A membrane-bound alkaline inorganic pyrophosphatase in isolated spinach chloroplasts

Review: "Pyrophosphate and pyrophosphatases in plants, their involvement in stress responses and their possible relationship to secondary metabolism"

Pyrophosphate (PPi) is produced as byproduct of biosynthesis in the cytoplasm, nucleus, mitochondria and chloroplast, or in the tonoplast and Golgi by membrane-bound H⁺-pumping pyrophosphatases (PPv). Inorganic pyrophosphatases (E.C. 3.6.1.1; GO:0004427) impulse various biosynthetic reactions by recycling PPi and are essential to living cells. Soluble and membrane-bound enzymes of high specificity have evolved in different protein families and multiple pyrophosphatases are encoded in all plant genomes known to date. The soluble proteins are present in cytoplasm, extracellular space, inside chloroplasts, and perhaps inside mitochondria, nucleus or vacuoles. The cytoplasmic isoforms may compete for PPi with the PPv enzymes and how PPv and soluble activities are controlled is currently unknown, yet the cytoplasmic PPi concentration is high and fairly constant. Manipulation of the PPi metabolism impacts primary metabolism and vice versa, indicating a tight link between PPi levels and carbohydrate metabolism. These enzymes appear to play a role in germination, development and stress adaptive responses. In addition, the transgenic overexpression of PPv has been used to enhance plant tolerance to abiotic stress, but the reasons behind this tolerance are not completely understood. Finally, the relationship of PPi to stress suggest a currently unexplored link between PPi and secondary metabolism.

Purification, physicochemical properties, and subcellular location of alkaline inorganic pyrophosphatase from sesame (Sesamum indicum L.) cotyledons

Abstract: Changes in the levels of inorganic pyrophosphatases (PPases) were monitored in germinating sesame seeds at regular intervals. Activities of acid and alkaline PPases increased markedly in cotyledons up to day 4, remained at the peak level up to day 7, and then showed a considerable decline thereafter. An alkaline PPase was isolated and purified from 5-day-old sesame cotyledons following acetone precipitation, ammonium sulfate fractionation, and chromatography on DEAE-Sephadex. Current protocol yielded about 20% recovery of total activity with a 6.4-fold purification. The enzyme was a monomer with a molecular mass of 20.8 kDa. Some of the properties of alkaline PPase including stability, substrate specificity, ion requirement, and amino acid composition were studied. Alkaline PPase showed maximum activity at pH 8.6 in the presence of Mg2+ and at 50 degrees C. However, the metal ion could not protect the enzyme against thermal denaturation. Alkaline PPase was highly specific for inorganic pyrophoaphate (PP) as substrate and the Km value was 0.7677 +/- 0.0528 mM. Full activation of the enzyme was achieved with a Mg2+/PPi ratio of 2. Divalent metal ions such as Ca2+, Cu2+, and Zn2+ inhibited PPase activity. Mg2+, partially relieved the inhibition caused by adenosine 5'-triphosphate. Studies related to the localization of alkaline PPase in microbodies revealed that the enzyme was distributed between glyoxysomes and mitochondria, with the former containing more of it.

Overexpression of C(4)-cycle enzymes in transgenic C(3) plants: a biotechnological approach to improve C(3)-photosynthesis

The process of photorespiration diminishes the efficiency of CO(2) assimilation and yield of C(3)-crops such as wheat, rice, soybean or potato, which are important for feeding the growing world population. Photorespiration starts with the competitive inhibition of CO(2) fixation by O(2) at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and can result in a loss of up to 50% of the CO(2) fixed in ambient air. By contrast, C(4) plants, such as maize, sugar cane and Sorghum, possess a CO(2) concentrating mechanism, by which atmospheric CO(2) is bound to C(4)-carbon compounds and shuttled from the mesophyll cells where the prefixation of bicarbonate occurs via phosphoenolpyruvate carboxylase (PEPC) into the gas-tight bundle-sheath cells, where the bound carbon is released again as CO(2) and enters the Calvin cycle. However, the anatomical division into mesophyll and bundle-sheaths cells ("Kranz"-anatomy) appears not to be a prerequisite for the operation of a CO(2) concentrating mechanism. Submerged aquatic macrophytes, for instance, can induce a C(4)-like CO(2) concentrating mechanism in only one cell type when CO(2) becomes limiting. A single cell C(4)-mechanism has also been reported recently for a terrestrial chenopod. For over 10 years researchers in laboratories around the world have attempted to improve photosynthesis and crop yield by introducing a single cell C(4)-cycle in C(3) plants by a transgenic approach. In the meantime, there has been substantial progress in overexpressing the key enzymes of the C(4) cycle in rice, potato, and tobacco. In this review there will be a focus on biochemical and physiological consequences of the overexpression of C(4)-cycle genes in C(3) plants. Bearing in mind that C(4)-cycle enzymes are also present in C(3) plants, the pitfalls encountered when C(3) metabolism is perturbed by the overexpression of individual C(4) genes will also be discussed.

Purification and characterization of thylakoid membrane-bound inorganic pyrophosphatase from Spinacia oleracia L

An inorganic pyrophosphatase (PPase) was purified from thylakoid membrane of spinach leaves to electrophoretic purity by methods including detergent solubilization, ammonium sulfate fractionation, and successive chromatographic techniques. Current protocol yielded about 10% recovery of total activity with a 30-fold purification. The specific activity of the purified enzyme was approximately 400 micromol PPi consumed/mg protein x h. This enzyme is a monomer with a molecular mass of 55 kDa. Several properties, including subunit composition, substrate specificity, ion requirements, inhibitor sensitivities, and amino acid composition, have been studied. Mg2+ is an essential cofactor for the thylakoid PPase. The preferred substrate for the hydrolytic reaction of PPase appears to be dimagnesium pyrophosphate. K+ could not stimulate the enzymatic activity of thylakoid PPase, while F- was a potent inhibitor. Group-specific modification of the thylakoid PPase demonstrates possible involvement of carboxylate residues in the enzymatic activity. Furthermore, antibodies raised against thylakoid PPase in a rabbit could inactivate the PPi hydrolysis of thylakoid and the purified enzyme, but not that of vacuolar H+-PPase, indicating both PPi hydrolases are structurally distinct.

Characterization of a mitochondrial inorganic pyrophosphatase in Saccharomyces cerevisiae

We have studied a mitochondrial inorganic pyrophosphatase (PPase) in the yeast Saccharomyces cerevisiae. The uncoupler FCCP (carbonyl cyanide p-trifluoromethoxyphenylhydrazone) and the ionophores valinomycin and nigericin stimulate the PPase activity of repeatedly washed yeast mitochondria 2-3-fold. We have previously cloned a yeast gene, PPA2, encoding the catalytic subunit of a mitochondrial PPase. Uncouplers stimulate the PPase activity several-fold in mitochondria from both cells that overexpress PPA2 from a high copy number plasmid and cells with normal expression. These results indicate that the PPA2 polypeptide functions as an energy linked and membrane associated PPase. The stimulation of mitochondrial PPase activity by FCCP, but not by valinomycin and nigericin, was greatly enhanced by the presence of DTT. The antibiotics Dio-9, equisetin and the F0F1-ATPase inhibitor oligomycin also increase mitochondrial PPase activity several fold. This stimulation is much higher, whereas basal PPase activity is lower, in isotonic than in hypotonic solution, which indicates that intact membranes are a prerequisite for maximal effects.

Inorganic-pyrophosphate-dependent phosphorylation of spinach thylakoid proteins

It has been shown for the first time that several photosystem-II thylakoid proteins and the main chlorophyll-a/b light-harvesting complex can be phosphorylated with inorganic pyrophosphate as phosphate donor. With pyrophosphate, as with ATP, the protein-kinase reaction is dependent on light or a strong reducing agent. The reaction which can be demonstrated in well-washed spinach thylakoids is dependent on electron transport and is controlled by the redox state of the plastoquinone pool. It is suggested that the pyrophosphate-dependent thylakoid protein phosphorylation is mediated by the same kinase which is responsible for the ATP-dependent protein phosphorylation. This pyrophosphate-dependent kinase activity may be derived from an evolutionary precursor from which ATP-dependent protein phosphorylation also developed.

Alkaline inorganic pyrophosphatase and starch synthesis in amyloplasts

The aim of this work was to see if amyloplasts contained inorganic pyrophosphatase. Alkaline pyrophosphatase activity, largely dependant upon MgCl2 but not affected by 100 μM ammonium molybdate or 60-100 mM KCl, was demonstrated in exracts of developing and mature clubs of the spadix of Arum maculatum L. and of suspension cultures of Glycine max L., but not in extracts of the developing bulb of Allium cepa L. The maximum catalytic activity of alkaline pyrophosphatase in the above tissues showed a positive correlation with starch synthesis, and in the first two tissues was shown to exceed the activity of ADPglucose pyrophosphorylase. Of the alkaline pyrophosphatase activity in lysates of protoplasts of suspension cultures of Glycine max, 57% was latent. Density-gradient centrifugation of these lysates showed a close correlation between the distribution of alkaline pyrophosphatase and the plastid marker, nitrite reductase. It is suggested that much, if not all, of the alkaline pyrophosphatase in suspension cultures of Glycine max is located in the plastids.

Mg(2+)-Dependent, cation-stimulated inorganic pyrophosphatase associated with vacuoles isolated from storage roots of red beet (Beta vulgaris L.)

Vacuoles isolated from storage roots of red beet (Beta vulgaris L.) posess a Mg(2+)-dependent, alkaline pyrophosphatase (PPase) activity which is further stimulated by salts of monovalent cations. The requirement for Mg(2+) is specific. Mn(2+) and Zn(2+) permitted only 20% and 12%, respectively, of the PPase activity obtained in the presence of Mg(2+) while Ca(2+), Co(2+) and Cu(2+) were ineffective. Stimulation of Mg(2+)-PPase activity by salts of certain monovalent cations was due to the cation and the order of effectiveness of the cations tested was K(+)=Rb(+)=NH 4 (+) >Cs(+). Salts of Li(+) and Na(+) inhibited Mg(2+)-PPase activity by 44% and 24%, respectively. KCl-stimulation of Mg(2+)-PPase activity was maximal with 60-100 mM KCl. There was a sigmoidal relationship between PPase activity and Mg(2+) concentrations which resulted in markedly non-linear Lineweaver-Burk plots. At pH 8.0, the optimal [Mg(2+)]:[PPi] ratio for both Mg(2+)-PPase and (Mg(2+)+KCl)-PPase activities was approximately 1:1, which probably indicates MgP2O7 (2-) is the true substrate.

Photophosphorylation as a function of illumination time. I. Effects of permeant cations and permeant anions

(1) Very brief periods of illumination do not initiate photophosphorylation in isolated chloroplast lamellae. The time of illumination required before any phosphorylation can be detected is inversely proportional to the light intensity. At very high intensities, phosphorylation is initiated after illumination for about 4 ms. (2) There is no similar delay in the initiation of electron transport. The rate of electron transport is very high at first but declines at about the time the capacity for ATP synthesis develops. When the chloroplasts are uncoupled with gramicidin the high initial rate persists. (3) Various ions which permeate the thylakoid membrane (K+ or Rb+ in the presence of valinomycin, SCN-, I-, or C1O4-) markedly increase the time of illumination required to initiate phosphorylation. Potassium ions in the presence of valinomycin increase the delay to a maximum of about 50 ms whereas thiocyanate ions increase the delay to a maximum of about 25 ms. The effects of K+ with valinomycin and the effect of SCN- are not additive. Permeant ions and combinations of permeant ions have little or no effect on phosphorylation during continuous illumination. (4) The reason for the threshold in the light requirement and the reason for the effect of permeant ions thereon are both obscure. However, it could be argued that the energy for phosphorylation initially resides in an electric potential gradient which is abolished by migration of ions in the field, leaving a more slowly developing proton concentration gradient as the main driving force for phosphorylation during continuous illumination. If so, the threshold in the presence of permeant ions should depend on internal hydrogen ion buffering.

Membrane-bound potassium and magnesium ion-stimulated inorganic pyrophosphatase from roots and cotyledons of sugar beet (Beta vulgaris L)

1. The 25 000-30 000 X g fraction from sugar beet leaf or roots contains, together with (Na+ plus K+)-activated ATPase, also K+-stimulated inorganic pyrophosphatase. 2. This inorganic pyrophosphatase is also stimulated by Rb+ and to a lesser degree by Li+ and Na+. 3. Na+ is at the same time an inhibitor to the K+-stimulation of the inorganic pyrophosphatase. 4. No signs of synergism for (Na+ plus K+) were found. 5. Optimum pH was at about 8.5.

[The influence of starch on the activity of pyrophosphatase from isolated spinach chloroplasts]

The activity of the inorganic pyrophosphatase from isolated spinach (Spinacia oleracea L.) chloroplasts is strongly dependent upon the addition of magnesium ions. Since the complex of the bivalent ion with inorganic pyrophosphate is the real substrate, a definite Mg(2+)/Na4P2O7-ratio is required for maximum activity. When the activity was measured in particle-free extracts from chloroplasts, this ratio was shown to be approximately 3. However, an increase up to 10 was observed in the presence of thylakoid membranes. Furthermore, the kinetics in the presence of broken chloroplasts becomes sigmoidal.The altered kinetics have been shown to be due to starch located in the thylakoids. The inhibitory effect is caused by amylose alone but not by amylopectin., Detailed kinetic analysis of the inhibition showed no influence of amylose on the Hill-coefficient. Since ethylenediaminetetracetic acid was shown to have similar effects as amylose, starch might regulate the pyrophosphatase activity by binding Mg(2+)-ions.