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1.
The localization of labeled amine in the heart of the bat after administration of tritiated norepinephrine (NE) was studied by means of electron microscope autoradiography. Monoamine oxidase was inhibited so that the distribution of amine in both neuronal (Uptake 1) and extraneuronal (Uptake 2) sites could be analyzed. Labeling was nonrandom in both the atrial and ventricular myocardium. The highest relative specific activity was found in neural processes which showed morphological criteria of terminal adrenergic axons. Analysis of the distribution of label around the labeled axonal varicosities indicated that the radioactive amine was more concentrated peripherally than centrally in these structures. Label was also found over cardiocytes in both atrium and ventricle. The pattern of this labeling indicated that the radioactive amine was associated with myofilaments. In the ventricle, I bands were most heavily labeled, indicating a probable association of radioactive amine with thin filaments. Labeling was prevented by administration of phenoxybenzamine and decreased only in cardiocytes by normetanephrine. The nonrandom distribution of labeled amine within cardiocytes supports the view that Uptake 2 represents not only a second mechanism of inactivation of the sympathetic neurotransmitter, but may also be involved in the mediation of some of the action of NE on cardiac muscle. 相似文献
2.
Summary Catecholamines (CA) were localized in stage 11–34 domestic fowl embryos by the formaldehyde-induced fluorescence (FIF) method after exposure in vivo or in vitro to CA (noradrenaline or -methylnoradrenaline), or the CA precursor l-DOPA. The effects of drugs known to alter CA metabolism in the adult were also investigated.Negligible FIF was observed in embryos which had not been exposed to CA. After CA loading, FIF could be seen in the neural tube and in non-neural tissues such as the notochord and gut mesenchyme and to a lesser degree in suprarenal area tissue, liver endothelium, sclerotome, and myotome. This FIF was inhibited by desmethylimipramine, a blocker of adult neuronal CA uptake (Uptake 1), but not by corticosterone, a blocker of adult extraneuronal CA uptake (Uptake 2). The notochord, dorsal pancreas and some blood cells were fluorescent after l-DOPA loading, and this FIF could be greatly diminished by the DOPA decarboxylase inhibitor RO4-4602.The pattern of FIF in the axial structures (neural tube and notochord) correlated with axial flexure in both position and time, and the intensity of fluorescence was strongest cranially and caudally, where flexure is most pronounced. The FIF in gut mesenchyme cells was closely related to the movement of the intestinal protals during early gut tube formation, and to the regions of the developing intestine that undergo intense morphogenesis during their early formation. 相似文献
3.
Beet discs aged in 0.5 mM CaSO 4 develop a capacity to absorb K + and Cl − from solutions of low concentration. The initial influx of these ions is described by a hyperbolic relationship with concentration in the range 0.01 to 0.5 mM KCl, which is identical with the system 1 absorption isotherm found in other tissues. A second hyperbolic isotherm, attributable to system 2, is found at higher concentrations (1-50 mM KCl). When the transport of labeled ion to the vacuole is studied by wash-exchanging the bulk of the cytoplasmic label following the absorption period, it is noted that in the range of system 1, isotope influx to the vacuole increases with time as the concentration of labeled ions in the cytoplasm increases, while in the range of system 2, influx to the vacuole is constant from the beginning. Diminution of the cytoplasmic specific activity during radio-isotope absorption by prefilling the cytoplasm with the analogous unlabeled salt, markedly reduces subsequent radioisotope uptake to the vacuole only in the range of system 1. These experiments suggest that the cytoplasm serves as a mixing chamber, and that the plasma membrane controls ion uptake to the tissue at low concentrations, indicating that the system 1 isotherm reflects ion movement into the cytoplasm through the plasma membrane. Flux experiments support this conclusion, showing that development with age of the system 1 isotherm corresponds to a quantitatively similar increase in plasma membrane influx in 0.2 mM KCl. At higher concentrations the outer membrane no longer rate-limits entry of ions to the vacuole. Isotope influx under these conditions, described by the system 2 isotherm, presumably reflects movement across the tonoplast. 相似文献
4.
The effects of metabolic and protein synthesis inhibitors on NH 4
+ uptake by Pisum arvense plants at low (0.05 mM) and high (1 mM) external ammonium concentration were studied. In short-time experiments cycloheximide
decreased the ammonium uptake rate at low level of NH 4
+ and increased the absorption of NH 4
+ from uptake medium containing high ammonium concentration. Arsenate and azide supplied into uptake solutions at low ammonium
concentration strongly decreased or completely suppressed the NH 4
+ uptake rate, respectively. When the experiments were carried out at high level of ammonium only azide decreased the uptake
rate of NH 4
+ and arsenate stimulated this process. Dinitrophenol very strongly repressed the uptake rate of NH 4
+ at both ammonium concentrations. After removing dinitrophenol from both solutions, neither at low nor high external ammonium
level the recovery of NH 4
+ uptake rate was achieved within 150 min or 3 h, respectively. The recovery of NH 4
+ uptake rate after removing azide was observed within 90 min and 3 h at low and high ammonium concentrations, respectively.
The regulation of NH 4
+ uptake by some inhibitors at low external ammonium level was investigated using plasma membrane vesicles isolated from roots
by two-phase partitioning. Orthovanadate completely suppressed the uptake of NH 4
+ by vesicles and quinacrine decreased the NH 4
+ uptake which 55 suggests that ammonium uptake depends on activities of plasma membrane-bound enzymes. On the other hand,
it was found that dinitrophenol completely reduced the NH 4
+ uptake by vesicles.
The various effects of inhibitors on ammonium uptake dependent on external ammonium concentration suggest the action of different
ammonium transport systems in Pisum arvense roots. The ammonium transport into root cells at low NH 4
+ level requires energy and synthesis of protein in the cytoplasm.
The research was supported by grant of KBN No. 6PO4C 068 08 相似文献
5.
ATP-dependent Ca 2+ uptake distinct from that of the mitochondria is found in both plasma membrane and microsomal membranes of rat kidney. Activity attributed to these fractions is enhanced by ammonium oxalate and is apparently insensitive to NaN 3. In contrast, rat kidney mitochondrial Ca 2+ uptake is blocked by NaN 3. The pH of optimal activity is significantly higher for the mitochondrial fraction. Microsomal membrane Ca 2+ uptake differs from that of the plasma membrane. Microsomal membranes are four times as active as the plasma membrane at high (5 mM) ATP levels. Apparent Km values for Mg 2+-ATP differ in the two preparations with a higher affinity for Mg 2+-ATP found in the plasma membrane Ca 2+ uptake activity of the plasma membrane preparation is readily inhibited by Na +. Sucrose gradient density fractionation indicates that the observed microsomal membrane Ca 2+ pump activity is associated with membrane vesicles derived from the endoplasmic reticulum. Ca 2+ pump activity of both plasma membrane and microsomal fraction is depressed din the adrenalectomized rat. This activity is not restored by a single natriuretic dose of aldosterone. 相似文献
6.
l. A kinetic analysis of [ 3H]noradrenaline ([ 3H]NA) and [ 3H]adrenaline ([ 3H]A) uptake in the photophores of Porichthys notatus revealed high (uptake,) and low affinity (uptake 2) components for both [ 3H]catecholamines at molarities of incubation between 10 −8 and 10 −5M.2. Uptake 1 for [ 3H]NA displayed a greater capacity than that for [ 3H]A. With [ 3H]NA, the contribution of uptake 2 to the overall uptake was small or insignificant at all but the highest concentration in the medium, whereas uptake 2 of [ 3H]A was relatively more substantial.3. Uptake, for either [ 3H]NA or [ 3H]A was temperature-, ouabain- and dinitrophenol-sensitive, as well as highly sodium- and potassium-dependent. Uptake 1 for [ 3h]a was significantly more affected by calcium omission than that for [ 3H]NA.4. Uptake, for both [ 3H]NA and [ 3h]a was comparably reduced by DMI, imipramine, dopamine and the converse, non-radioactive catecholamine. Serotonin reduced the accumulation of the two catecholamines to an extent that was directly ([ 3h]na) or inversely ([ 3h]a) proportional to its concentration. The adrenergic antagonists propranolol and phentolamine showed selectivity against uptake 1 for [ 3H]NA and [ 3h]a, respectively.5. These and previous radioautographic results indicate that there are functionally distinct, differentially distributed, carrier-mediated, high-affinity transport systems for NA and A in the photophores of Porichthys. 相似文献
7.
Abstract: Rats were treated with reserpine, desmethylimipramine, or carrier, either alone or in combination with tropolone. Either 10 min ( t1) or 1 h ( t2) after intraventricular injection of [ 3H]noradrenaline, they were decapitated. The total 3H activity and the recovery of [ 3H]noradrenaline were determined in tissue extracts from various brain regions. Maximum total 3H activity was measured at t1 in all tropolone-treated rats; the mean sum of these results served as an estimate of the initial tissue concentration of [ 3H]noradrenaline. At t1, 40–50% of the sum of [ 3H]noradrenaline and its metabolites was recovered unchanged in normal rats; reserpine and DMI reduced the recovery to 18–27%. In all groups, the decline of [ 3H]noradrenaline was retarded after t1. Inhibition of catechol- O-methyltransferase by tropolone caused consistently elevated [ 3H]noradrenaline levels, but did not affect the metabolic rate after t1 when compared with similarly pretreated, but tropolone-free rats. Thus, if catechol- O-methyltransferase was inhibited during the injection of [ 3H]noradrenaline, a higher percentage of the amine had been taken up into spaces with a slow noradrenaline turnover. The maximum increase was seen when the neuronal uptake, was inhibited by desmethylimipramine. This supported the hypothesis that an additional extraneuronal space exists, in addition to the known intraneuronal and extraneuronal compartments, which has a slow noradrenaline turnover. The tropolone effect on the noradrenaline recovery possibly shows that there might be a saturable “methylating system,” similar to that described for the periphery, in which catechol- O-methyltransferase is linked to the extraneuronal uptake 2. By affecting the access of noradrenaline to non-neuronal cells it might influence the rate of noradrenaline elimination from the intercellular space. 相似文献
8.
In microsomes from 24-hour-old radish ( Raphanus sativus L.) seedlings ATP-dependent Ca 2+ uptake occurs only in inside-out plasma membrane vesicles (F Rasi-Caldogno, MC Pugliarello, MI De Michelis [1987] Plant Physiol 83: 994-1000). A Ca 2+-dependent ATPase activity can be shown in the same microsomes, when assays are performed at pH 7.5. The Ca 2+-dependent ATPase is stimulated by the Ca 2+ ionophore A 23187 and is localized at the plasma membrane. Ca 2+-dependent ATPase activity and ATP-dependent Ca 2+ uptake present very similar saturation kinetics with erythrosin B (50% inhibition at about 0.1 micromolar), free Ca 2+ (half-maximal rate at about 70 nanomolar), and MgATP ( Km 15-20 micromolar). Ca 2+ uptake can be sustained by GTP or ITP at about 60% the rate measured in the presence of ATP; only very low Ca 2+ uptake is sustained by CTP or UTP and none by ADP. These results indicate that the Ca 2+-ATPase described in this paper is the enzyme which drives active transport of Ca 2+ at the plasma membrane of higher plants. 相似文献
9.
—The kinetics of plasma choline (Ch) and the uptake of plasma Ch into the brain were studied by means of intravenous infusion of [ 2H 4]Ch at various rates into anaesthetized and conscious rats. [ 2H 4]Ch levels in both arterial and venous plasma at steady state were linearly related to the infusion rate; however, unlabelled Ch levels were independent of infusion rate. [ 2H 4]Ch levels were higher in the arterial plasma than in the venous plasma, while unlabelled Ch levels were higher in the venous plasma than in the arterial plasma. It was concluded that Ch is being generated in the brain and is released into the venous effluent. The supply of Ch to the plasma is not decreased if the plasma Ch level is increased. The clearance and turnover of Ch in the compartment of its initial distribution are 75 ml kg -1 min -1 and 716 nmol kg -1 min -1, respectively. The uptake of plasma Ch into the brain is not saturated even at very high levels of plasma Ch. 相似文献
10.
Uptake of phosphate ions by 1 mm segments of isolated maize root cortex layers was studied. Cortex segments (from roots of 8 days old maize plants) absorb phosphate ions from 1 mM KH 2PO 4 in 0.2 mM CaSCO 4 at the average rate of 34.3 ±3.2 μg P i g ?1 (fr. m.) h ?1, i.e. 0.35 ± 0.02 μmol P i g ?1 (fr. m.) h ?1. Phosphate uptake considerably increases after a certain period of “augmentation”, i.e. washing in aerated 0.2 mM CaSO 4. This increase is completely blocked by the presence of 10 μg ml ?1 cycloheximide. The relation of uptake rate to phosphate concentration in the medium was shown to have 3 phases in the concentration range of 0.02 - 40 mM. Transition points were found between 0.8–1 mM and 10–20 mM. Following K m and V max values were found: K m[mM] : 0.37 - 3.82 - 27.67 V max[μg P i g ?1 (fr. m.) h ?1] : 3.33 - 39.40 - 66.67 We have found no sharp pH optimum for phosphate uptake. It proceeds at almost constant rate till pH 6.0 and then the uptake rate drops with increasing pH. At low phosphate concentrations (1 mM) the lowest uptake rate was found at 5 and 13 °C, while the uptake is higher at 5 °C than at 13 °C at phosphate concentrations higher than 1 mM. At these concentrations uptake rate at 35 °C is lower than at 25 °C. Phosphate uptake considerably decreased in anaerobic conditions. DNP and iodoacetate (0.1 mM) completely blocked phosphate uptake from 1 mM KH 2PO 4, while uptake from 5 and 10 mM KH 2PO 4 was left unaffected by these substances. The inhibitors of active - SH groups NEM and PCMB inhibited phosphate uptake: 10 ?3 M NEM by 81.6%, 10 4 M NEM by 42% and 10 ?4 M PCMB by 42%. 相似文献
11.
Microsomal vesicles from 24-hour-old radish ( Raphanus sativus L.) seedlings accumulate Ca 2+ upon addition of MgATP. MgATP-dependent Ca 2+ uptake co-migrates with the plasma membrane H +-ATPase on a sucrose gradient. Ca 2+ uptake is insensitive to oligomycin, inhibited by vanadate (IC 50 40 micromolar) and erythrosin B (IC 50 0.2 micromolar) and displays a pH optimum between pH 6.6 and 6.9. MgATP-dependent Ca 2+ uptake is insensitive to protonophores. These results indicate that Ca 2+ transport in these microsomal vesicles is catalyzed by a Mg 2+-dependent ATPase localized on the plasma membrane. Ca 2+ strongly reduces ΔpH generation by the plasma membrane H +-ATPase and increases MgATP-dependent membrane potential difference (Δψ) generation. These effects of Ca 2+ on ΔpH and Δψ generation are drastically reduced by micromolar erythrosin B, indicating that they are primarily a consequence of Ca 2+ uptake into plasma membrane vesicles. The Ca 2+-induced increase of Δψ is collapsed by permeant anions, which do not affect Ca 2+-induced decrease of ΔpH generation by the plasma membrane H +-ATPase. The rate of decay of MgATP-dependent ΔpH, upon inhibition of the plasma membrane H +-ATPase, is accelerated by MgATP-dependent Ca 2+ uptake, indicating that the decrease of ΔpH generation induced by Ca 2+ reflects the efflux of H + coupled to Ca 2+ uptake into plasma membrane vesicles. It is therefore proposed that Ca 2+ transport at the plasma membrane is mediated by a Mg 2+-dependent ATPase which catalyzes a nH +/Ca 2+ exchange. 相似文献
12.
NH 4+ and NO 3? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO 3?up take followed saturable kinetics (K 2=4.9 μg-at N t ?1, V max= 2.85 μg- at N, g(wet) ?1. h ?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO 3? uptake was reduced one-half in the presence of NH 4+, but presence of NO 3? had no effect on NH 4+ uptake. Darkness reduced both NO 3? and NH 4+ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO 3? uptake followed saturable kinetices: K 2=13.1 μg-at N. l ?1. V max=3.05 μg-at N. g(wet) ?1. h ?1.NH 4+ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K 2=5.3 μg-at N.1–1, V max= 2.38 μg-at N G (wet) ?1.h ?1: at higher concentration uptake increased lincarly with concentrations. NO 3?and NH 4+ were taken up simulataneously: presence of one form did not affect uptake of the other. 相似文献
13.
Protoplasts isolated from beetroot tissue took up glucose preferentially whereas sucrose was transported more slowly. The 14C-label from [ 14C]glucose and [ 14C]sucrose taken up by the cells could be detected rapidly in phosphate esters and, after feeding of [ 14C]glucose was found also in sucrose. The temperature-dependent uptake process (activation energy E A about 50 kJ · mol –1) seems to be carrier mediated as indicated by its substrate saturation and, for glucose, by competition experiments which revealed positions C 1, C 5 and C 6 of the D-glucose molecule as important for effective uptake. The apparent K m(20° C) for glucose (3-O-methylglucose) was about 1 mM whereas for sucrose a significantly lower apparent affinity was determined (K m about 10 mM). When higher concentrations of glucose (5 mM) or sucrose (20 mM) were administered, the uptake process followed first-order kinetics. Carrier-mediated transport was inhibited by N,N-dicyclohexylcarbodiimide, Na-orthovanadate, p–chloromercuribenzenesulfonic acid, and by uncouplers and ionophores. The uptake system exhibited a distinct pH optimum at pH 5.0. The results indicate that generation of a proton gradient is a prerequisite for sugar uptake across the plasma membrane. Protoplasts from the bundle regions in the hypocotyl take up glucose at higher rates than those derived from bundle-free regions. The results favour the idea that apoplastic transport of assimilates en route of unloading might be restricted to distinct areas within the storage organ (i.e. the bundle region) whereas distribution in the storage parenchyma is symplastic.Abbreviations CCCP
Carbonylcyanide m–chlorophenylhydrazone
- DCCD
N,N-dicyclohexylcarbodiimide
- DOG
deoxyglucose
- Mes
2-(N-morpholino)ethanesulfonic acid
- 3-OMG
3-O-methylglucose
- PCMBS
p–chloromercuribenzenesulfonic acid
- SDS
Sodium dodecyl sulfate
- Tris
2-amino-2-(hydroxymethyl)-1,3-propanediol 相似文献
14.
Managanese (Mn) is an essential trace element at low concentrations, but at higher concentrations is neurotoxic. It has several
chemical and biochemical properties similar to iron (Fe), and there is evidence of metabolic interaction between the two metals,
particularly at the level of absorption from the intestine. The aim of this investigation was to determine whether Mn and
Fe interact during the processes involved in uptake from the plasma by the brain and other organs of the rat. Dams were fed
control (70 mg Fe/kg), Fe-deficient (5–10 mg Fe/kg), or Fe-loaded (20 g carbonyl Fe/kg) diets, with or without Mn-loaded drinking
water (2 g Mn/L), from day 18–19 of pregnancy, and, after weaning the young rats, were continued on the same dietary regimens.
Measurements of brain, liver, and kidney Mn and nonheme Fe levels, and the uptake of 54Mn and 59Fe from the plasma by these organs and the femurs, were made when the rats were aged 15 and 63 d. Organ nonheme Fe levels
were much higher than Mn levels, and in the liver and kidney increased much more with Fe loading than did Mn levels with Mn
loading. However, in the brain the increases were greater for Mn. Both Fe depletion and loading led to increased brain Mn
concentrations in the 15-d/rats, while Fe loading also had this effect at 63 d. Mn loading did not have significant effects
on the nonheme Fe concentrations. 54Mn, injected as MnCl 2 mixed with serum, was cleared more rapidly from the circulation than was 59Fe, injected in the form of diferric transferrin. In the 15-d-rats, the uptake of 54Mn by brain, liver, kidneys, and femurs was increased by Fe loading, but this was not seen in the 63-d rats. Mn supplementation
led to increased 59Fe uptake by the brain, liver, and kidneys of the rats fed the control and Fe-deficient diets, but not in the Fe-loaded rats.
It is concluded that Mn and Fe interact during transfer from the plasma to the brain and other organs and that this interaction
is synergistic rather than competitive in nature. Hence, excessive intake of Fe plus Mn may accentuate the risk of tissue
damage caused by one metal alone, particularly in the brain. 相似文献
15.
Summary Plasma membrane vesicles, which are mostly right side-out, were isolated from corn leaves by aqueous two-phase partitioning method. Characteristics of Ca 2+ transport were investigated after preparing inside-out vesicles by Triton X-100 treatment. 45Ca 2+ transport was assayed by membrane filtration technique. Results showed that Ca 2+ transport into the plasma membrane vesicles was Mg-ATP dependent. The active Ca 2+ transport system had a high affinity for Ca 2+( K
m
(Ca 2+)=0.4 m) and ATP( K
m
(ATP)=3.9 m), and showed pH optimum at 7.5. ATP-dependent Ca 2+ uptake in the plasma membrane vesicles was stimulated in the presence of Cl – or NO
3
–
. Quenching of quinacrine fluorescence showed that these anions also induced H + transport into the vesicles. The Ca 2+ uptake stimulated by Cl – was dependent on the activity of H + transport into the vesicles. However, carbonylcyanide m-chlorophenylhydrazone (CCCP) and VO
4
3–
which is known to inhibit the H + pump associated with the plasma membrane, canceled almost all of the Cl –-stimulated Ca 2+ uptake. Furthermore, artificially imposed pH gradient (acid inside) caused Ca 2+ uptake into the vesicles. These results suggest that the Cl –-stimulated Ca 2+ uptake is caused by the efflux of H + from the vesicles by the operation of Ca 2+/H + antiport system in the plasma membrane. In Cl –-free medium, H + transport into the vesicles scarcely occurred and the addition of CCCP caused only a slight inhibition of the active Ca 2+ uptake into the vesicles. These results suggest that two Ca 2+ transport systems are operating in the plasma membrane from corn leaves, i.e., one is an ATP-dependent active Ca 2+ transport system (Ca 2+ pump) and the other is a Ca 2+/H + antiport system. Little difference in characteristics of Ca 2+ transport was observed between the plasma membranes isolated from etiolated and green corn leaves. 相似文献
16.
Nitrate and NO 2− transport by roots of 8-day-old uninduced and induced intact barley ( Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO 3− and NO 2− was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km, possibly a constitutive uptake system, and a linear system for both NO 3− and NO 2−. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO 3− and NO 2− uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of NO 3− and NO 2− uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of NO 3− uptake ranged from 34 to 36 micromolar and of NO 2− uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO 3− and NO 2− inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that NO 3− was a more effective inhibitor than NO 2−. Nitrate and NO 2− transport by both the low and high Km systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited. 相似文献
17.
The photoacoustic response of the photosynthetic apparatus to a short light pulse consists of three components: heat evolution, O 2 evolution and CO 2 uptake. Recent attempts of deconvoluting the individual components by curve-fitting by means of model curves [Kolbowski et al. (1990) Photosynth Res 25: 309–316] suffered from the fact that the model curve for CO 2 uptake changed its curve shape with CO 2 concentration. Here, it is shown that good fits can be obtained if a stretching factor is incorporated into the fitting routine which adjusts the shape of the uptake model curve. The relationship between CO 2 uptake und H + transport across the thylakoid membrane was investigated by experiments in different CO 2 concentrations from 0 to 7%. It was found that under limiting conditions (7% CO 2) the flux ratio CO 2: O 2 was close to 4. This was compared with the value expected from the stoichiometries of the linear electron transport chain. 相似文献
18.
Fe uptake in dicotyledonous plants is mediated by a root plasma membrane-bound ferric reductase that reduces extracellular Fe(III)-chelates, releasing Fe 2+ ions, which are then absorbed via a metal ion transporter. We previously showed that Fe deficiency induces an increased capacity to absorb Fe and other micronutrient and heavy metals such as Zn 2+ and Cd 2+ into pea ( Pisum sativum L.) roots [Cohen et al. (1998) Plant Physiol 116:1063–1072). To investigate the molecular basis for this phenomenon, an Fe-regulated transporter that is a homologue of the Arabidopsis IRT1 micronutrient transporter was isolated from pea seedlings. This cDNA clone, designated RIT1 for root iron transporter, encodes a 348 amino acid polypeptide with eight putative membrane-spanning domains that is induced under Fe deficiency and can functionally complement yeast mutants defective in high- and low-affinity Fe transport. Chelate buffer techniques were used to control Fe 2+ in the uptake solution at nanomolar activities representative of those found in the rhizosphere, and radiotracer methodologies were employed to show that RIT1 is a very high-affinity 59Fe 2+ uptake system ( K
m =54–93 nM). Additionally, radiotracer ( 65Zn, 109Cd) flux techniques were used to show that RIT can also mediate a lower affinity Zn and Cd influx ( K
m of 4 and 100 M, for Zn 2+ and Cd 2+, respectively). These findings suggest that, in typical agricultural soils, RIT1 functions primarily as a high-affinity Fe 2+ transporter that mediates root Fe acquisition. This is consistent with recent findings with Arabidopsis IRT1 knockout mutants that strongly suggest that this transporter plays a key role in root Fe uptake and nutrition. However, the ability of RIT1 to facilitate Zn and Cd uptake when these metals are present at elevated concentrations suggests that RIT1 may be one pathway for the entry of toxic metals into the food chain. Furthermore, the finding that plant Fe deficiency status may promote heavy metal uptake via increased expression of this transporter could have implications both for human nutrition and also for phytoremediation, the use of terrestrial plants to sequester toxic metals from contaminated soil. 相似文献
19.
In the range 10 ?6M - 5 × 10 ?2M uptake of K + in excised roots of barley ( Hordeum vulgare L. cv. Herta) with low and high K content could in both cases be represented by an isotherm with four phases. Uptake, especially in the range of the lower phases, was reduced in high K roots through decreases in V max and increases in K m. Similar data for other plants are also shown to be consistent with multiphasic kinetics. The concentrations at which transitions occurred were not affected by the K status, indicating the existence of separate uptake and transition sites. Uptake was markedly reduced in the presence of 10 ?5M 2,4-dinitrophenol, especially at low K + concentrations, but the isotherms remained multiphasic. This contraindicates major contributions from a non-carrier-mediated, passive flux. A tentative hypothesis for multiphasic ion uptake envisions a structure which changes conformation as a result of all-or-none changes in a separate transition site. The structure is “tight” at low external ion concentrations (low V max. low K m. active uptake, allosteric regulation) and “loose” at high concentrations (high V max- high K m- facilitated diffusion, no regulation). 相似文献
20.
Ca 2+ stimulates the binding of a variety of prostaglandins (PG) to liver mitochondria. Optimal binding is observed at slightly acidic pH and at concentrations of Ca 2+ between 200 and 500 μm. The stimulation of the binding requires the active transport of Ca 2+ in mitochondria and is only observed in the absence of permeant anions. The maximal amount of PG bound is about 3 nmol/mg of mitochondrial protein. All PG tested induce efflux of the Ca 2+ taken up by mitochondria without impairing the ability of mitochondria to actively accumulate it. Optimal stimulation of the efflux of Ca 2+ requires concentrations of PG higher than those used in the PG-binding experiments and is associated with an evident uncoupling of the respiration that follows a Ca 2+-induced O 2 uptake jump. The “uncoupling” by PG is explained by postulating the entrance of protonated PG into mitochondria, followed by their exit from the organelle as 2:1 complexes with Ca 2+. 相似文献
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