptake into both the endothelial and smooth muscle cells. These results suggest that the uptake of inositol into vascular smooth muscle is: (1) dependent upon an inward Na(+)-gradient; (2) carrier mediated, and (3) inhibited by alpha 1 adrenoceptor agonists.