From δ-aminolevulinic acid to chlorophylls and every step in between: in memory of Constantin (Tino) A. Rebeiz, 1936-2019

Constantin A. (Tino) Rebeiz, a pioneer in the field of chlorophyll biosynthesis, and a longtime member of the University of Illinois community of plant biologists, passed away on July 25, 2019. He came to the USA at a time that was difficult for members of minority groups to be in academia. However, his passion for the complexity of the biochemical origin of chlorophylls drove a career in basic sciences which extended into applied areas of environmentally friendly pesticides and treatment for skin cancer. He was a philanthropist; in retirement, he founded the Rebeiz Foundation for Basic Research which recognized excellence and lifetime achievements of selected top scientists in the general area of photosynthesis research. His life history, scientific breakthroughs, and community service hold important lessons for the field.

Partial characterization of a phosphorylated intermediate associated with the plasma membrane ATPase of corn roots

The phosphorylated protein associated with a deoxycholate-extracted plasma membrane fraction from corn (Zea mays L. var WF9 x Mol7) roots was characterized in order to correlate its properties with those of plasma membrane ATPase. Its phosphorylation, like that of plasma membrane ATPase, was dependent on Mg(2+), substrate specific for ATP, insensitive to azide, oligomycin, or molybdate, and sensitive to N,N'-dicyclohexylcarbodiimide, diethylstilbestrol, or vanadate. Monovalent cations affected the phosphorylation of the protein in a manner consistent with their stimulatory effects on ATPase. For K(+), this was shown to occur through an increase in the turnover of the phosphoenzyme. Analysis of the phosphorylated protein by NaDodSO(4)/polyacrylamide gel electrophoresis revealed the presence of a single labeled polypeptide with a molecular weight of about 100,000. Phosphorylation of this polypeptide was dependent on Mg(2+), sensitive to K(+), and inhibited by vanadate. It is concluded that this polypeptide represents the catalytic subunit of the plasma membrane ATPase. These results are discussed in terms of a model for the coupling of metabolic energy to H(+) and K(+) transport in higher plants.

Characterization of a red beet protein homologous to the essential 36-kilodalton subunit of the yeast V-type ATPase

V-type proton-translocating ATPases (V-ATPases) (EC 3.6.1.3) are electrogenic proton pumps involved in acidification of endomembrane compartments in all eukaryotic cells. V-ATPases from various species consist of 8 to 12 polypeptide subunits arranged into an integral membrane proton pore sector (Vo) and a peripherally associated catalytic sector (V1). Several V-ATPase subunits are functionally and structurally conserved among all species examined. In yeast, a 36-kD peripheral subunit encoded by the yeast (Saccharomyces cerevisiae) VMA6 gene (Vma6p) is required for stable assembly of the Vo sector as well as for V1 attachment. Vma6p has been characterized as a nonintegrally associated Vo subunit. A high degree of sequence similarity among Vma6p homologs from animal and fungal species suggest that this subunit has a conserved role in V-ATPase function. We have characterized a novel Vma6p homolog from red beet (Beta vulgaris) tonoplast membranes. A 44-kD polypeptide cofractionated with V-ATPases upon gel-filtration chromatography of detergent-solubilized tonoplast membranes and was specifically cross-reactive with anti-Vma6p polyclonal antibodies. The 44-kD polypeptide was dissociated from isolated tonoplast preparations by mild chaotropic agents and thus appeared to be nonintegrally associated with the membrane. The putative 44-kD homolog appears to be structurally similar to yeast Vma6p and occupies a similar position within the holoenzyme complex.

Reversal of the red beet tonoplast H(+)-ATPase by a pyrophosphate-generated proton electrochemical gradient

The reversal of the tonoplast H(+)-ATPase to mediate ATP synthesis was investigated in tonoplast vesicles isolated from red beet (Beta vulgaris L.) storage tissue. Our approach involved use of the H(+)-PP(i)ase to establish a proton electrochemical gradient (delta muH+) across the tonoplast vesicle membrane to drive the H(+)-ATPase in reverse. However, an initial problem with this approach was the presence of an adenylate kinase activity in the tonoplast fraction that interfered with measurement of ATP synthesis as a coupling between the H(+)-ATPase and H(+)-PP(i)ase. Inclusion of the adenylate kinase inhibitor p1p5-di(adenosine)pentaphosphate (Ap5A) in assays at 50 microM led to a complete inhibition of this activity and allowed measurement of ATP synthesis coupled to PPi hydrolysis. When measured in the presence of Ap5A, PPi-dependent ATP synthesis was blocked by Triton X-100 and inhibited by gramicidin D, imidodiphosphate, nitrate, and bafilomycin A. These results are consistent with PPi-dependent ATP synthesis occurring as a coupled process involving a delta muH+ established across the membrane. Furthermore, the observation that ATP synthesis is inhibited by inhibitors of the tonoplast H(+)-ATPase (nitrate and bafilomycin A) would suggest that this enzyme is involved in the synthetic reaction and can operate in reverse to synthesize ATP from ADP and Pi. A thermodynamic analysis of coupling between the H(+)-PP(i)ase and H(+)-ATPase suggests that PPi-driven ATP synthesis could only occur under these reaction conditions if the H+/substrate stoichiometries for the H(+)-PP(i)ase and H(+)-ATPase were 1 and 2, respectively. These values are consistent with transport stoichiometries previously determined for these enzymes in red beet tonoplast vesicles using kinetic methods.

Energy transduction in tonoplast vesicles from red beet (Beta vulgaris L.) storage tissue: H+/substrate stoichiometries for the H(+)-ATPase and H(+)-PPase

The H+/substrate stoichiometries of the tonoplast H(+)-ATPase and H(+)-PPase were determined by a kinetic approach. Using red beet (Beta vulgaris L.) tonoplast vesicles, rates of substrate-dependent H+ transport were estimated by (I) a mathematical model describing the time course of delta pH formation, (II) the rate of H+ leakage following H+ pump inhibition at a steady state delta pH, and (III) the initial rate of alkalinization of the external medium. When compared with rates of substrate hydrolysis measured under identical conditions, all three methods yielded an H+/ATP stoichiometry of 2 while the H+/PPi stoichiometry was determined to be 1 using methods I and II. Experimental limitations did not permit an analysis of the H+/PPi stoichiometry by method III. From these results and the estimated level of substrate and product typically found in the cytoplasm of plant cells, it is suggested that the H(+)-ATPase and H(+)-PPase as primary H(+)-pumps are poised toward net substrate hydrolysis under in vivo conditions thereby operating in parallel to generate a proton electrochemical gradient across the tonoplast.

Characterization of a H+/NO3- symport associated with plasma membrane vesicles of maize roots using 36ClO3- as a radiotracer analog

The mechanism of NO3- transport was examined in isolated plasma membrane vesicles from maize (Zea mays L., hybrid B73 X LH 51) roots using 36ClO3- as a radiotracer analog for NO3-. When an acid-exterior delta pH was imposed across the vesicle membrane, uptake of 36ClO3- was stimulated and the time course of radiolabel uptake displayed an overshoot phenomenon characteristic of the coupling of one solute gradient ot the movement of another solute. Evidence supporting delta pH as the driving force for 36ClO3- uptake included a dependence of the overshoot peak and initial rate of 36ClO3- uptake on the magnitude of the imposed delta pH, the occurrence of delta pH-driven 36ClO3- uptake in the presence of KSCN/valinomycin, and the ability of an imposed delta pH to drive 36ClO3- uptake when radiolabel was equilibrated across the membrane. When delta pH-driven 36ClO3- transport was examined in the presence of NO3-, radiolabel uptake was inhibited in a competitive manner. This was consistent with the carrier having the capacity to use either ClO3- or NO3- and supports the use of this radiotracer as an analog for NO3- in transport studies. When delta pH-driven 36ClO3- uptake was examined as a function of 36ClO3- concentration and delta pH, saturation kinetics were observed and the magnitude of the imposed delta pH affected the Km but not the Vmax for 36ClO3- uptake. This suggested an ordered binding mechanism where 36ClO3- would bind to the protonated form of the carrier prior to translocation. Radiolabeled 36ClO3- uptake was inhibited by treatment of the vesicles with phenylglyoxal, suggesting the involvement of arginine moieties in the process of transport. Taken together, these results support the presence of a H+/NO3- symport carrier at the plasma membrane which could be involved in mediating energy-dependent NO3- uptake into plant cells.

Determination of H/ATP Stoichiometry for the Plasma Membrane H-ATPase from Red Beet (Beta vulgaris L.) Storage Tissue

The H(+)/ATP stoichiometry was determined for the plasma membrane H(+)-ATPase from red beet (Beta vulgaris L., var Detroit Dark Red) storage tissue associated with native vesicles. The determination of H(+)/ATP stoichiometry utilized a kinetic approach where rates of H(+) influx, estimated by three different methods, were compared to rates of ATP hydrolysis measured by the coupled enzyme assay under identical conditions. These methods for estimating H(+) influx were based upon either determining the initial rate of alkalinization of the external medium from pH 6.13, measuring the rate of vesicle H(+) leakage from a steadystate pH gradient after stopping the H(+)-ATPase or utilizing a mathematical model which describes the net transport of H(+) at any given point in time. When the rate of H(+) influx estimated by each of these methods was compared to the rate of ATP hydrolysis, a H(+)/ATP stoichiometry of about 1 was observed. In consideration of the maximum free energy available from ATP hydrolysis (DeltaG(atp)), this value for H(+)/ATP stoichiometry is sufficient to account for the magnitude of the proton electrochemical gradient observed across the plasma membrane in vivo.

Modification of the Red Beet Plasma Membrane H-ATPase by Diethylpyrocarbonate

Incubation of the red beet (Beta vulgaris L.) plasma membrane H(+)-ATPase with micromolar concentrations of diethylpyrocarbonate (DEPC) resulted in inhibition of both ATP hydrolytic and proton pumping activity. Enzyme activity was restored when DEPC-modified protein was incubated with hydroxylamine, suggesting specific modification of histidine residues. Kinetic analyses of DEPC inhibition performed on both membrane-bound and solubilized enzyme preparations suggested the presence of at least one essential histidine moiety per active site. Inclusion of either ATP (substrate) or ADP (product and competitive inhibitor) in the modification medium reduced the amount of inhibition observed in the presence of DEPC. However, protection was not entirely effective in returning activity to noninhibited control values. These results suggest that the modified histidine does not reside directly in the ATP binding region of the enzyme, but is more likely involved in enzyme regulation through subtle conformational effects.

Ca-translocating ATPase of the plant plasma membrane

For Ca(2+) to function as a second messenger in signal transduction, it is essential that plant cells maintain low cytoplasmic Ca(2+) levels relative to internal organelles and the apoplast. At the plasma membrane, Ca(2+) is actively transported out of the cytoplasm and current evidence supports the involvement of a primary Ca(2+)-translocating ATPase in mediating this energy-dependent process. This review examines the preliminary biochemical characterization of this transport enzyme.

Electrostatic Changes in Lycopersicon esculentum Root Plasma Membrane Resulting from Salt Stress

Salinity-induced alterations in tomato (Lypersicon esculentum Mill. cv Heinz 1350) root plasma membrane properties were studied and characterized using a membrane vesicle system. Equivalent rates of MgATP-dependent H(+)-transport activity were measured by quinacrine fluorescence (DeltapH) in plasma membrane vesicles isolated from control or salt-stressed (75 millimolar salt) tomato roots. However, when bis-[3-phenyl-5-oxoisoxazol-4-yl] pentamethine was used to measure MgATP-dependent membrane potential (DeltaPsi) formation, salt-stressed vesicles displayed a 50% greater initial quench rate and a 30% greater steady state quench than control vesicles. This differential probe response suggested a difference in surface properties between control and salt-stressed membranes. Fluorescence titration of vesicles with the surface potential probe, 8-anilino-1-napthalenesulphonic acid (ANS) provided dissociation constants (K(d)) of 120 and 76 micromolar for dye binding to control and salt-stressed vesicles, respectively. Membrane surface potentials (Psi(o)) of-26.0 and -13.7 millivolts were calculated for control and salt-stressed membrane vesicles from the measured K(d) values and the calculated intrinsic affinity constant, K(i). The concentration of cations and anions at the surface of control and salt-stressed membranes was estimated using Psi(o) values and the Boltzmann equation. The observed difference in membrane surface electrostatic properties was consistent with the measured differences in K(+)-stimulated kinetics of ATPase activity between control and salt-stressed vesicles and by the differential ability of Cl(-) ions to stimulate H(+)-transport activity. Salinity-induced changes in plasma membrane electrostatic properties may influence ion transport across the plasma membrane.

Further Characterization of the Red Beet Plasma Membrane Ca-ATPase Using GTP as an Alternative Substrate

The GTP-driven component of Ca(2+) uptake in red beet (Beta vulgaris L.) plasma membrane vesicles was further characterized to confirm its association with the plasma membrane Ca(2+)-translocating ATPase and assess its utility as a probe for this transport system. Uptake of (45)Ca(2+) in the presence of GTP demonstrated similar properties to those previously observed for red beet plasma membrane vesicles utilizing ATP with respect to pH optimum, sensitivity to orthovanadate, dependence on Mg:substrate concentration and dependence on Ca(2+) concentration. Calcium uptake in the presence of GTP was also strongly inhibited by erythrosin B, a potent inhibitor of the plant plasma membrane Ca(2+)-ATPase. Furthermore, after treatment with EGTA to remove endogenous calmodulin, the stimulation of (45)Ca(2+)-uptake by exogenous calmodulin was nearly equivalent in the presence of either ATP or GTP. Taken together these results support the proposal that GTP-driven (45)Ca(2+) uptake represents the capacity of the plasma membrane Ca(2+)-translocating ATPase to utilize this nucleoside triphosphate as an alternative substrate. When plasma membrane vesicles were phosphorylated with [gamma-(32)P]-GTP, a rapidly turning over, 100 kilodalton phosphorylated peptide was observed which contained an acyl-phosphate linkage. While it is proposed that this peptide could represent the catalytic subunit of the plasma membrane Ca(2+)-ATPase, it is noted that this molecular weight is considerably lower than the 140 kilodalton size generally observed for plasma membrane Ca(2+)-ATPases present in animal cells.

A modified method for the production of 36ClO3- for use in plant nitrate transport studies

A modified method for the production and purification of 36ClO3- in high yield is described. This procedure, involving the electrolytic oxidation of 36Cl- in a cell with simple electrode design and purification of the electrolysis products (36Cl-, 36ClO3-, and 36ClO4-) by aqueous column chromatography, allows for the recovery of about 80% of the initial radiolabel as 36ClO3-. The method is rapid and suitable for the production of this radiolabeled anion for use as a tracer analog for nitrate in plant membrane transport experiments.

Change in Target Molecular Size of the Red Beet Plasma Membrane ATPase during Solubilization and Reconstitution

The plasma membrane ATPase from red beet (Beta vulgaris L.) storage tissue associated with either native plasma membrane vesicles, a detergent-solubilized enzyme preparation or reconstituted liposomes was subjected to radiation inactivation analysis to determine if changes in target molecular size occurred with modification of its amphipathic environment. For each preparation of the enzyme, the decline in ATP hydrolytic activity with increasing dose of gamma-ray radiation demonstrated a simple exponential profile indicating the presence of a single target size. Analysis of the radiation inactivation profiles for the plasma membrane associated, solubilized, and reconstituted enzyme revealed target molecular sizes of 225 kilodaltons (kD), 129 kD, and 218 kD, respectively. These results suggest that the plasma membrane associated and reconstituted ATPase preparations consist of enzyme present as a dimer of 100 kD subunits while the solubilized enzyme is present in the monomeric form. These results also indicate that the 100 kD catalytic subunit most likely represents the minimal unit of ATP hydrolytic activity.

Modification of an essential arginine residue associated with the plasma membrane ATPase of red beet (Beta vulgaris L.) storage tissue

The dicarbonyl compounds, phenylgloxyl and 2,3-butanedione were used to demonstrate the presence of an essential arginine residue in the mechanism of the red beet (Beta vulgaris L.) plasma membrane ATPase. Treatment of the red beet ATPase with either of these reagents resulted in an inhibition of ATP hydrolytic activity protectable by the inclusion of either ATP or ADP during inhibitor incubation. Ligands of the ATP hydrolytic reaction also protected against phenylglyoxyl inhibition and affected the ability of ADP to protect against inhibition by this reagent. Kinetic analysis of 2,3-butanedione and phenylglyoxyl inhibition suggested the presence of a single arginine residue susceptible to attack by these reagents. As similar results with these arginine modification reagents were found for both the plasma membrane-associated and solubilized forms of the ATPase, it is apparent that the function of this arginyl moiety is not affected by detergent treatment and removal of the enzyme from the membrane.

A small scale procedure for the isolation of transport competent vesicles from plant tissues

A microscale method for the isolation of selectively sealed microsomal membrane fractions from plant tissue is presented. The method is based on differential centrifugation in a table top microcentrifuge to accommodate small sample size (10-25 g tissue) and the addition of KI or KCl in the homogenization medium for isolating selectively sealed plasma membrane or tonoplast vesicles. This microscale procedure was found to be useful in isolating membranes from red beet (Beta vulgaris) storage tissue, sugar beet (Beta vulgaris) storage tissue, corn (Zea mays) roots, and soybean (Glycine max) roots. This paper also describes the ability to further purify an enriched red beet plasma membrane fraction on a discontinuous sucrose density gradient, in a microcentrifuge, that is highly competent in ATP-dependent H+-transport. The speed and wide applicability of this procedure make it ideal when a large number of samples need to be processed.

Isolation of sealed plasma membrane vesicles from Phytophthora megasperma f. sp. glycinea: II. Partial characterization of Ca2+ transport and glyceollin effects

Calcium uptake was examined in sealed plasma membrane vesicles isolated from the plant pathogenic fungus, Phytophthora megasperma f. sp. glycinea. Calcium uptake was ATP-dependent and by the addition of various ionophores in the presence of ATP, it was shown that Ca2+ transport was mediated by a nH+/Ca2+ antiport. Further evidence for this antiport mechanism included Ca2+ uptake driven by an imposed pH gradient and the observation that calcium could dissipate a steady-state pH gradient across the vesicle membrane. Transport mediated by the nH+/Ca2+ antiport was optimal at pH 7.0, and demonstrated saturation kinetics for Ca2+ with a Km of about 7 microM. Glyceollin, a soybean phytoalexin, was found to inhibit Ca2+ transport consistent with its ability to increase H+ conductance. In the presence of glyceollin, calcium leakage from Phytophthora membrane vesicles also increased. This study provides basic information about calcium transport in a plant pathogenic fungus as well as demonstrating a possible mode of action of a phytoalexin.

Isolation of sealed plasma membrane vesicles from Phytophthora megasperma f. sp. glycinea. I. Characterization of proton pumping and ATPase activity

Sealed vesicles were isolated from a plant pathogenic fungus Phytophthora megasperma f. sp. glycinea using a modification of a method previously developed for plant plasma membrane vesicle isolation. Vanadate-sensitive, proton pumping microsomal membrane vesicles were resolved on a linear sucrose density gradient and found to comigrate with a vanadate-sensitive ATPase. Both the proton pumping and ATPase activity of these vesicles had a pH optimum of 6.5 and demonstrated similar properties with respect to substrate specificity and inhibitor sensitivity. These properties were in agreement with previously published data on the Phytophthora plasma membrane ATPase. In contrast with previous reports there was no K+ stimulation of the plasma membrane ATPase and the Km for Mg:ATP (1:1 concentration ratio) was higher (2.5 mM). A comparison of anion (potassium salts) effects upon delta pH and delta psi formation in sealed Phytophthora plasma membrane vesicles revealed a correspondence between the relative ability of anions to stimulate proton transport and to reduce delta psi. The relative order for this effect was KCl greater than KBr much greater than KMes, KNO3, KClO3, K2SO4. This study presents a method for the isolation of sealed vesicles from Phytophthora hyphae. It also provides basic information on the plasma membrane H+-ATPase and its associated proton pumping activity.

Phosphorylation and dephosphorylation reactions of the red beet plasma membrane ATPase studied in the transient state

The reaction mechanism of the solubilized red beet (Beta vulgaris L.) plasma membrane ATPase was studied with a rapid quenching apparatus. Using a dual-labeled substrate ([gamma-(32)P]ATP and [5',8-(3)H]ATP), the presteady-state time course of phosphoenzyme formation, phosphate liberation and ADP liberation was examined. The time course for both phosphoenzyme formation and ADP liberation showed a rapid, initial rise while the timecourse for phosphate liberation showed an initial lag. This indicated that ADP was released with formation of the phosphoenzyme while phosphate was released with phosphoenzyme breakdown. Phosphoenzyme formation was Mg(2+)-dependent and preincubation of the enzyme with free ATP followed by the addition of Mg(2+) increased the rate of phosphoenzyme formation 2.3-fold. This implied that phosphoenzyme formation could result from a slow reaction of ATP binding followed by a more rapid reaction of phosphate group transfer. Phosphoenzyme formation was accelerated as the pH was decreased, and the relationship between pH and the apparent first-order rate constants for phosphoenzyme formation suggested the role of a histidyl residue in this process. Transient kinetics of phosphoenzyme breakdown confirmed the presence of two phosphoenzyme forms, and the discharge of the ADP-sensitive form by ADP correlated with ATP synthesis. Potassium chloride increased the rate of phosphoenzyme turnover and shifted the steady-state distribution of phosphoenzyme forms. From these results, a minimal catalytic mechanism is proposed for the red beet plasma membrane ATPase, and rate constants for several reaction steps are estimated.

Chemical Equivalence of Phosphoenzyme Reaction States in the Catalytic Mechanism of the Red Beet (Beta vulgaris L.) Plasma Membrane ATPase

A comparison of two phosphoryl enzyme reaction states associated with the plasma membrane ATPase of red beet (Beta vulgaris L.) storage tissue was carried out to determine if their differences in reactivity toward ADP and K(+) was related to a structural difference in the site of phosphorylation. Using a pulse labeling method it was possible to produce preparations where either the ADP-sensitive and -insensitive phosphoenzyme forms or the ADP-insensitive phosphoenzyme form alone were trapped as trichloroacetic acid denatured protein. Following complete digestion with Pronase, both preparations yielded radioactive tripeptides with similar properties with respect to pH stability of the covalent bond linking the phosphate to the peptide, isoelectric point, and migration on cellulose thin layer plates. Since the preparation containing both intermediate reaction states behaved in a uniform manner during analysis and displayed properties similar to the preparation containing only the ADP-insensitive phosphoenzyme form, it was proposed that both phosphoenzyme forms were chemically equivalent and derived from the same region of the catalytic active site. The observation that ethyleneimine treatment of both preparations followed by trypsin digestion resulted in the production of tripeptides similar to the Pronase fragments would support this proposal since it suggests that the tripeptides from both phosphoenyzme states contain a lysine residue on the C terminal end and are adjacent to a cysteine residue on the N-terminal end. The chemical equivalence of these two phosphoenzyme reaction states suggests that their differences in reactivity toward ligands may be related to conformational changes associated with the catalytic and transport mechanism of this enzyme.

Pyridine nucleotide oxidation by a plasma membrane fraction from red beet (Beta vulgaris L.) storage tissue

The potential role of pyridine nucleotide oxidation in the energization and/or regulation of membrane transport was examined using sealed plasma membrane vesicles isolated from red beet (Beta vulgaris L.) storage tissue. In this system, pyridine nucleotide oxidation, which was enhanced in the presence of ferricyanide, occurred. In the presence or absence of ferricyanide, the oxidation of NADH was several-fold greater than the oxidation of NADPH, indicating that it was the preferred substrate for oxidation in this system. Ferricyanide reduction coupled to NADH oxidation did not require the transmembrane movement of reducing equivalents since ferricyanide incorporated inside the vesicles could not be reduced by NADH added externally to the vesicles, unless the vesicles were made leaky by the addition of 0.05% (v/v) Triton X-100. Using fluorescent probes for the measurement of transmembrane pH gradients and membrane potentials, it was determined that NADH oxidation did not result in the production of a proton electrochemical gradient or have any effect upon the proton electrochemical gradient produced by the plasma membrane H+-ATPase. The oxidation of NADH in the presence of ferricyanide did result in the acidification of the reaction medium. This acidification was unaffected by the addition of Gramicidin D and stimulated by the addition of 0.05% (v/v) Triton X-100, suggesting a scalar (nonvectorial) production of protons in the oxidation/reduction reaction. The results of this study suggest that the oxidation of pyridine nucleotides by plasma membrane vesicles is not related to energization of transport at the plasma membrane or modulation of the activity of the plasma membrane H+-ATPase.

Calcium Transport in Sealed Vesicles from Red Beet (Beta vulgaris L.) Storage Tissue : II. Characterization of Ca Uptake into Plasma Membrane Vesicles

Calcium uptake was examined in sealed plasma membrane vesicles isolated from red beet (Beta vulgaris L.) storage tissue using (45)Ca(2+). Uptake of (45)Ca(2+) by the vesicles was ATP-dependent and radiotracer accumulated by the vesicles could be released by the addition of the calcium ionophore A23187. The uptake was stimulated by gramicidin D but slightly inhibited by carbonylcyanide m-chlorophenylhydrazone. Although the latter result might suggest some degree of indirect coupling of (45)Ca(2+) uptake to ATP utilization via deltamuH(+), no evidence for a secondary H(+)/Ca(2+) antiport in this vesicle system could be found. Following the imposition of an acid-interior pH gradient, proton efflux from the vesicle was not enhanced by the addition of Ca(2+) and an imposed pH gradient could not drive (45)Ca(2+) uptake. Optimal uptake of (45)Ca(2+) occurred broadly between pH 7.0 and 7.5 and the transport was inhibited by orthovanadate, N,N'-dicyclohexylcarbodiimide, and diethylstilbestrol but insensitive to nitrate and azide. The dependence of (45)Ca(2+) uptake on both calcium and Mg:ATP concentration demonstrated saturation kinetics with K(m) values of 6 micromolar and 0.37 millimolar, respectively. While ATP was the preferred substrate for driving (45)Ca(2+) uptake, GTP could drive transport at about 50% of the level observed for ATP. The results of this study demonstrate the presence of a unique primary calcium transport system associated with the plasma membrane which could drive calcium efflux from the plant cell.

Calcium Transport in Sealed Vesicles from Red Beet (Beta vulgaris L.) Storage Tissue : I. Characterization of a Ca-Pumping ATPase Associated with the Endoplasmic Reticulum

Calcium transport was examined in microsomal membrane vesicles from red beet (Beta vulgaris L.) storage tissue using chlorotetracycline as a fluorescent probe. This probe demonstrates an increase in fluorescence corresponding to calcium accumulation within the vesicles which can be collapsed by the addition of the calcium ionophore A23187. Calcium uptake in the microsomal vesicles was ATP dependent and completely inhibited by orthovanadate. Centrifugation of the microsomal membrane fraction on a linear 15 to 45% (w/w) sucrose density gradient revealed the presence of a single peak of calcium uptake which comigrated with the marker for endoplasmic reticulum. The calcium transport system associated with endoplasmic reticulum vesicles was then further characterized in fractions produced by centrifugation on discontinous sucrose density gradients. Calcium transport was insensitive to carbonylcyanide m-chlorophenylhydrazone indicating the presence of a primary transport system directly linked to ATP utilization. The endoplasmic reticulum vesicles contained an ATPase activity that was calcium dependent and further stimulated by A23187 (Ca(2+), A23187 stimulated-ATPase). Both calcium uptake and Ca(2+), A23187 stimulated ATPase demonstrated similar properties with respect to pH optimum, inhibitor sensitivity, substrate specificity, and substrate kinetics. Treatment of the red beet endoplasmic reticulum vesicles with [gamma-(32)P]-ATP over short time intervals revealed the presence of a rapidly turning over 96 kilodalton radioactive peptide possibly representing a phosphorylated intermediate of this endoplasmic reticulum associated ATPase. It is proposed that this ATPase activity may represent the enzymic machinery responsible for mediating primary calcium transport in the endoplasmic reticulum linked to ATP utilization.

Proton Transport in Plasma Membrane and Tonoplast Vesicles from Red Beet (Beta vulgaris L.) Storage Tissue : A Comparative Study of Ion Effects on DeltapH and DeltaPsi

The proton transport properties of plasma membrane and tonoplast vesicles isolated from red beet (Beta vulgaris L.) storage tissue were examined and compared. Membrane vesicles isolated with 250 millimolar KCl in the homogenization media and recovered at low density following sucrose density gradient centrifugation displayed characteristics of proton transport (nitrate inhibition, no inhibition by orthovanadate, pH optimum of 7.75, pyrophosphate-driven proton transport) which were consistent with a tonoplast origin. When the KCl in the homogenization medium was replaced by 250 millimolar KI, sealed membrane vesicles were recovered at higher densities in sucrose gradients and displayed properties (orthovanadate sensitivity, no inhibition by nitrate, pH optimum of 6.5) consistent with a plasma membrane origin. A comparison of anion effects (potassium salts) upon DeltapH and DeltaPsi revealed a direct correspondence between the relative ability of anions to stimulate proton transport and reduce DeltaPsi. For tonoplast vesicles, the relative order for this effect was KI > KBr >/= KCl > KClO(3) > K(2)SO(4) while for plasma membrane vesicles, a different order KI > KNO(3) >/= KBr >/= KClO(3) > KCl > K(2)SO(4) was observed. Proton transport in plasma membrane and tonoplast vesicles was inhibited by fluoride; however, plasma membrane vesicles appeared to be more sensitive to this anion. In order to correlate anion effects in the two vesicle fractions with anion transport, the kinetics of anion stimulation of steady-state pH gradients established in the absence of monovalent ions was examined. Anions were added as potassium salts and the total potassium concentration (100 millimolar) was maintained through the addition of K(+)/Mes. For plasma membrane vesicles, chlorate and nitrate displayed saturation kinetics while chloride displayed stimulation of proton transport which followed a linear profile. For tonoplast vesicles, the kinetics of chloride stimulation of proton transport displayed a saturable component. The results of this study indicate differences in proton transport properties of these two vesicle types and provide information on conditions where proton transport in the two fractions can be optimized.

Vanadate-dependent oxidation of pyridine nucleotides in rat liver microsomal membranes

An enzymatic Na3VO4-dependent system for the oxidation of reduced pyridine nucleotides in purified rat liver microsomes was characterized. The system has a pH optimum of 6.5, and appears to be specific for vanadate, since activity in the presence of a related transition metal, molybdate, was not detected. Vanadate-dependent oxidation occurred with a concomitant consumption of O2 and, contrary to previous reports, preferred NADPH over NADH. At pH 6.5, the NADPH/NADH oxidase activity ratio was greater than 2:1. Sodium vanadate-dependent oxidation of NADH was inhibited by rotenone, antimycin A, NaN3, and NaCN. Conversely, Na3VO4-dependent NADPH oxidation was slightly affected by rotenone, but was insensitive to antimycin A, NaN3, NaCN, or quinacrine. Vanadate-dependent oxidation of either pyridine nucleotide was inhibited by the addition of either superoxide dismutase or catalase, indicating that both superoxide and hydrogen peroxide may be intermediates in the process. Linear sucrose gradient purification of the microsomes showed that the vanadate-dependent system for NADPH oxidation resides primarily in the endoplasmic reticulum. These studies indicate the existence of separate and distinct enzymatic systems for vanadate-stimulated oxidation of NADPH and NADH in mammalian microsomal membranes, and argue against an exclusive role of endogenous superoxide in the process.

Selective production of sealed plasma membrane vesicles from red beet (Beta vulgaris L.) storage tissue

Modification of our previous procedure for the isolation of microsomal membrane vesicles from red beet (Beta vulgaris L.) storage tissue allowed the recovery of sealed membrane vesicles displaying proton transport activity sensitive to both nitrate and orthovanadate. In the absence of a high salt concentration in the homogenization medium, contributions of nitrate-sensitive (tonoplast) and vanadate-sensitive (plasma membrane) proton transport were roughly equal. The addition of 0.25 M KCl to the homogenization medium increased the relative amount of nitrate-inhibited proton transport activity while the addition of 0.25 M KI resulted in proton pumping vesicles displaying inhibition by vanadate but stimulation by nitrate. These effects appeared to result from selective sealing of either plasma membrane or tonoplast membrane vesicles during homogenization in the presence of the two salts. Following centrifugation on linear sucrose gradients it was shown that the nitrate-sensitive, proton-transporting vesicles banded at low density and comigrated with nitrate-sensitive ATPase activity while the vanadate-sensitive, proton-transporting vesicles banded at a much higher density and comigrated with vanadate-sensitive ATPase. The properties of the vanadate-sensitive proton pumping vesicles were further characterized in microsomal membrane fractions produced by homogenization in the presence of 0.25 M KI and centrifugation on discontinuous sucrose density gradients. Proton transport was substrate specific for ATP, displayed a sharp pH optimum at 6.5, and was insensitive to azide but inhibited by N'-N-dicyclohexylcarbodiimide, diethylstilbestrol, and fluoride. The Km of proton transport for Mg:ATP was 0.67 mM and the K0.5 for vanadate inhibition was at about 50 microM. These properties are identical to those displayed by the plasma membrane ATPase and confirm a plasma membrane origin for the vesicles.

Membrane Transport in Isolated Vesicles from Sugarbeet Taproot: III. Effects of Fluoride on ATPase Activity and Transport

The effects of fluoride on the tonoplast type ATPase and transport activities associated with sealed membrane vesicles isolated from sugarbeet (Beta vulgaris L.) storage tissue were examined. This anion had two distinct effects upon the proton-pumping vesicles. When ATP hydrolysis was measured in the presence of gramicidin D, significant inhibition (approximately 50%) only occurred when the fluoride concentration approached 50 millimolar. In contrast, the same degree of inhibition of proton transport occurred when the fluoride concentration was about 24 millimolar. Effects on proton pumping at this concentration of fluoride could be attributed to an inhibition of chloride movement which serves to dissipate the vesicle membrane potential. Valinomycin could partially restore ATPase activity in sealed vesicles which were inhibited by fluoride and this restoration occurred with a reduction in the membrane potential. Fluoride demonstrated a competitive interaction with chloride-stimulation of proton transport and inhibited the uptake of radioactive chloride into sealed vesicles. When the vesicles were allowed to develop a pH gradient in the absence of KCl, and KCl was subsequently added, fluoride reduced enhancement of the existing pH gradient by KCl. The results are consistent with a chloride carrier that is inhibited by fluoride.

Ca uptake by endoplasmic reticulum from zucchini hypocotyls : the use of chlorotetracycline as a probe for ca uptake

Ca(2+) uptake into microsomal vesicles was measured using the fluorescent probe chlorotetracycline. The Ca(2+) uptake was ATP-dependent and did not occur in the presence of the calcium ionophore A23187. There was a linear relationship between the rate of ATP-dependent fluorescence increase using chlorotetracycline and ATP-dependent (45)Ca(2+) uptake, indicating that chlorotetracycline can be used as a quantitative probe for Ca(2+) uptake. The fluorescent probe allows measurements to be made in real time, and avoids the use of radioisotopes. Ca(2+) transport was associated with endoplasmic reticulum on linear gradients when the endoplasmic reticulum was in either rough or smooth form. The Ca(2+) uptake had a pH optimum of 7.5, a K(m) for ATP of 0.1 millimolar, a K(m) for Ca(2+) of about 70 nanomolar, and was stimulated 2-fold by calmodulin. Vanadate inhibited uptake completely at a concentration of 50 micromolar, half-maximally at 5 micromolar. Carbonyl cyanide 4-(trifluoromethoxy)-phenyl-hydrazone, oligomycin, azide, and nitrate caused only slight inhibition. Dicyclohexylcarbodiimide (DCCD) stimulated slightly at concentrations as high as 400 micromolar. The hormones gibberellic acid, indoleacetic acid, and abscisic acid at 10 micromolar had no significant effect. Myo-inositol 1,4,5-trisphosphate did not cause release of Ca(2+) after uptake. The properties of the enzyme suggest that it has a functional role in regulating cytosolic Ca(2+) levels. Based on the lack of an effect by hormones, it may not act as a mediator of second messenger roles of Ca(2+). The inhibition by vanadate and slight stimulation by DCCD may be useful as a ;signature' for this endoplasmic reticulum Ca(2+) uptake system.

Intermediate reaction states of the red beet plasma membrane ATPase

The phosphorylation reaction for the plasma membrane ATPase of red beet (Beta vulgaris L.) was examined in order to further understand the mechanism of this enzyme. The level of steady-state phosphorylation had a pH optimum of about 6.0 while ATPase activity (32Pi production) measured under identical conditions had a pH optimum of 7.0. Phosphoenzyme decomposition was accelerated as both the pH and temperature were increased. The former effect may account for the observed difference between the pH optimum for phosphorylation and ATPase. Although the kinetics of K+ stimulation of ATP hydrolysis have been observed to be complex, the kinetics of K+ stimulation of phosphoenzyme turnover were observed to be simple Michaelis-Menten. An antagonism was observed between MgATP and K+ for the stimulation of phosphoenzyme turnover. Increased MgATP concentration reduced the degree of K+ stimulation of phosphoenzyme turnover and ATPase activity. These effects could be explained by the observation that two forms of phosphoenzyme occur during ATP hydrolysis. One form is discharged by ADP while the other form is ADP insensitive. Potassium stimulation of phosphoenzyme breakdown occurs primarily because of effects on the ADP-insensitive phosphoenzyme form. These results are consistent with a mechanism of ATP hydrolysis involving interconversions of conformational states.

Membrane transport in isolated vesicles from sugarbeet taproot : I. Isolation and characterization of energy-dependent, h-transporting vesicles

Sealed membrane vesicles were isolated from homogenates of sugarbeet (Beta vulgaris L.) taproot by a combination of differential centrifugation, extraction with KI, and dextran gradient centrifugation. Relative to the KI-extracted microsomes, the content of plasma membranes, mitochondrial membranes, and Golgi membranes was much reduced in the final vesicle fraction. A component of ATPase activity that was inhibited by nitrate co-enriched with the capacity of the vesicles to form a steady state pH gradient during the purification procedure. This suggests that the nitrate-sensitive ATPase may be involved in driving H(+)-transport, and this is consistent with the observation that H(+)-transport, in the final vesicle fraction was inhibited by nitrate. Proton transport in the sugarbeet vesicles was substrate specific for ATP, insensitive to sodium vanadate and oligomycin but was inhibited by diethylstilbestrol and N,N'-dicyclohexylcarbodiimide. The formation of a pH gradient in the vesicles was enhanced by halide ions in the sequence I(-) > Br(-) > Cl(-) while F(-) was inhibitory. These stimulatory effects occur from both a direct stimulation of the ATPase by anions and a reduction in the vesicle membrane potential. In the presence of Cl(-), alkali cations reduce the pH gradient relative to that observed with bis-tris-propane, possibly by H(+)/alkali cation exchange. Based upon the properties of the H(+)-transporting vesicles, it is proposed that they are most likely derived from the tonoplast so that this vesicle preparation would represent a convenient system for studying the mechanism of transport at this membrane boundary.

Membrane Transport in Isolated Vesicles from Sugarbeet Taproot : II. Evidence for a Sucrose/H-Antiport

The process of sucrose transport was investigated in sealed putative tonoplast vesicles isolated from sugarbeet (Beta vulgaris L.) taproot. If the vesicles were allowed to develop a steady state pH gradient by the associated transport ATPase and 10 millimolar sucrose was added, a transient flux of protons out of the vesicles was observed. The presence of an ATPase produced pH gradient allowed [(14)C]sucrose transport into the vesicles to occur at a rate 10-fold higher than the rate observed in the absence of an imposed pH gradient. Labeled sucrose accumulated into the sealed vesicles could be released back to the external medium if the pH gradient was dissipated with carbonylcyanide-m-chlorophenyl hydrazone (CCCP). When the kinetics of ATP dependent [(14)C]sucrose uptake were examined, the kinetic profile followed the simple Michaelis-Menten relationship and a Michaelis constant of 12.1 millimolar was found. When a transient, inwardly directed sucrose gradient was imposed on the vesicles in the absence of charge compensating ions, a transient interior negative membrane potential was observed. This membrane potential could be prevented by the addition of CCCP prior to sucrose or dissipated by the addition of CCCP after sucrose was added. These results suggest that an electrogenic H(+)/sucrose antiport may be operating on the vesicle membrane.

Target molecular size of the red beet plasma membrane ATPase

Radiation inactivation of the red beet (Beta vulgaris L.) plasma membrane ATPase was carried out using gamma-ray radiation from a (137)Cs source. Inactivation of vanadate-sensitive ATPase activity by gamma-ray radiation followed an exponential decline with increasing total dose, indicating a single target size calculated to have a molecular weight of about 228,000. Since the catalytic subunit of the red beet plasma membrane ATPase has been demonstrated to have a molecular weight of about 100,000 by dodecyl-sulfate gel electrophoresis following (32)P-phosphorylation, it is suggested that the native enzyme may exist, at least, as a dimer of catalytic subunits.

Structural relatedness of three ion-transport adenosine triphosphatases around their active sites of phosphorylation

Three membrane-bound adenosine triphosphatases were investigated for homology in the sequence of four amino acids about the active site of phosphorylation. The ATPases were as follows: sodium-potassium-dependent ATPase from dog kidney, Na,K-ATPase; hydrogen-potassium-dependent ATPase from hog gastric mucosa, H,K-ATPase, an ATPase similar to Na,K-ATPase; and an ATPase activity in the plasma membrane of corn, Zea mays, roots (CR-ATPase), a higher plant ATPase. A membrane preparation containing an ATPase of Acholeplasma laidlawii, a prokaryote, (AL) was also investigated. For most of the experiments, the preparations were phosphorylated from [gamma-32P]ATP, denatured in acid, and subjected to proteolytic digestion. Radioactive phosphopeptides were separated by high voltage paper electrophoresis and characterized by sensitivity to chemical reagents. In gastric H,K-ATPase, the aspartate residue at the active site was determined directly by labeling with [3H]borohydride. A common sequence around the active site was found for Na,K-ATPase, H,K-ATPase, and CR-ATPase. This sequence, -Cys-(Ser/Thr)-Asp(P)-Lys-, is similar to that in the calcium ion-transport ATPase of sarcoplasmic reticulum. The AL membrane preparation showed an acylphosphate that turned over rapidly after a chase of labeled membranes with unlabeled ATP. The corresponding sequence was different from that of the three ATPases. An acylphosphate was on two polypeptides with molecular weights of about 80,000 and 60,000; these appear not to correspond to subunits of a Na+-stimulated ATPase in this organism (Lewis, R. N. A. H., and McElhaney, R. N. (1983) Biochim. Biophys. Acta 735, 113-122).

Vanadate-dependent NADH oxidation in microsomal membranes of sugar beet

Microsomal membranes isolated from sugar beet (Beta vulgaris L. var. GWD-2) storage tissue were found to contain a Na3VO4-dependent system for the oxidation of NADH. The system was demonstrated to be enzymatic in nature and specific for Na3VO4. Maximal Na3VO4-dependent NADH oxidation was observed at pH 6.5, when Na3VO4 was present at 200 microM and when NADH was present at 100 microM. The oxidation activity was insensitive to rotenone and antimycin A but was inhibited by NaN3, NaCN, and quinacrine. Sodium vanadate-dependent NADH oxidation occurred with a concomitant uptake of O2 from the assay solution. Both NADH oxidation and O2 consumption were dependent upon the presence of Na3VO4, inhibited by manganese, and preferred NADH to NADPH. Catalase prevented Na3VO4-dependent O2 consumption but accelerated NADH oxidation. The effects of manganese and catalase suggest that superoxide anion and hydrogen peroxide may be involved in this process. While it is unclear as to the physiological significance of Na3VO4-dependent NADH oxidation in plant cells, the presence of this system indicates that caution must be exercised when coupled ATPase assays depending upon NADH oxidation are used with plant membranes in the presence of Na3VO4.

Characterization of the solubilized plasma membrane ATPase of red beet

The plasma membrane ATP-phosphohydrolase (ATPase) from red beet (Beta vulgaris L.) storage tissue was solubilized with the zwitterionic detergent Zwittergent 3-14 from a plasma membrane-enriched fraction which was extracted with the anionic detergent, sodium deoxycholate. For both the extraction of extraneous proteins by deoxycholate and the solubilization of active plasma membrane ATPase by Zwittergent 3-14, the optimal concentration of detergent was 0.1% (weight per volume) with a detergent to protein ratio of 1.0 (milligram per milligram). The properties of the solubilized ATPase were found to be similar to the membrane-bound enzyme with respect to pH optimum, substrate specificity, inhibitor sensitivity, and kinetics of K(+) stimulation. The solubilized ATPase preparation formed a rapidly turning over phosphoenzyme, the breakdown velocity of which was increased in the presence of 50 millimolar KCl. Solubilization with 0.1% Zwittergent 3-14 following extraction with 0.1% deoxycholate resulted in an increase in both ATPase activity and steady state phosphoenzyme level; however, a direct correspondence between the increase in ATPase activity and phosphorylation level did not exist. It is proposed that this discrepancy may be the result of a detergent-mediated modification of kinetic rate constants in the mechanism of the enzyme.

Density gradient localization of plasma membrane and tonoplast from storage tissue of growing and dormant red beet : characterization of proton-transport and ATPase in tonoplast vesicles

Membranes from homogenates of growing and of dormant storage roots of red beet (Beta vulgaris L.) were centrifuged on linear sucrose gradients. Vanadate-sensitive ATPase activity, a marker for plasma membrane, peaked at 38% to 40% sucrose (1.165-1.175 grams per cubic centimeter) in the case of growing material but moved to as low as 30% sucrose (1.127 grams per cubic centimeter) during dormancy.A band of nitrate-sensitive ATPase was found at sucrose concentrations of 25% to 28% or less (around 1.10 grams per cubic centimeter) for both growing and dormant material. This band showed proton transport into membrane vesicles, as measured by the quenching of fluorescence of acridine orange in the presence of ATP and Mg(2+). The vesicles were collected on a 10/23% sucrose step gradient. The phosphate hydrolyzing activity was Mg dependent, relatively substrate specific for ATP (ATP > GTP > UTP > CTP = 0) and increased up to 4-fold by ionophores. The ATPase activity showed a high but variable pH optimum, was stimulated by Cl(-), but was unaffected by monovalent cations. It was inhibited about 50% by 10 nanomolar mersalyl, 20 micromolar N,N'-dicyclohexylcarbodiimide, 80 micromolar diethylstilbestrol, or 20 millimolar NO(3) (-); but was insensitive to molybdate, vanadate, oligomycin, and azide. Proton transport into vesicles from the 10/23% sucrose interface was stimulated by Cl(-), inhibited by NO(3) (-), and showed a high pH optimum and a substrate specificity similar to the ATPase, including some proton transport driven by GTP and UTP.The low density of the vesicles (1.10 grams per cubic centimeter) plus the properties of H(+) transport and ATPase activity are similar to the reported properties of intact vacuoles of red beet and other materials. We conclude that the low density, H(+)-pumping ATPase of red beets originated from the tonoplast. Tonoplast H(+)-ATPases with similar properties appear to be widely distributed in higher plants and fungi.

Evidence for a beta-Aspartyl Phosphate Residue in the Phosphorylated Intermediate of the Red Beet Plasma Membrane ATPase

A borohydride reduction method was used to identify the phosphorylated amino acid in the phospho-enzyme of the red beet (Beta vulgaris L.) plasma membrane ATPase. Plasma membrane fractions were phosphorylated with unlabeled ATP in the presence of MgSO(4) at pH 6.5 and then treated with sodium [(3)H]borohydride. The borohydride-treated samples were subjected to hydrolysis in 6 normal HCl at 110 degrees C for 22 hours and then analyzed by high voltage paper electrophoresis and thin layer chromatography. This analysis demonstrated the formation of labeled homoserine as the major reduction product when phosphorylated membrane samples were treated with sodium [(3)H]borohydride. This suggests that the phosphoryl group in the plasma membrane ATPase of red beet storage tissue is attached to the beta-carboxyl side chain of an aspartic acid residue in the active site of the enzyme.

Role of magnesium in the plasma membrane ATPase of red beet

The phosphorylation technique was used to assess the role of Mg in the red beet (Beta vulgaris L.) plasma membrane ATPase. When an excess of ethylenediaminetetraacetate (Tris salt, pH 6.5) was added to phosphorylation reactions at steady-state, the phosphorylation level declined exponentially and the rate constant for dephosphorylation was similar to that observed when phosphorylation reactions were chased with unlabeled ATP. When KCl was included with the EDTA chase, a 2.4-fold increase in the turnover of the phosphoenzyme was observed. Thus, the formation of the phosphorylated intermediate but not its breakdown requires free Mg to be present. When an excess of unlabeled ATP containing MgSO(4) was added to plasma membranes incubated for 20 seconds with [gamma-(32)P]ATP in the absence of MgSO(4), a burst of phosphorylation was observed that declined exponentially. The rate constant for this decline was similar to that observed for phosphoenzyme turnover after initial labeling in the presence of MgSO(4). Extrapolation of this kinetic plot to zero time indicated that ATP binding can occur when MgSO(4) is absent. It is proposed that Mg has a specific role in the transphosphorylation reaction of the terminal phosphate group of ATP to the enzyme.

Plasma membrane ATPase of red beet forms a phosphorylated intermediate

When a plasma membrane-enriched fraction isolated from red beet (Beta vulgaris L.) was incubated in the presence of 40 micromolar [gamma-(32)P] ATP, 40 micromolar MgSO(4) at pH 6.5, a rapidly turning over phosphorylated protein was formed. Phosphorylation of the protein was substrate-specific for ATP, sensitive to diethylstilbestrol and vanadate, but insensitive to azide. When the dephosphorylation reaction was specifically studied, KCl was found to increase the turnover of the phosphorylated protein consistent with its stimulatory effect upon plasma membrane ATPase. The protein-bound phosphate was found to be most stable at a pH between 2 and 3 and under cold temperature, suggesting that the protein phosphate bond was an acyl-phosphate. When the phosphorylated protein was analyzed with lithium dodecyl sulfate gel electrophoresis, a labeled polypeptide with a molecular weight of about 100,000 daltons was observed. Phosphorylation of this polypeptide was rapidly turning over and Mg-dependent. It is concluded that the phosphorylation observed represents a reaction intermediate of the red beet plasma membrane ATPase.

Characterization of a k-stimulated adenosine triphosphatase associated with the plasma membrane of red beet

A membrane fraction enriched with a magnesium-dependent, monovalent cation-stimulated ATPase was isolated from red beet (Beta vulgaris L.) storage roots by a combination of differential centrifugation, extraction with KI, and sucrose density gradient centrifugation. This fraction was distinct from endoplasmic reticulum, Golgi, mitochondrial, and possibly tonoplast membranes as determined from an analysis of marker enzymes. The ATPase activity associated with this fraction was further characterized and found to have a pH optimum of 6.5 in the presence of both Mg(2+) and K(+). The activity was substrate specific for ATP and had a temperature optimum near 40 degrees C. Kinetics with Mg:ATP followed a simple Michaelis-Menten relationship. However the kinetics of K(+)-stimulation were complex and suggestive of negative cooperativity. When monovalent cations were present at 2.5 millimolarity, ATPase was stimulated in the sequence K(+) > Rb(+) > Na(+) > Li(+) but when the concentration was raised to 50 millimolarity, the sequence changed to K(+) >/= Na(+) >/= Rb(+) > Li. The activity was not synergistically stimulated by combinations of Na(+) and K(+). The enzyme was insensitive to NaN(3), oligomycin, ouabain, and sodium molybdate but sensitive to N,N'-dicyclohexylcarbodiimide, diethylstilbestrol, and sodium vanadate. Based on the similarity between the properties of this ATPase activity and those from other well characterized plant tissues, it has been concluded that this membrane fraction is enriched with plasma membrane vesicles.

Phosphorylation of the adenosine triphosphatase in a deoxycholate-treated plasma membrane fraction from corn roots

The ATP phosphohydrolase (ATPase) activity of a corn (Zea mays L., WF9 x Mo17) root plasma membrane fraction was enriched almost 2-fold by selective extraction with 0.1% (w/v) deoxycholate. The detergent treatment solubilized about 30% of the total membrane protein and some ATP hydrolyzing activity that was not K(+)-stimulated, but the major portion of the ATPase activity could be pelleted with membranes. The properties of the ATPase associated with the detergent-extracted plasma membrane fraction were similar to those for the ATPase of the untreated plasma membrane fraction with respect to substrate specificity, pH optimum, kinetics with MgATP, ion stimulation, and inhibitor sensitivity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed only minor differences in protein composition resulting from the detergent treatment.The plasma membrane fraction from corn roots contained an endogenous protein kinase activity. This was shown by the time course of phosphate incorporation and by the labeling of a number of protein bands on SDS-polyacrylamide gel electrophoresis. The deoxycholate treatment removed measurable protein kinase activity and allowed the demonstration of a rapidly turning over covalent phosphorylated intermediate associated with the detergent-extracted plasma membrane fraction. The phosphorylated intermediate was present as a 100,000 dalton polypeptide and may represent the catalytic subunit of the plasma membrane K(+)-ATPase.

Isolation of tonoplast vesicles from tobacco protoplasts

Vacuoles were isolated from protoplasts of Nicotiana glutinosa by the method of Mettler and Leonard (Plant Physiol 1979 64: 1114-1120) with minor modifications so that the number of intact protoplasts contaminating the vacuole preparation was reduced to less than 1% (by number). Isopycnic centrifugation of a [(3)H]choline-labeled, sonicated vacuole preparation on linear 5 to 40% sucrose gradients indicated that tonoplast vesicles equilibrated at a density of about 1.12 grams per cubic centimeter. When tonoplast vesicles were isolated on discontinuous sucrose density gradients substrate specific ATPase activity was not found to be associated with this membrane fraction. These results are discussed in terms of the energetics of ion transport through the tonoplast membrane.

Ion Transport in Isolated Protoplasts from Tobacco Suspension Cells: III. Membrane Potential

The membrane electrical potential difference was measured in cultured cells and isolated protoplasts of tobacco (Nicotiana glutinosa L.) by inserting a microelectrode into cells held fast by a suction micropipette. The potential difference (+/- standard deviation) for unplasmolyzed tobacco cells was -52 +/- 12 millivolts, for cells in 0.3 molar mannitol, -50 +/- 11 millivolts; and for cells plasmolyzed in 0.7 molar mannitol, -49 +/- 12 millivolts all inside negative. The potential difference for isolated protoplasts in 0.7 molar mannitol was -49 +/- 16 millivolts, inside negative. In both cultured cells and protoplasts, the addition of 0.1 millimolar KCN caused a depolarization of the membrane potential. It was concluded that plasmolysis and enzymic release of the protoplast had no significant effect on the membrane potential of cultured tobacco cells.