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Dithiol Oxidant and Disulfide Reductant Dynamically Regulate the Phosphorylation of Light-Harvesting Complex II Proteins in Thylakoid Membranes1

Martinsuo, Päivi; Pursiheimo, Saijaliisa; Aro, Eva-Mari; Rintamäki, Eevi
Fonte: The American Society for Plant Biologists Publicador: The American Society for Plant Biologists
Tipo: Artigo de Revista Científica
Publicado em /09/2003 EN
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484.21555%
Light-induced phosphorylation of light-harvesting chlorophyll a/b complex II (LHCII) proteins in plant thylakoid membranes requires an activation of the LHCII kinase via binding of plastoquinol to cytochrome b6f complex. However, a gradual down-regulation of LHCII protein phosphorylation occurs in higher plant leaves in vivo with increasing light intensity. This inhibition is likely to be mediated by increasing concentration of thiol reductants in the chloroplast. Here, we have determined the components involved in thiol redox regulation of the LHCII kinase by studying the restoration of LHCII protein phosphorylation in thylakoid membranes isolated from high-light-illuminated leaves of pumpkin (Cucurbita pepo), spinach (Spinacia oleracea), and Arabidopsis. We demonstrate an experimental separation of two dynamic activities associated with isolated thylakoid membranes and involved in thiol regulation of the LHCII kinase. First, a thioredoxin-like compound, responsible for inhibition of the LHCII kinase, became tightly associated and/or activated within thylakoid membranes upon illumination of leaves at high light intensities. This reducing activity was completely missing from membranes isolated from leaves with active LHCII protein phosphorylation...

Effects of Temperature on H+ Uptake And Release During Circadian Rhythmic Movements of Excised Samanea Motor Organs 1

Lee, Youngsook; Satter, Ruth L.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /02/1988 EN
Relevância na Pesquisa
564.7701%
A previous study revealed that Samanea saman leaflets open more completely and close less completely as temperature is increased. We now demonstrate that, as temperature is increased, extensor cells release more H+ during their swelling phase (opening), but flexor motor cells release less H+ during their swelling phase (closure).

Characterization of an ATPase Associated with the Inner Envelope Membrane of Amyloplasts from Suspension-Cultured Cells of Sycamore (Acer pseudoplatanus L.) 1

Harinasut, Poontariga; Takabe, Tetsuko; Akazawa, Takashi; Tagaya, Mitsuo; Fukui, Toshio
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /09/1988 EN
Relevância na Pesquisa
482.16156%
Amyloplast envelope membranes isolated from cultured, white-wild cells of sycamore (Acer pseudoplatanus L.) have been found to contain a Mg2+-ATPase, ranging in specific activity from 5 to 30 nanomoles per minute per milligram protein. This ATPase hydrolyzes a broad range of nucleoside triphosphates, whereas it hydrolyzes nucleoside mono- and diphosphates poorly, if at all. The ATPase activity was stimulated by several divalent cations, including Mg2+, Mn2+ and Ca2+, whereas it was not affected by Sr2+, K+, or Na+. The Km for total ATP was 0.6 millimolar, and the activity showed a broad pH optimum between 7.5 and 8.0. The ATPase was insensitive to N,N′-dicyclohexylcarbodiimide and oligomycin, but it was inhibited by vanadate. All these characteristics are basically similar to those reported previously for the Mg2+-ATPase of the chloroplast inner-envelope membrane. Likewise, the amyloplast envelope enzyme was shown to be located specifically on the inner envelope membrane. The amyloplast envelope membranes were chemically modified with a series of unique affinity labeling reagents, the adenosine polyphosphopyridoxals (M Tagaya, T Fukui 1986 Biochemistry 25: 2958-2964). About 90% of the ATPase activity was lost when the envelope membranes were preincubated with 0.1 millimolar adenosine triphosphopyridoxal. Notably...

Tonoplast Stability and Survival of Isolated Vacuoles in Different Buffers 1

De Leon, Jose Luis Diaz; Daie, Jaleh; Wyse, Roger
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /10/1988 EN
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564.7701%
The mechanism of sucrose transport into vacuoles isolated from leaf tissue has been studied only in barley (Hordeum vulgare) mesophyll cells. In this tissue, sucrose transport was reported to be a facilitated diffusion. We have observed a facilitated diffusion of sucrose into vacuoles isolated from this tissue. However, no pH dependence was observed. Evidence is presented indicating that the pH dependence of sucrose uptake into vacuoles may be an artifact, reflecting tonoplast instability and survival of isolated vacuoles in different buffers. Apparently vacuoles do not withstand exposure to some commonly used buffers.

Diclofop-Methyl Increases the Proton Permeability of Isolated Oat-Root Tonoplast 1

Ratterman, Denise Marie; Balke, Nelson E.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /10/1989 EN
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483.6852%
Diclofop-methyl (methyl ester of 2-[4-(2′,4′-dichlorophenoxy)phenoxy]propionate; 100 micromolar) and diclofop (100 micromolar) inhibited both ATP- and PPi-dependent formation of H+ gradients by tonoplast vesicles isolated from oat (Avena sativa L., cv Dal) roots. Diclofop-methyl (1 micromolar) significantly reduced the steady-state H+ gradient generated in the presence of ATP. The ester (diclofop-methyl) was more inhibitory than the free acid (diclofop) at pH 7.4, but this relative activity was reversed at pH 5.7. Neither compound affected the rate of ATP or PPi hydrolysis by the proton-pumping enzymes. Diclofop-methyl (50, 100 micromolar), but not diclofop (100 micromolar), accelerated the decay of nonmetabolic H+ gradients established across vesicle membranes. Diclofop-methyl (100 micromolar) did not collapse K+ gradients across vesicle membranes. Both the (+)- and (−)-enantiomers of diclofop-methyl dissipated nonmetabolic H+ gradients established across vesicle membranes. Diclofop-methyl, but not diclofop (each 100 micromolar), accelerated the decay of H+ gradients imposed across liposomal membranes. These results show that diclofop-methyl causes a specific increase in the H+ permeability of tonoplast.

Evidence for Cotransport of Nitrate and Protons in Maize Roots 1: I. Effects of Nitrate on the Membrane Potential

McClure, Peter R.; Kochian, Leon V.; Spanswick, Roger M.; Shaff, Jon E.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /05/1990 EN
Relevância na Pesquisa
564.7701%
The electrical response of nitrate-grown maize (Zea mays L.) roots to 0.1 millimolar nitrate was comprised of two sequential parts: a rapid and transient depolarization of the membrane potential, followed by a slower, net hyperpolarization to a value more negative than the original resting potential. The magnitude of the response was smaller in roots of seedlings grown in the absence of nitrate, but, within 3 hours of initial exposure to 0.1 millimolar nitrate, increased to that of nitrate-grown roots. Chloride elicited a separate electrical response with a pattern similar to that of the nitrate response. However, the results presented in this study strongly indicate that the electrical response to nitrate reflects the activity of a nitrate-inducible membrane transport system for nitrate which is distinct from that for chloride. Inhibitors of the plasmalemma H+-ATPase (vanadate, diethylstilbestrol) completely inhibited both parts of the electrical response to nitrate, as did alkaline external pH. The magnitude of the initial nitrate-dependent, membrane potential depolarization was independent of nitrate concentration, but the subsequent nitrate-dependent hyperpolarization showed saturable dependence with an apparent Km of 0.05 millimolar. These results support a model for nitrate uptake in maize roots which includes a depolarizing NO3−/H+ symport. The model proposes that the nitrate-dependent membrane potential hyperpolarization is due to the plasma membrane proton pump...

Cause for Dark, Chilling-Induced Inactivation of Photosynthetic Oxygen-Evolving System in Cucumber Leaves 1

Shen, Jian-Ren; Terashima, Ichiro; Katoh, Sakae
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /08/1990 EN
Relevância na Pesquisa
482.16156%
Effects on oxygen evolution of the storage of detached cucumber (Cucumis sativus) leaves at 0°C in the dark were investigated with thylakoids and oxygen-evolving photosystem II membranes isolated from stored leaves. The cold and dark treatment of leaves selectively inactivated electron transport on the oxidizing side of photosystem II. Photosystem II membranes isolated from treated leaves were largely depleted of two proteins of 20 and 14 kilodaltons, which correspond to the extrinsic 23- and 17- kilodalton proteins of spinach functioning in oxygen evolution. The manganese content of photosystem II membranes was also markedly reduced by the treatment. Thus, the inactivation of oxygen evolution induced by the dark, chilling treatment is ascribed to solubilization of the 20- and 14-kilodalton proteins and extraction of manganese.

Photoacoustic Measurements in Vivo of Energy Storage by Cyclic Electron Flow in Algae and Higher Plants 1

Herbert, Stephen K.; Fork, David C.; Malkin, Shmuel
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /11/1990 EN
Relevância na Pesquisa
565.06055%
Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem II is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.

Substrate Regulation of Single Potassium and Chloride Ion Channels in Arabidopsis Plasma Membrane

Lew, Roger R.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /02/1991 EN
Relevância na Pesquisa
565.06055%
Patch clamp measurements of excised inside-out patches of Arabidopsis thaliana plasma membrane reveal at least two ion channels which conduct either potassium or chloride. The conductance of the potassium channel ranged from 5 to 70 picosiemens depending on KCl concentration. The conductance increased linearly with increasing cytoplasmic-side [KCl]; the extent of this dependence declined as extracytoplasmic-side [KCl] was increased. This indicates that substrate regulation of the potassium channel is a consequence of the molecular architecture of the channel: in particular, multi-ion binding sites within the channel pore. The chloride channel conductance (ranging from 5-40 picosiemens) was independent of cytoplasmic-side [KCl] until a threshold concentration of about 300 millimolar was reached. Such behavior is expected only if the channel is allosterically regulated by cytoplasmic-side K+ and/or Cl−. The median open times of either channel (about 200 milliseconds for the potassium channel and 20 milliseconds for the chloride channel) were unaffected by substrate concentrations.

N-Cyclo-N′-(4-Dimethylamino-α-Naphthyl)Carbodiimide Inhibits Membrane-Bound and Partially Purified Tonoplast ATPase from Maize Roots

Brauer, David; Tu, Shu-I
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /03/1991 EN
Relevância na Pesquisa
564.7701%
Certain carboxylic acid groups within the primary structure of proton translocating proteins are thought to be involved in the proton pathway. In this report, the effects of a lipophilic carboxylic acid reactive reagent, N-cyclo-N′(4-dimethylamino-α-naphthyl)carbodiimide (NCD-4), on the two types of proton pumps in maize (Zea mays L.) root microsomes were investigated. NCD-4 was found to inhibit the vacuolar-type H+-ATPase in microsomal preparations; however, the plasma membrane-type H+-ATPase was unaffected. The H+-ATPase in highly purified tonoplast vesicles was also inhibited by NCD-4. Inhibition was dependent on the concentration and length of exposure to the reagent. However, there was little, if any, increase in the fluorescence of treated vesicles, indicating few carboxylic acid residues were reacting. Inhibition of the tonoplast H+-ATPase by NCD-4 was examined further with a partially purified preparation. The partially purified H+-ATPase also showed sensitivity to the NCD-4, supporting the hypothesis that this carboxylic acid reagent is an inhibitor of the tonoplast ATPase from maize roots.

Identification and Analysis of a Barley cDNA Clone Encoding the 31-Kilodalton LHC IIa (CP29) Apoprotein of the Light-Harvesting Antenna Complex of Photosystem II 1

Morishige, Daryl T.; Thornber, J. Philip
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /01/1992 EN
Relevância na Pesquisa
564.7701%
The light-harvesting complex (LHC) of photosystem II is composed of several different pigment-binding apoproteins. We have identified a cDNA clone LHCIIa-1 encoding the 31-kilodalton LHC IIa (CP29, Chl a/b-P1) apoprotein of barley (Hordeum vulgare). Direct protein microsequencing of an internal peptide fragment from the LHC IIa apoprotein has been used to identify unequivocally the cDNA clone as that coding for the LHC IIa apoprotein. Microsequencing of the 28-kilodalton LHC IIc protein (CP26) showed only minor sequence similarity to the LHC IIa protein, indicating that they are two different gene products. LHCIIa-1 codes for a protein of 286 amino acid residues (molecular weight, 31,308), which displays strong similarities to other pigment-binding LHC proteins, and yet contains an additional 42 amino acid residue segment. Two regions of strong intramolecular sequence similarity are also observed.

Primary Structure of Cytochrome b5 from Cauliflower (Brassica oleracea L.) Deduced from Peptide and cDNA Sequences 1

Kearns, Ellen V.; Keck, Pamela; Somerville, Chris R.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /07/1992 EN
Relevância na Pesquisa
564.7701%
Cytochrome b5 is a microsomal protein that functions as an intermediate electron donor in fatty acid desaturation and other oxidation/reduction reactions. cDNA clones were isolated from cauliflower (Brassica oleracea L.) by using oligonucleotides based on the partial amino acid sequence of the protein. The deduced amino acid sequence of the polypeptide exhibited approximately 30% sequence identity with the homologous protein from vertebrates.

Thylakoid Organization in the Chromophyte Alga Ochromonas danica 1: Isolation and Characterization of a New Pigment-Protein Complex

Gibbs, Pamela B.; Biggins, John
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /09/1991 EN
Relevância na Pesquisa
565.06055%
This report describes the isolation and preliminary characterization of a new pigment-protein complex from the chromophyte alga, Ochromonas danica. The pigment-protein complex was obtained by extracting a thylakoid membrane preparation with the zwitterionic detergent lauryldimethylamine oxide followed by ultracentrifugation on sucrose gradients. The pigment-protein complex has been characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, absorption spectroscopy, and low temperature (77 Kelvin) chlorophyll fluorescence spectroscopy. A polypeptide with a monomeric molecular weight of 31,000 as determined by denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis was the major constituent of this pigment-protein complex. The major pigment in this complex was chlorophyll a, although an as yet unidentified carotenoid was also present. There was no evidence for the presence of chlorophyll c.

Nonsedimentable Microvesicles from Senescing Bean Cotyledons Contain Gel Phase-Forming Phospholipid Degradation Products 1

Yao, Kening; Paliyath, Gopinadhan; Thompson, John E.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /10/1991 EN
Relevância na Pesquisa
485.1079%
A mixture of liquid-crystalline and gel-phase lipid domains is detectable by wide angle x-ray diffraction in smooth microsomal membranes isolated from senescent 7-day-old cotyledons, whereas corresponding membranes from young 2-day-old cotyledons are exclusively liquid-crystalline. The gel-phase domains in the senescent membranes comprise phospholipid degradation products including diacylglycerols, free fatty acids, long-chain aldehydes, and long-chain hydrocarbons. The same complement of phospholipid degradation products is also present in nonsedimentable microvesicles isolated from senescent 7-day-old cotyledons by filtration of a 250,000g, 12-hour supernatant through a 300,000 dalton cut-off filter. The phospholipid degradation products in the microvesicles form gel-phase lipid domains when reconstituted into phospholipid liposomes. Nonsedimentable microvesicles of a similar size, which are again enriched in the same gel-phase-forming phospholipid degradation products, are also generated in vitro from smooth microsomal membranes isolated from 2-day-old cotyledons when Ca2+ is added to activate membrane-associated lipolytic enzymes. The Ca2+-treated membranes do not contain detectable gel-phase domains, suggesting that the phospholipid degradation products are completely removed by microvesiculation. The observations collectively indicate that these nonsedimentable microvesicles serve as a vehicle for moving phospholipid degradation products out of membrane bilayers into the cytosol. As noted previously (Yao K...

Transport of Arginine and Aspartic Acid into Isolated Barley Mesophyll Vacuoles 1

Martinoia, Enrico; Thume, Monika; Vogt, Esther; Rentsch, Doris; Dietz, Karl-Josef
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /10/1991 EN
Relevância na Pesquisa
565.06055%
The transport of arginine into isolated barley (Hordeum vulgare L.) mesophyll vacuoles was investigated. In the absence of ATP, arginine uptake was saturable with a Km of 0.3 to 0.4 millimolar. Positively charged amino acids inhibited arginine uptake, lysine being most potent with a Ki of 1.2 millimolar. In the presence of free ATP, but not of its Mg-complex, uptake of arginine was drastically enhanced and a linear function of its concentration up to 16 millimolar. The nonhydrolyzable adenylyl imidodiphosphate, but no other nucleotide tested, could substitute for ATP. Therefore, it is suggested that this process does not require energy and does not involve the tonoplast ATPase. The ATP-dependent arginine uptake was strongly inhibited by p-chloromercuriphenylsulfonic acid. Furthermore, hydrophobic amino acids were inhibitory (I50 phenylalanine 1 millimolar). Similar characteristics were observed for the uptake of aspartic acid. However, rates of ATP-stimulated aspartic acid transport were 10-fold lower as compared to arginine transport. Uptake of aspartate in the absence of ATP was negligible.

Asymmetric distribution of cytochrome P-450 and NADPH–cytochrome P-450 (cytochrome c) reductase in vesicles from smooth endoplasmic reticulum of rat liver

Cooper, Michael B.; Craft, John A.; Estall, Margaret R.; Rabin, Brian R.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 15/09/1980 EN
Relevância na Pesquisa
484.1315%
1. The topography of cytochrome P-450 in vesicles from smooth endoplasmic reticulum of rat liver has been examined. Approx. 50% of the cytochrome is directly accessible to the action of trypsin in intact vesicles whereas the remainder is inaccessible and partitioned between luminal-facing or phospholipid-embedded loci. Analysis by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis reveals three major species of the cytochrome. Of these, the variant with a mol.wt. of 52000 is induced by phenobarbitone and this species is susceptible to trypsin. 2. After trypsin treatment of smooth membrane, some NADPH–cytochrome P-450 (cytochrome c) reductase activity remains and this remaining activity is enhanced by treatment with 0.05% deoxycholate, which renders the membranes permeable to macromolecules. In non-trypsin-treated control membranes the reductase activity is increased to a similar extent. These observations suggest an asymmetric distribution of NADPH–cytochrome P-450 (cytochrome c) reductase in the membrane. 3. As compared with dithionite, NADPH reduces only 44% of the cytochrome P-450 present in intact membranes. After tryptic digestion, none of the remaining cytochrome P-450 is reducible by NADPH. 4. In the presence of both a superoxide-generating system (xanthine plus xanthine oxidase) and NADPH...

Co-evolution of primordial membranes and membrane proteins

Mulkidjanian, Armen Y.; Galperin, Michael Y.; Koonin, Eugene V.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
498.0672%
Studies of the past several decades have provided major insights into the structural organization of biological membranes and mechanisms of many membrane molecular machines. However, the origin(s) of the membrane(s) and membrane proteins remain enigmatic. We discuss different concepts of the origin and early evolution of membranes, with a focus on the evolution of the (im)permeability to charged molecules, such as proteins and nucleic acids, and small ions. Reconstruction of the evolution of F-type and A/V-type membrane ATPases (ATP synthases), which are either proton or sodium-dependent, might help understand not only the origin of membrane bioenergetics, but also of membranes themselves. We argue that evolution of biological membranes occurred as a process of co-evolution of lipid bilayers, membrane proteins and membrane bioenergetics.

Differential Mobility of Pigment-Protein Complexes in Granal and Agranal Thylakoid Membranes of C3 and C4 Plants1[OA]

Kirchhoff, Helmut; Sharpe, Richard M.; Herbstova, Miroslava; Yarbrough, Robert; Edwards, Gerald E.
Fonte: American Society of Plant Biologists Publicador: American Society of Plant Biologists
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
484.1315%
The photosynthetic performance of plants is crucially dependent on the mobility of the molecular complexes that catalyze the conversion of sunlight to metabolic energy equivalents in the thylakoid membrane network inside chloroplasts. The role of the extensive folding of thylakoid membranes leading to structural differentiation into stacked grana regions and unstacked stroma lamellae for diffusion-based processes of the photosynthetic machinery is poorly understood. This study examines, to our knowledge for the first time, the mobility of photosynthetic pigment-protein complexes in unstacked thylakoid regions in the C3 plant Arabidopsis (Arabidopsis thaliana) and agranal bundle sheath chloroplasts of the C4 plants sorghum (Sorghum bicolor) and maize (Zea mays) by the fluorescence recovery after photobleaching technique. In unstacked thylakoid membranes, more than 50% of the protein complexes are mobile, whereas this number drops to about 20% in stacked grana regions. The higher molecular mobility in unstacked thylakoid regions is explained by a lower protein-packing density compared with stacked grana regions. It is postulated that thylakoid membrane stacking to form grana leads to protein crowding that impedes lateral diffusion processes but is required for efficient light harvesting of the modularly organized photosystem II and its light-harvesting antenna system. In contrast...

Omega-3 supplementation alters mitochondrial membrane composition and respiration kinetics in human skeletal muscle

Herbst, E A F; Paglialunga, S; Gerling, C; Whitfield, J; Mukai, K; Chabowski, A; Heigenhauser, G J F; Spriet, L L; Holloway, G P
Fonte: Blackwell publishing Ltd Publicador: Blackwell publishing Ltd
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
493.46145%
Studies have shown increased incorporation of omega-3 fatty acids into whole skeletal muscle following supplementation, although little has been done to investigate the potential impact on the fatty acid composition of mitochondrial membranes and the functional consequences on mitochondrial bioenergetics. Therefore, we supplemented young healthy male subjects (n = 18) with fish oils [2 g eicosapentaenoic acid (EPA) and 1 g docosahexanoic acid (DHA) per day] for 12 weeks and skeletal muscle biopsies were taken prior to (Pre) and following (Post) supplementation for the analysis of mitochondrial membrane phospholipid composition and various assessments of mitochondrial bioenergetics. Total EPA and DHA content in mitochondrial membranes increased (P < 0.05) ∼450 and ∼320%, respectively, and displaced some omega-6 species in several phospholipid populations. Mitochondrial respiration, determined in permeabilized muscle fibres, demonstrated no change in maximal substrate-supported respiration, or in the sensitivity (apparent Km) and maximal capacity for pyruvate-supported respiration. In contrast, mitochondrial responses during ADP titrations demonstrated an enhanced ADP sensitivity (decreased apparent Km) that was independent of the creatine kinase shuttle. As the content of ANT1...

Phloem Mobility of Xenobiotics: I. Mathematical Model Unifying the Weak Acid and Intermediate Permeability Theories

Kleier, Daniel A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /03/1988 EN
Relevância na Pesquisa
565.06055%
A passive diffusion model has been developed which simultaneously accounts for the dependence of phloem mobility on permeability and acid dissociation. The model is consistent with the observation that the addition of an acid moiety to an otherwise phloem immobile compound may enhance that compound's ability to move in the phloem. However, acid trapping in the basic phloem is not the only enhancement factor. Acid functionalization also lowers the effective permeability usually towards its optimum value. The unified theory predicts that for a given acid dissociation constant there is an optimum permeability and conversely for a given permeability there is an optimum dissociation constant.